Triggered on rising or falling voltage (user
One-shot or alternating (pulse) operation
Pulse mode can be set for variable on/off
Precise 0.1s to 16.5-minute timing
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.
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.