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Versatile Special Function Timer

This automatic timer has a wide range of applications whereby a timing cycle is initiated in response to a trigger signal. There are quite a few uses for it in a car or you could use it on a model railway layout, in a home security system or whatever.

Design by JOHN CLARKE

Main Features

  • Triggered on rising or falling voltage (user selectable)

  • One-shot or alternating (pulse) operation

  • Pulse mode can be set for variable on/off periods

  • Precise 0.1s to 16.5-minute timing period

  • Relay output with dual double-throw contacts rated at 5A

  • LED indicator for timing

  • This project was originally presented as a "Versatile Auto Timer" in our book, "Performance Electronics for Cars" but since it has considerably wider applications than in cars, we have decided to give it a wider audience by publishing it in SILICON CHIP with an updated microcontroller, the PIC16F628A.

    Since it is based on a microcontroller, it can be easily configured to give a wide range of times and triggering options. As well, it can run as a "one-shot", giving a fixed ON time for a device after being triggered or it can cycle the device on and off repeatedly after being triggered.

    Click for larger image
    Fig.1: the circuit is based on a PIC16F628A microcontroller that's programmed to provide a timed output after being triggered. The output at pin 2 drives a double-pole relay via transistor Q2.

    There are a number of triggering operations. For example, you could just use a pushbutton to start the timer or it might be triggered by the application or removal of more than 6V to the input.

    As you might imagine, there are any number of applications for this timer in a car. For example, it could run a fan for 10 minutes at the push of a button or it could run the ventilation fan for a couple of minutes every 10 minutes when the car is locked in a sunny car park. When you come back to the car, it would not be stifling inside and there would not have been too much drain on your battery.

    Or what if you have a model railway layout with points switching? Say you have just changed over the points and you want lights to flash and bells to sound at a road crossing for three minutes after? That’s a job for this timer. There are many others.

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