Fuse Monitor

The idea for this project may have come to me in a flash of inspiration, and its a very simple way to check if a fuse has blown without removing it from its holder. The simplicity of this circuit uses just two components, but with just one resistor and an LED this circuit gives visual indication of when a fuse has blown. LED1 is normally off, being "short circuited " by the fuse, F1. Should the inevitable "big-bang" happen in your workshop then LED1 will illuminate and led you know all about it! Please note that the LED will only illuminate under fault conditions, i.e. with a short circuit or shunt on the load. In this case the current is reduced to a safe level by R1.

Circuit diagram:

Fuse Monitor Circuit Diagram

Author: Andy Collinson - Copyright: Zen

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Battery Self Discharge Indicator

If the lead acid battery is not using for long time, self discharge takes place at the rate of 4 % per week at 27 degree. For example, a 125 Ah tubular battery self discharge at the rate of 5 Amps current per week if it is not in charge / discharge cycles. This add on circuit can be used to detect the self discharge of the 12 volt lead acid battery below the safe level of 11.6 volts. It is ideal to monitor the charge level in the battery, it the battery is not using for long period. The circuit uses only a few components and its working is simple. A PNP transistor T1 act as a switch to light the LED, if the battery voltage drops below the safe level. The base bias of T1 is controlled by a Zener diode ZD. Its rating is 10 volt 1 W.

Circuit diagram:

Battery Self Discharge Indicator Circuit Diagram

The Zener diodes usually requires 1.6 volts excess than its rated value to enter into the “Avalanche state”. So, as long as the battery voltage is above 11.6 (10+1.6), Zener conducts and keeps the base of T1 high. Since T1 is a PNP transistor, it will not conduct till its base becomes negative. So LED remains dark. When the voltage in the battery reduces below 11 volts, Zener turns off and the base of T1 becomes negative. T1 then conducts and LED lights. So the battery can be charged again to keep it in top condition. Preset VR can be used to set the exact point at which LED turns on.
Author: Dr.Mohan Kumar
Copyright: Electroschmatics

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Battery Voltage Indicator Circuit

Monitors battery voltage, Three-LED Display

Connecting this circuit to the battery of your vehicle, you will always know at a glance the approximate voltage available. An indication of battery voltage is useful to the motorist for monitoring the battery's capacity to deliver current, and as a check on the efficiency of the dynamo or alternator. Threshold voltages of the Leds are set by means of two Zener Diodes (D6 & D10) plus two further Diodes wired in series (D4, D5 and D8, D9 respectively) adding a step of about 1.3V to the nominal Zener voltage.
Circuit diagram:

Battery Voltage Indicator Circuit Diagram

Parts:
R1 = 1k
R2 = 100K
R3 = 1k
R4 = 3.3K
R5 = 3.3K
R6 = 1k
R7 = 3.3K
R8 = 3.3K
Q1 = BC547
Q2 = BC547
Q3 = BC557
D1 = Red Led
D2 = Amber Led
D3 = 1N4148
D4 = 1N4148
D5 = 1N4148
D6 = BZX79C10
D7 = Green Led
D8 = 1N4148
D9 = 1N4148
D10 = BZX79C12

Notes:

• Red LED D1 is on when battery voltage is 11.5V or less. This indicates a low battery charge.
• Amber LED D2 is on when battery voltage is comprised in the 11.5 - 13.5V range. This indicates that the battery is good if the motor is off. When motor is running, this indicates no charge from dynamo or alternator.
• Green LED D7 is on when battery voltage is 13.5V or more. This indicates a normal condition when motor is running and dynamo or alternator is charging.

Source:Extremecircuits.net

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Low Battery Indicator II

This circuit indicates the remaining battery life bAy varying the duty cycle and flash rate of an LED as the battery voltage decreases. In fact, the circuit actually indicates five battery conditions: (1) a steady glow assures indicates that the battery is healthy; (2) a 2Hz flicker (briefly off) indicates that the battery is starting to show age; (3) a 5Hz 50% duty-cycle flash is a warning that you should have a spare battery on hand; (4) a brief flicker on at a 2Hz rate indicates the battery's last gasp; and (5) when the LED is continuously off, it's time to replace the battery. IC1 is wired as an oscillator/comparator, with a nominal fixed voltage reference of about 1.5V on its pin 2 (inverting) input (actually, it varies between about 1.7V and 1.4V depending on the hysteresis provided via R6).

Circuit diagram:

Low battery indicator Circuit Diagram

This reference voltage is derived from a voltage divider consisting of resistors R4 & R5, which are connected across the 5V rail derived from regulator REG1, and feedback resistor R6. Similarly, IC1's pin 3 input (non-inverting) is connected to a voltage divider consisting of R1 & R2 which are across the 9V battery. Using the component values shown, the circuit will switch LED1 from being continuously on to flash mode when the 9V battery drops to about 6.5V. Subsequently, LED1 is continuously off for battery voltages below 5.5V.

Naturally, you can tweak the resistor values in the divider network for different voltage thresholds as desired. In operation, the circuit oscillates only when the sampled battery voltage (ie, the voltage on pin 3) is between the upper and lower voltage thresholds set on pin 2. Capacitor C3 provides the timing. Above and below these limits, IC1 simply functions as a comparator and holds LED1 continuously on or off. Finally, to precisely set the "dead-battery" threshold, make R4 adjustable to offset the variations in regulator tolerance.

Author: Ashish Nand - Copyright: fgf

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Water Level Indicator Circuit Schematic

Simple, two-wire, remote monitoring unit, Three-LED level display, 9V battery powered

The whole project was developed on a friend's request. Its purpose was to remotely monitor the water-level in a metal tank located in the attic by means of a very simple control unit placed in the kitchen, some floors below.Mains requirements were:

1. No separate supply for the remote circuit
2. Main and remote units connected by a thin two-wire cable
3. Simple LED display for the main unit
4. Battery operation to avoid problems related to mains supply and water proximity
5. As the circuit was battery operated a low current consumption was obviously welcomed

The very small remote unit is placed near the tank and measures the water level in three ranges by means of two steel rods. Each range will cover one third of the tank capacity:

Almost empty - signaled by means of a red LED (D3) in the control unit display
About half-level - signaled by means of a yellow LED (D2) in the control unit display
Almost full - signaled by means of a green LED (D1) in the control unit display

Circuit diagram:

Water-level Indicator Circuit Diagram

Circuit operation:

When the water-level is below the steel rods, no contact is occurring from the metal can and the rods, which are supported by a small insulated (wooden) board. The small circuit built around IC1 draws no current and therefore no voltage drop is generated across R5. IC2A, IC2B and Q1 are wired as a window comparator and, as there is zero voltage at input pins #2 and #5, D3 will illuminate. When the water comes in contact with the first rod, pin #13 of IC1 will go high, as its input pins #9 to #12 were shorted to negative by means of the water contact.

Therefore, R4 will be connected across the full supply voltage and the remote circuit will draw a current of about 9mA. This current will cause a voltage drop of about 0.9V across R5 and the window comparator will detect this voltage and will change its state, switching off D3 and illuminating D2. When the water will reach the second rod, also pin #1 of IC1 will go high for the same reason explained above. Now either R3 and R4 will be connected across the full supply voltage and the total current drawing of the remote circuit will be about 18mA.

The voltage drop across R5 will be now about 1.8V and the window comparator will switch off D2 and will drive D1. The battery will last very long because the circuit will be mostly in the off state. Current is needed only for a few seconds when P1 is pushed to check the water-level and one of the LEDs illuminates.

Parts:
R1 = 15K 1/4W Resistors
R2 = 15K 1/4W Resistors
R3 = 1K 1/4W Resistors
R4 = 1K 1/4W Resistors
R5 = 100R 1/4W Resistor
R6 = 47K 1/4W Resistor
R7 = 3.3K 1/4W Resistors
R8 = 3.3K 1/4W Resistors
R9 = 2.7K 1/4W Resistors
R10 = 15K 1/4W Resistors
R12 = 15K 1/4W Resistors
R13 = 3.3K 1/4W Resistors
R14 = 2.7K 1/4W Resistors
R15 = 2.7K 1/4W Resistors
D1 = 3mm Green LED
D2 = 3mm Yellow LED
D3 = 3mm Red LED
C1 = 470nF 63V Polyester or Ceramic Capacitor
J1 = Two ways output sockets
J2 = Two ways output sockets
P1 = SPST pushbutton
B1 = 9V PP3 Battery
Q1 = BC547 45V 100mA NPN Transistor
IC1 = 4012 Dual 4 input NAND gate IC
IC2 = LM393 Dual Comparator IC
Two steel rods of appropriate length

Notes:

• The two steel rods must be supported by a small insulated (wooden) board
• IC1 and R1-R4 are mounted on a small board placed near or on the steel rods support
• The two-wire cable connecting the remote circuit board to the main control board, i.e. J1 to J2, can be of any size and type (preferably thin for obvious reasons). It can be very long, if necessary.
• The circuit can be used also with non-metal tanks, provided a third steel rod having the height of the tank will be added and connected to pin #7 of IC1, R3, R4 and J1.
• The 4012 chip was chosen because it contains two gates and was at hand, but you can use two of the gates contained into 4001, 4011, 4093, 4049, 4069 etc. chips, provided all inputs of each gate are tied together and all inputs of unused gates are connected to the positive rail, leaving output pins open.

Source:Extremecircuits.net

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Low Battery Indicator I

Here is the circuit diagram of low battery indicator from silicon chip electronics. This simple circuit lights LED1 when the battery voltage drops below the setting set by trimpot VR1. In effect, VR1 and associated resistors bias Q1 on which holds Q2 and the LED off. When the voltage drops below the set value, Q1 turns off, allowing Q2 to turn on and light the LED. The circuit is suitable for nominal battery voltages up to 12V.

Circuit diagram:

Low battery indicator Circuit Diagram

Author: I. Ross - Copyright: Silicon Chip

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Maximum Minimum Voltage Indicator

This circuit indicates which of three voltages in the range from about about -4V to about +4V - at A, B and C - is the highest by lighting one of three indicator LEDs. Alternatively, it can be wired to indicate the lowest of three voltages or to indicate both the highest and lowest voltages. Op amps IC1a, IC1b & IC1c are wired as comparators, while the three indicator LEDs and their series 1kO current limiting resistors are strung across the op amp outputs to implement the appropriate logic functions.

Circuit diagram:

Maximum Minimum Voltage Indicator Circuit Diagram

For example, LED A will light only when pin 8 of IC1c is low (ie, A greater B) and pin 7 of IC1b is high (ie, A greater C). Similarly, LED B will light only when pin 8 of IC1c is high (ie, B greater A) and pin 1 of IC1a is low (ie, B greater C). LED C works in similar fashion if the voltage at C is the highest. Note that if all the LEDs and their parallel 1N4148 diodes are reversed, the circuit will indicate the lowest of the three input voltages. And if each 1N4148 diode is replaced by a LED, the circuit will indicate both the highest and lowest inputs.

Author: Andrew Partridge - Copyright: Silicon Chip

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Clipping Indicator For Audio Amplifiers

A clipping indicator is a useful accessory on any audio amplifier. It indicates when the amplifier has reached its limit and is clipping the peaks of the audio signal. In practice, quite a lot of clipping can occur before you can hear it. So why is it necessary to know when an amplifier is clipping if you can't notice it? The answer is that clipping "squares up" the waveform and square waves contain lots of higher-frequency harmonics which can easily damage the tweeters in loudspeaker systems. This circuit is a true clipping indicator as opposed to the level indicators that are commonly used in preamplifier stages.

The problem with level indicators is that an amplifier's maximum output power is not constant. That's because the amplifier's supply rails are not regulated and so the maximum power available at any given instant varies, depending on the applied signal. The circuit is quite simple and is based on two BD140 PNP transistors and zener diode ZD1. During normal operation, Q1 is turned on via ZD1 and R1. As a result, Q2 is held off (since its base is pulled high) and so LED1 is also off. However, if the output signal subsequently rises to within 4.7V of the positive supply rail, Q1 turns off since it no longer has any forward bias on its base.

Circuit diagram:

Clipping Indicator Circuit Diagram

As a result Q2's base is now pulled low via R2 and so Q2 turns on and lights LED1. (Note: the 0.6V drop across Q1's base/emitter is ignored here because ZD1 conducts before its rated voltage due to the very low current involved). Why choose 4.7V below the power rail as the turn-on point? The reason is that, due to the drive limitations and the nature of emitter followers, they can be expected to have at least 4V across them when they saturate (ie, clip). ZD1 can be increased to a 5V or 6.2V type if the circuit is to be used with a monster amplifier.

The value of R3 should be customized according to the amplifier's supply rail, so that LED1 operates with the correct brightness. To do that, first measure the amplifier's positive supply voltage, then use Ohms Law (R = V/I) to calculate the value of R3 for a current of about 20mA. As it stands, this circuit can only be used to monitor the positive-going half-cycles of the audio waveform. If you want to monitor the negative half-cycles as well, you will have to build a second circuit with the following changes: (1) reverse both LED1 and ZD1; and (2) use BD139 (NPN) transistors for Q1 & Q2. Note that, in both cases, you should use the earth inside the amplifier, as the speaker negative may not be earth (such as in a bridged output).

Author: Philip Chugg - Copyright: Silicon Chip Electronics Magazine

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Peak Indicator

Small, portable and experimental circuit, Simple unit

A simple circuit, peak indicator of tops of musical signal. Each time where the level of signal exceeds the level + 4dB, turns on led D1. It is useful in each channel of console of sound, in final amplifiers or in that other application, to we needed. With the prices of circuit, the indicate begins with levels above + 4 dB (1.25V rms). For adaptation in different levels of signal, we can use a trimmer, before capacitor C1.
Circuit Diagram:

Peak Indicator Circuit Diagram

Parts:
R1 = 10K
R2 = 1.2K
R3 = 220K
R4 = 4.7K
R5 = 4.7K
C1 = 47uF-25V
C2 = 2.2uF-25V
Q1 = BC550C
Q2 = BC550C
D1 = Red LED

Notes:

• It can be assembled on a general purpose PCB.
• It can be powered from a 12V-15V regulated power supply.

Source:Extremecircuits.net

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Audio Clipping Indicator

Detects clipping in preamp stages, mixers, amplifiers etc., Single LED display - 9V Battery supply unit

This circuit was intended to be used as a separate, portable unit, to signal by means of a LED when the output wave form of a particular audio stage is "clipping" i.e. is reaching the onset of its maximum permitted peak-to-peak voltage value before an overload is occurring. This will help the operator in preventing severe, audible distortion to be generated through the audio equipment chain. This unit is particularly useful in signaling overload of the input stages in mixers, PA or musical instruments amplification chains, but is also suited to power amplifiers. A careful setting of Trimmer R5 will allow triggering of the LED with a wide range of peak-to-peak input voltages, in order to suit different requirements. Unfortunately, an oscilloscope and a sine wave frequency generator are required to accurately setup this circuit. Obviously, the unit can be embedded into an existing mixer, preamp or power amplifier, and powered by the internal supply rails in the 9 - 30V range. The power supply can also be obtained from higher voltage rails provided suitable R/C cells are inserted. SW1 and B1 must obviously be omitted.

Circuit diagram:

Audio Clipping Indicator Circuit Diagram

Parts:

R1_______________1M 1/4W Resistor (See Notes)
R2,R3,R8_______100K 1/4W Resistors
R4,R6___________10K 1/4W Resistors
R5_______________5K 1/2W Trimmer Cermet or Carbon
R7_______________2K2 1/4W Resistor
R9______________22K 1/4W Resistor
R10______________1K 1/4W Resistor (See Notes)
C1,C4__________220nF 63V Polyester Capacitors
C2_______________4p7 63V Ceramic Capacitor (See Notes)
C3_____________220µF 25V Electrolytic Capacitor
C5______________10µF 25V Electrolytic Capacitor (See Notes)
D1,D2________1N4148 75V 150mA Diodes
D3______________LED (Any dimension, shape and color)
Q1____________BC547 45V 100mA NPN Transistor
IC1___________TL062 Dual Low current BIFET Op-Amp (or TL072, TL082)
SW1____________SPST Toggle or Slide Switch (See Text)
B1_______________9V PP3 Battery (See Text)

Circuit operation:

The heart of the circuit is a window comparator formed by two op-amps packaged into IC1. This technique allows to detect precisely and symmetrically either the positive or negative peak value reached by the monitored signal. The op-amps outputs are mixed by D1 and D2, smoothed by C4, R7 and R8, and feed the LED driver Q1 with a positive pulse. C5 adds a small output delay in order to allow detection of very short peaks.

Notes:

• With the values shown, the circuit can be easily set up to detect sine wave clipping from less than 1V to 30V peak-to-peak (i.e. 15W into 8 Ohms). If you need to detect higher output peak-to-peak voltages, R1 value must be raised. On the contrary, if the circuit will be used to detect only very low peak-to-peak voltages, it is convenient to lower R1 value to, say, 220K omitting C2. In this way, the adjustment of R5 will be made easier.
• Using a TL062 chip at 9V supply, stand-by current drawing is about 1.5mA and less than 10mA when the LED illuminates. With TL072 or TL082 chips, current drawing is about 4.5mA and 13mA respectively.
• When using power supplies higher than 12V, the value of R10 must be raised accordingly.
• When using power supplies higher than 25V, the working voltage value of C5 must be raised to 35 or 50V.

Source:Extremecircuits.net

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Telephone In-Use Indicator

This circuit will illuminate a LED if one of your telephones is in use. It should work in all countries (Including UK) that have a standing line voltage above 48 Volts DC. Please note that it is illegal to make a physical permanent connection to your telephone line in some countries (this includes the UK and Ireland). If building this circuit it is advisable to use a plugin cord so that the unit can be unplugged should a fault occur. If in doubt consult either your telephone or cable operator.

Circuit diagram:

Telephone In-Use Indicator Circuit Diagram

If all extension phones are on-hook and the line voltage is around 48 V, Q1 will conduct thus effectively shorting the gate of Q2 to its source, so it will be off and the LED will be disabled. Lifting the handset of any phone on the line causes the line voltage to drop to 5-15 V. The gate voltage of Q1, equal to some 6% of the line voltage, will then be too low and Q1 will be turned off. So Q2's gate is now biased at approximately 1/2 of the line voltage, Q2 turns on and the LED indicates that the line is in use. The circuit itself is practically invisible to the other telephone devices using the same line. LED1 must be low-current and its current-limiting resistor must be 2k2 or more.

The other components' ideal values may vary slightly, depending on the local telephone line parameters. The circuit is powered off the telephone line. If other types of MOSFETs are used, the 500k trimmer can be adjusted to ensure that Q1 is biased fully on while the line is not in use (LED1 off), and vice versa. If Q2 is not a BS108 but some other 200 V MOSFET with a higher G-S threshold voltage, it might be necessary to increase the value of the lower (or decrease the value of the upper) one of the two resistors connected to the gate of Q2. Plain (bipolar junction) transistors can be used instead and the circuit also works fine.

But the resistor values are then much lower - letting ten times more microamps of current pass through while the line is not in use, and even this MOSFET design still could not meet formal minimum on-hook DC resistance specifications. Both prototypes' PCBs were 4x1 cm. The current-limiting resistor for LED1 is 2k2 in both cases. DO NOT ground any of the leads or conducting surfaces in this circuit. A more reliable design would also include some kind of over-voltage protection etc.

Warning:

In their normal course of operation, telephone lines can deliver life-threatening voltages! Do not attempt to build any of the circuits/projects unless you have the expertise, skill and concentration that will help you avoid an injury. There are also legal aspects and consequences of connecting things to telephone lines, which vary from country to country. Keep away from telephone lines during a lightning storm!

Source: Zen

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Two-LED Voltage Indicator

There are many applications where the accuracy of a digital or analogue (bar graph) is not required but something better than a simple low/high indicator is desirable. A battery charge level indicator in a car is a good example. This simple circuit requiring only two LEDs (preferably one with a green and red LED in a single package), a cheap CMOS IC type 4093 and a few resistors should ful-fil many such applications. With a suitable sensor, the indicator will display the relevant quantity as a colour ranging from red through orange and yellow to green. IC1.A functions as an oscillator running at about 10 kHz with the component values given, although this is not critical.

Assuming for the moment that R1 is not commented, the output of IC1.A is a square wave with almost 50% duty cycle. The voltage at the junction of R2 and C1 will be a triangular wave (again, almost) with a level determined by the difference in the two threshold voltages of the NAND Schmitt trigger gate IC1.A. IC1.B, IC1.C and IC1.D form inverting and noninverting buffers so that the outputs of IC1.C and IC1.D switch in complementary fashion. With a 50% duty cycle, the red and green LEDs will be driven on for equal periods of time so that both will light at approximately equal brightness resulting in an orange-yellow display. With R1 in circuit, the actual input voltage to IC1.

Circuit diagram:

Two-LED Voltage Indicator Circuit Diagram

A will consist of the triangular waveform added to the dc input Vin. As the input voltage varies, so will the oscillator duty cycle causing either the red or the green LED to be on for longer periods and so changing the visible colour of the combi-LED. The actual range over which the effect will be achieved is determined by the relative values of R1 and R2, enabling the circuit to be matched to most supply voltages. With the component values given and a supply of 8 volts, the LED will vary from fully red to fully green in response to input voltages of 2.5 V and 5.6 V respectively. To monitor a car battery voltage, the battery itself could be used to power the circuit provided a zener diode and dropper resistor are added to stabilise the IC supply voltage.

This is shown in dashed outlines in the circuit diagram. With an 8.2 V zener the dropper resistor should be around 220 ? and R1 has to be reduced to 4.7 k. The LED brightness is determined by R4. As a rule of thumb, R4 = (Vsupply – 2) / 3[k] and remember that the 4093 can only supply a few mA’s of output current. Applications of this little circuit include ‘non critical’ ones such as go/non-go battery testers, simple temperature indicators, water tank level indicators, etc.
Author: Bart Trepak - Copyright: Elektor 2004

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Yes-No Indicator Has Zero Standby Current

This circuit produces a random "Yes" or "No" with a single button press - indicated by the illumination of a red or green LED. The circuit has two advantages over similar circuits. First, it uses just a single momentary contact pushbutton, so no on-off switch is required. When the pushbutton is pressed, an oscillator comprising the 10nF capacitor and 22kΩ resistor at pins 1 & 2 is almost immediately stopped by FET Q1, which pulls the oscillator's timing capacitor to the positive rail. However, the 220nF capacitor and 470kΩ resistor in the gate circuit of Q1 introduce a tenth of a second's delay, so that about 250 oscillations take place before the clock is stopped.

Due to variations in charge on the circuit's capacitors, as well as voltage and temperature variations, and the unpredictability of when the pushbutton will be pressed, randomness is assured. The circuit has a high degree of randomness because it takes advantage of a near-perfect complementary square waveform at pins 10 and 11 of the 4047 IC. The oscillator frequency (available at pin 13) is passed through an internal divide-by-2 circuit in the 4047. This appears at pin 10 (Q), and is inverted at pin 11 (Q-bar), thus assuring a near perfect 50:50 duty cycle for the two LEDs.

Circuit diagram:

Yes-No Indicator Circuit Diagram

Note:

However, that the "impartiality" of the circuit is partly contingent on the value of the 10nF capacitor and on a reasonably equal current flow through both LEDs. Over five trials, the Yes-No Indicator scored 142 Yes, 158 No, with Yes falling behind No in the fourth trial. Because the circuit only works while switch S1 is pressed, standby current is zero, therefore a miniature 12V battery may be used to power it. In this case the circuit could be used thousands of times before the battery would run flat. The circuit has a further potential use. If the LEDs are omitted and a piezo (capacitive) sounder is wired directly to pins 10 and 11, it will produce a loud beep when equipment is turned on, and will continue to draw less than 0.5mA until it is switched off. The frequency of the beep may be changed by altering the value of the 10nF capacitor and its duration by altering the value of the 220nF capacitor.

Author: Thomas Scarborough - Copyright: Silicon Chip Electronics

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DC or AC Voltage Indicator Circuit

Detects 1.8 to 230 Volts DC or AC, Minimum parts counting

This circuit is not a novelty, but it proved so useful, simple and cheap that it is worth building. When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED will illuminate. Reversing polarities the Green LED will illuminate. Connecting the probes to an AC source both LEDs will go on.The bulb limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from about 30V, shining more brightly as voltage increases. Therefore, due to the bulb filament behavior, any voltage in the 1.8 to 230V range can be detected without changing component values.

Circuit diagram:

DC or AC Voltage Indicator Circuit Diagram

Parts:
P1 = Red Probe
P2 = Black Probe
D1 = 5 or 3mm. Red LED
D2 = 5 or 3mm. Green LED
LP = 1220V 6W Filament Lamp Bulb

Note:

• A two colors LED (Red and Green) can be used in place of D1 & D2.

Source:Extremecircuits.net

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Audio Indicator Using LM741

This circuit can be used to remotely monitor a loudspeaker, alarm, or audio source for presence of an audio waveform. It can also be directly connected across loudspeaker terminals used as a peak indicator. If you need to monitor some audio signal at a location that is either soundproof are too far away to actually hear the signal then you can use a circuit similar to this one. A remote indication that the audio source is active is provided visually, here in the form of a LED. Referring to the above circuit, the 10k preset is a level control which should be adjusted so that when the source ( audio signal or alarm ) is producing the desired amount of noise, the LED lights. The input stage is an inverting x100 amplifier made with a 741 op-amp.

Circuit diagram:

Audio Indicator Circuit Diagram Using LM741

Author: Andy Collinson

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Blown Fuse Indicator

This blown fuse indicator will work with a wide range of DC supply voltages from 5V to 50V. It illuminates LED1 when the fuse blows. With the fuse intact, Q1 is held off and there is no bias current available for the base of Q2. So the LED is off. When the fuse blows, a small current flows via the base-emitter junctions of Darlington transistor Q1, through its base resistor R1 and then via the load. Typically this current will be around 20μA and this turns on Q1 which provides base current to Q2 which then turns on to illuminate the LED.

Circuit diagram:

Blown Fuse Indicator Circuit Diagram

The emitter current of Q2 is limited by Q3 which turns when the current reaches about 10mA, to shunt base current away from Q2. The three resistor values not given in the circuit are dependent on the supply voltage and can be calculated from the following simple equations:

• R1(kΩ) = V(DC)/0.02 = 560kΩ for 12V DC
• R2(kΩ) = V(DC)/2 = 5.6kΩ for 12V DC
• R3(Ω) = V(DC)/0.02 = 560Ω for 12V DC

R3 should be included for voltages above about 20V otherwise the heat dissipation in Q2 will be too great. At lower voltages it can be omitted. Any general purpose NPN transistors can be used for Q2 and Q3, provided they will handle the DC supply voltage. The PNP Darlington, Q1, could be an MPSA65, available from Dick Smith Electronics (Cat Z-2088).

Author: Keith Gooley - Copyright: Silicon Chip

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Mains Indicator

It is not always immediately obvious whether a power-consuming appliance is switched on or not. Examples are the lamp in the attic or the shed, or electric heating in an awkward place. A nice solution would be to connect an LED directly in series with the appliance, unfortunately you’d better duck for cover if you tried… The obvious solution would be to place a (power-) resistor in series with the load and connect an LED with series resistor across it.

However, this solution has significant disadvantages, for instance, the power loss is relatively large (easily a few watts). In addition, the value of the resistor should be adjusted depending on the magnitude of the current. It would be better to insert two anti-parallel diodes in the power lead. Unfortunately, the voltage drop is too low to power an LED. It does work with 6 diodes, for that matter, but the power loss is then also 3 times greater.

Circuit diagram:

Mains Indicator Circuit Diagram

We therefore chose a solution with two diodes, followed by a 4 times voltage multiplier in the form of a cascade rectifier. That is an energy friendly solution. The current through the LED is automatically limited by the internal impedance of the cascade rectifier. The impedance isn’t that small, despite the large electrolytic capacitors. Use a low-current LED, otherwise the LED will probably not be bright enough.

Parts layout:

Parts Layout Of Mains Indicator

The 1N5404 used here can handle up to 3 A (3 A × 230 V = 690 W). If the power is less than 200 W, you could use two 1N4004s instead. The voltage across the diodes is a square wave with an amplitude of about 1.3 Vpp. The voltage multipliers are used to turn this into the LED voltage. This will only work if the voltage drop across the diodes in the multipliers isn’t too large. That is why these diodes are Schottky diodes. These only have about a 0.35 V voltage drop.

PCB layout:

PCB Layout Of Mains Indicator

Exactly which type of Schottky diode that you use is not too important. You are free to experiment with the value of the electrolytic capacitors. The larger their value, the greater is the amount of current that can be delivered. Keep in mind that working with mains voltage can be fatal. Build the circuit in such away that there is no risk that live parts can be touched and maintain isolation distances of 6 mm (also in air). For the same reason, use a 5 mm LED (not a 3 mm one!) and fit it as far into the enclosure as possible. Mount the PCB in the enclosure with nylon bolts.

COMPONENTS LIST
Capacitors:
C1-C4 = 220µF 6.3V
Semiconductors:
D1,D2 = 1N5401
D3-D6 = BAT85 (or any other Schottky diode)
D7 = LED, low current
Miscellaneous:
K1 = 2-way PCB terminal block, lead pitch 5mm

Author: Karel Walraven - Copyright: Elektor Electronics

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Earth Fault Indicator

The security of many electrical devices depends today on the availability of an earthed mains outlet. We should remember that these are connected to the frame or to the metal housing of the equipment and so it routes to the protective earth (PE) connections. In this setup, mains voltage, however small, will cause the differential circuit breaker to trip. The circuit breaker is part of any modern electrical installation. This type of security device may however become defective due to common corrosion as we have seen many times on various older household devices, as well as on construction sites.

Actually, since these devices are frequently in wet conditions, the screw and/or lug used to connect the earth wire to the device frame corrodes gradually and ends up breaking or causing a faulty contact. The remedy is then worse than the problem because the user, thinking that he/she is protected by earth, does not take special precautions and risks his/her life. However, all that’s needed is an extremely simple system to automatically detect any break in the earth connection; so simple that we ask ourselves why it is not already included as part of all factory production for appliances that carry any such risk, as we have discussed above.

We propose it as a project for you to build using this schematic. The live wire (L) of the mains power supply is connected to diode D1 which ensures simple half-wave rectification which is sufficient for our use. The current which is available is limited to a very low value by resistor R2. If the appliance earth connection to which our circuit is installed is efficient, this current is directed to earth via resistor R1 and the rest of the circuit is inactive due to insufficient power. If the earth connection is disconnected, the current supplied by D1 and R2 charges up capacitor C1.

Circuit diagram:

Earth Fault Indicator Circuit Diagram

When the voltage at the terminals of the capacitor reaches about 60 volts, neon indicator light La1 is turned on and emits a flashing light which discharges capacitor C1 at the same time. This phenomenon is reproduced indefinitely as long as the earth connection has not been restored, and the neon light continues to flash to attract attention in case of danger. Building the project is not particularly difficult but, since it is a project aimed at human safety, we must take the maximum of precautions concerning the choice of components utilised. Therefore, C1 must have an operating voltage of at least 160 volts while R2 must be a 0.5-watt resistor, not for reasons of power dissipation, but in order to maintain the voltage.

The neon light can be any type, possibly used, or it may be part of an indicator light to make it easier to attach to the protected appliance. In the second case, we must obviously get rid of its series resistor which would prevent proper operation here. During installation of the circuit in the appliance to be protected, we should also clearly mark Live (L) and Neutral (N) (for example, seek Live with a simple screwdriver) because inverting these two wires at this point will disable proper operation. The final point, which is self-evident considering the principle used here: the earth connection for our setup must be hooked up to the frame of the appliance to be protected at a different point than where the normal earth wire is connected.
Author: Christian Tavernier - Copyright: Elektor Electronic Magazine

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Battery Indicator Circuit For The Caravan

This i-TRIXX circuit can prevent a whole lot of trouble for those of you who go on holiday in a caravan. It would be a significant damper on your holiday spirit when you are ready to leave the camping and discover that you have used your battery too much and that you are now unable to start the car. This annoyance can be avoided if you were warned early enough by an illuminated LED when the charge in the battery threatens to become too low.

A quiet, out of the way, in the countryside camping is what modern people look for to be able to unwind. However, we do not want to be completely deprived of all our creature comforts. We don’t cope very long without electric light or a TV! And in the absence of a mains power outlet the car battery has to function as energy source, with the risk that later on there will be too little left to start the engine. The little circuit presented here gives you an early warning when the battery voltage (and therefore its stored energy) threatens to become too low.

Circuit diagram:

Caravan Battery Indicator Circuit Diagram

The setting of T1 and T2 determines whether LED D2 will light up when the battery voltage drops below a certain level. Junction FET T3 is used as a current source in order to try to keep the current through the LED as constant as possible. In this way the indicator remains lit even when the battery is in a state of very deep discharge (< 4 volt). The LED is a good low-current type that is still very bright at a very small current (1 to 2 mA). Voltage divider R1 and R2 has been calculated such that T1 will start to conduct when the voltage of the battery is greater than 12 V.

If you think this threshold is too high (or: if you think that you can still start your car with a lower battery voltage), then you can reduce the value of R2 or replace it with a 50-k preset (connected as an adjustable resistor). When T1 conducts, the base current to T2 is interrupted and the collector of T2 will become high through R4. In this state T3 does not conduct and the LED is off. When the battery voltage drops below 12 V, T1 will block and T2 will start to conduct. R5 is now connected to ground via T2 which turns T3 into a current source of about 2 mA that drives the LED.

There is, of course, a transition region during which the current through the LED slowly increases; after all, T1 and T2 do not switch with in?nite gain! In our prototype the LED changed from fully off to fully on at a voltage variation from 12 to 11 V. As a bonus, a partially illuminated LED gives a rough indication as to how much the voltage actually is. Diode D1 prevents the circuit from inadvertently giving up the ghost if the circuit is connected incorrectly to the battery (reverse polarity). In practice, because of variations in the specifications of the transistors, the threshold and the current level through the LED can be different.

Test the circuit thoroughly before using it. If you want a brighter indicator, you can increase the current through the LED by replacing T3 with a BF245B or BF245C. When the LED is off, the current through the circuit is barely 30 µA at a battery voltage of 14.4 V. With the LED is on and at a battery voltage of about 10 V, the current consumption is about 2 mA. Even with an illuminated LED, the circuit is not likely to be the cause of a flat battery. Even a good quality battery will have a self discharge rate which is many times greater than the maximum current consumption of this circuit!
Elektor Electronics 12-2006

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110 and 220V AC LED Voltage Indicator

Useful for power lines control, Simple, transformerless circuitry

This circuit, designed on request, has proven to be useful to indicate when the voltage in a power supply line is changing from 120V to 240Vac. It can be used in different circumstances and circuits, mainly when an increase in ac or dc supply voltage needs to be detected. D3 illuminates when the line voltage is approaching 120V and will remain in the on state also at 240V supply. On the other hand, D6 will illuminate only when the line voltage is about 240V and will stay on because the latching action of Q1, Q2 and related components. C1, D1 and D2 provide a low dc voltage in the 4.5V - 6V range in order to allow proper operation of latch circuit and LEDs.

Circuit diagram:

Parts:

R1_____________470R 1/2W Resistor
R2_____________220K 1/4W Resistor
R3,R7__________470R 1/4W Resistors
R4_______________1K 1/4W Resistor
R5_______________2K2 1/4W Resistor
R6_____________330R 1/4W Resistor
C1_____________330nF 630V Polyester Capacitor
C2______________10µF 25V Electrolytic Capacitor
D1,D2________1N4007 1000V 1A Diode
D3,D6___________LEDs (Color and shape at will)
D4_________BZX79C10 10V 500mW Zener Diode (See Notes)
D5___________1N4148 75V 150mA Diode
Q1____________BC547 45V 100mA NPN Transistor
Q2____________BC557 45V 100mA PNP Transistor

Notes:

• D4 value could require some adjustment in order to allow precise switching of the circuit at the chosen voltage. If the case, please try values in the 8.2V - 15V range.
• Warning! The circuit is connected to 240Vac mains, then some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when plugged and enclose it in a plastic box.

Source: www.RedCircuits.com

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