Quick Counter For Young Children

This circuit is a toy to encourage young children to count. Power is turned on by switch S1, then S2 is closed. This makes nine LEDs flash slowly. S2 is then opened and the LEDs go out. Pressing pushbutton PB1 turns on a random number of LEDs - briefly - during which time they are to be counted. The number counted can be checked by pressing PB2 which turns the same LEDs on for as long as needed. Then repeat. The circuit works as follows: IC3 is a 4049 hex inverter connected as three oscillators running at different rates. It is turned on by closing switch S2a. The clock pulses from IC3 drive both halves of IC1 and one half of IC2, both being 4015 dual 4-stage shift registers. Each shift register has four outputs which go high in order: 1, 1 and 2; 1 and 2 and 3; 1 and 2 and 3 and 4. However as output 4 is connected to the reset line of its own half - the shift register resets to zero. Outputs 1, 2 & 3 of all three shift registers are connected to nine LEDs, the cathodes of which go to a common rail.

Circuit diagram:

Quick Counter Circuit Diagram For Young children

This rail is connected to ground via S2b when switch S2 is closed. When S2 is opened the three oscillators stop but a random number of LEDs is still connected to the high outputs of the 4015s. That number can be viewed briefly by pressing PB1 which pulses the 7555 timer in monostable mode, to give a short duration output which drives Q1 and connects the LED cathodes to 0V. The viewing time is adjustable by VR1. Checking a count is done by pressing PB2 which holds the same LEDs on as long as desired. The LEDs are set in a 3 x 3 grid with the connection scattered, ie, the first row is not the three LEDs from the first half of IC1. Note that, unlike the usual dice, a number such as 5 can appear in many formats, so pattern recognition is no help. Also note that this is not a nine output true dice - because the numbers do not come up with equal frequency.

Author: A. Lowe - Copyright: Silicon Chip

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LED Noughts And Crosses

Here’s a twist to the age-old game of noughts and crosses. Instead of pen and paper, it uses nine 10mm tri-colour LEDs arranged in a 3 x 3 grid. One player has nine red buttons while the other player has nine green, set out in identical grids. The aim, of course, is to make three LEDs in a row glow the same colour – red or green! Pushing a red button causes the LED in the equivalent position on the grid to glow red in colour. Likewise, pushing a green button lights the equivalent green LED.

Circuit diagram:

LED Noughts And Crosses Circuit Diagram

If a player pushes a button for a LED that is already glowing red or green, then that LED changes to yellow, exposing the false move! All the LEDs are then turned off, ready for the next game, by pressing the "Clear" button. For simplicity, the circuit shows only one tri-colour LED and a pair of opposing buttons. This circuit fragment must be repeated another eight times to create a complete 3 x 3 grid. A brief press on a button fires the associated SCR and turns on the LED. The common (cathode) lead of all LEDs is connected to the 0V rail via the normally-closed contacts of the "Clear" pushbutton (S3).

Author: A. J. Lowe - Copyright: Silicon Chip Electronics

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Soldering Iron Tip Preserver

Although 60/40 solder melts at about 200&degC, the tip temperature of a soldering iron should be at about 370&degC. This is necessary to make a good quick joint, without the risk of overheating delicate components because the iron has to be kept on the joint for too long. Unfortunately, at this temperature, the tip oxidises rapidly and needs constant cleaning. That's where this circuit can help - it keeps the soldering tip to just below 200&degC while the iron is at rest.

Oxidisation is then negligible and the iron can be brought back up to soldering temperature in just a few seconds when needed. In addition, normal soldering operation, where the iron is returned to rest only momentarily, is unaffected because of the thermal inertia of the iron. Two 555 timers (IC1 & IC2) form the heart of the circuit. IC1 is wired as a monostable and provides an initial warm-up time of about 45 seconds to bring the iron up to temperature. At the end of this period, its pin 3 output switches high and IC2 (which is wired in astable configuration) switches the iron on - via relay RLY1 - for about one second in six to maintain the standby temperature.

Circuit diagram:

Soldering Iron Tip Preserver Circuit Diagram

The presence of the iron in its stand is sensed by electrical contact between the two and some slight modification of the stand may be necessary to achieve this. When the iron is at rest, Q1's base is pulled low and so Q1 is off. Conversely, when the iron is out of its stand, Q1 turns on and pulls pins 2 & 6 of IC2 high, to inhibit its operation. During this time, pin 3 of IC2 is low and so the iron is continuously powered via RLY1's normally closed (NC) contacts. Note that the particular soldering iron that the circuit was designed for has its own 24V supply transformer. Other irons may need different power supply arrangements. The warm-up time and standby temperature can be varied by altering R2 and R5, as necessary.
Author: Alan March - Copyright: Silicon Chip Electronics

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Gym Agility: A Simple Strategy Game

This simple circuit is a two-person game of strategy and speed - and potentially, agility and athletic fitness. Each player has a row of four LEDs before him/her. Beside each LED, there is a pushbutton which, when pressed, lights up the corresponding LED. The aim of the game is for a player to illuminate all four of their LEDs in a row, in which case the circuit declares a winner. However, there is a catch. As soon as you light one of your own LEDs, the other player's corresponding LED goes out - and vice versa.

The game begins by giving each player two illuminated LEDs. Consider now that this game is scaled up and used in a gym. If the LEDs in the circuit are directly replaced with N-channel power MOSFETs, then 12V globes can be illuminated (a MOSFET's gate is wired in place of a LED's anode, the source goes to negative, and the load is wired between the drain and positive). If four large pushbuttons are mounted on one wall and four on another, this could become a game of agility - if not a physical tussle to keep the other player away from critical push-buttons.

Circuit diagram:

Gym Agility Circuit Diagram

Here's how the circuit works:

Schmitt NAND gate IC1a and IC1b (4093) form a simple bistable latch. When one output (pin 3) goes "high", the other output (pin 4) goes "low" and vice versa. The main advantage of using a bistable latch (as opposed to a flipflop) is that it does not suffer from switch bounce. Four such bistable latches are fed to inputs A-D of IC2. However, for the sake of simplicity, only one of these is shown; ie, IC1a-IC1b. We now need to identify when all four bistable latches go either "high" or "low".

This is done using IC2, a 4067 16-channel multiplexer. When inputs A-D are all "low" (binary 0000), this opens decimal channel 0. Conversely, when all are "high" (binary 1111), this opens decimal channel 15. Channels 0 and 15 thus trigger a win for one side or the other, by taking pins 9 or 16 of IC2 "low". Finally, if the game is quite hectic, a win might only last for a fraction of a second before it is lost again. Therefore, IC1c and IC1d are wired as timers, which do not permit any further play until a win has been reported for one or two seconds - either via LED3 or LED4. During this time, however, the players' buttons may be pressed to reset the game to two LEDs all.

Author: Thomas Scarborough - Copyright: Silicon Chip Electronics Magazine

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Heart Rate Monitor

Strictly speaking, this simple circuit shouldn't work! How could anyone expect an ordinary light dependent resistor photo cell to 'see' through a fingertip in natural daylight and detect the change in blood flow as the heart pulsates? The secret is a high gain circuit, based on a dual op amp IC which can be either the low power LM358 or the JFET TL072. The LDR is connected in series across the 9V battery supply via a 100kO resistor (R1) and the minute signal caused by the blood pulsing under the skin is fed to the non-inverting (+) input, pin 3, of IC1a via a 0.µF capacitor.

Pin 3 is biased by a high impedance voltage divider consisting of two 3.3MO resistors. The feedback resistors to pin 2 set the gain to 11 times. The output of IC1a is fed via a 0.47µF capacitor and 220kO resistor to IC1b. This is configured as an inverting op amp with a gain of 45 so that the total circuit gain is about 500. The output of IC1b is used to drive an analog meter which may be a multimeter set to the 10V DC range or any panel meter in series with a resistor to limit the current to less than its full-scale deflection. The prototype used an old VU meter with a 47kO resistor fitted in series.

Circuit diagram:

Heart Rate Monitor Circuit Diagram

Note that the unit was designed to use the Dick Smith Electronics light dependent resistor (Z-4801). Other LDRs may require a change in the value of resistor R1. A light source such as a high brightness LED is not required. All that is needed is a reasonably well-lit room, preferably natural daylight, to produce a healthy swing of the needle. Only when the hands are very cold does it make it a little more difficult to accurately count the pulses. To check your heart rate, carefully position your thumb or finger over the LDR and count the meter fluctuations for a period of 15 seconds. Then multiply the result by four to obtain your pulse rate. The circuit can not be used if you are walking or running, etc.
Author: Tony Lee
Copyright: Silicon Chip Electronics Magazine

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