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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Shower/egg timer uses red & green LEDs

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This circuit can be regarded as an electronic version of the 4-minute sandglass timers which were available some time ago. Instead of transferring sand from an upper to a lower glass bulb, this unit has two arrays of eight LEDs – a vertical column of eight green LEDs and a horizontal row of eight red LEDs, all controlled by an LM3914 bargraph display chip.

When it starts, all the green LEDs are lit and all the red LEDs are off. As it times out, the green LEDs progressively go out, starting from the top, and the red LEDs light up, starting from the left. When the last green LED goes out and the last red LED turns on, a buzzer sounds to indicate the end of the timing period.

In essence, the circuit works by monitoring the discharge of the 3300µF capacitor connected to pin 5 of IC1 via a 470kΩ resistor. To start, the capacitor is charged to the positive supply by pressing switch S1. The LM3914 operates in the conventional way as far as the green LEDs are concerned, with all LEDs alight when the voltage across the capacitor is at the maximum level.

Each green LED is connected in series with the internal LED of a 4N25 optocoupler. For example, LED15 is in series with the LED of OPTO8. Hence, when LED15 is on, OPTO8 is also on and this holds the base of Q8 low and so LED16 is off. When LED15 turns off, as the 3300µF capacitor discharges, OPTO8 turns off and this allows Q8 and red LED16 to turn on.

This process occurs gradually with more green LEDs turning off and more red LEDs turning on until finally, LED1 turns off and allows Q1 to turn on. This lights LED2 and also sounds the buzzer driven from the collector of Q1.

To set up the timer, first adjust trimpot VR3 until the voltage at pin 4 is 0.85V. This sets the reference voltage for the internal resistor chain within IC1. That done, disconnect the 3300µF capacitor, the 220kΩ resistor and trimpot VR2.

Now connect an adjustable power supply to the point marked “A” and set the voltage to 5.64V. This sets the level at which the highest green LED will go off. Adjust trimpot VR2 until that same LED just comes on. Thus IC1 is set to indicate between 6V and more than 4V, because only eight of the possible 10 outputs are used.

Now reconnect the capacitor, VR1, etc and press the button for a second to fully charge the capacitor. Check the time until the buzzer sounds. If necessary, adjust VR1 so that the last red LED comes on and the buzzer sounds after four minutes – or whatever time you desire.

A. J. Lowe,
Bardon, Qld. ($45)

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