Home > Circuits > Versatile Timer IC
Modified:21:47, 22 October 2013

Long time delays are tricky to achieve accurately. An astable feeding a multi-stage binary counter can provide long delays but to provide a 24hour delay with a 1Hz astable would need (24 x 60 x 60 = ) 86400 divider/counter which is at least 16 binary stages.

The simple answer is to program a small PIC with useful timed outputs that can be used directly or divided down further for delays of weeks or even months!! Bear in mind however that this has a less than average accuracy - of about 5% as this IC's internal oscillator is not precision set, but it suffices for most applications and may only need re-adjustment now and then.

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Basic circuit

The output frequencies were selected as follows:
Pin No:
Output Cycle Time
1 second
1 hour
12 hours
24 hours
48 hours
Note that the outputs start off low and go high after half the cycle time, thus an equal 50/50 mark/space ratio. These outputs can drive counters or be used directly.

Pin 4 is used to reset the IC. A low on this input will reset the timing.



This circuit uses the IC as a timer, in that after 24 hours, the 48hour output goes high, being the second part of its cycle. When this happens, Q1 conducts and activates the buzzer.

Note that after another 24 hours the buzzer will switch off again and the cycle will repeat again.

Pin 3 would provide a 12 hour delay and pin 5 a 6 hour delay.


This circuit uses the IC as a clock, in that the output pulses are counted. The counter in this case is a 4017 decade counter where each clock pulse advances a high from one output to the next. The clock is connected to the hourly cycle output so the outputs indicate hours.

It is important to note the need for the inverter at the clock input. This is because the counter advances on the rising edge of the signal and this happens halfway through any outputs cycle, so an inverter must be used, or a counter that advances on the negative edge. Without the inverter the outputs would go high after 0.5hrs, 1.5hrs... etc instead of 1hr, 2hrs, 3hrs.

A switch can be connected to select any outputs to trigger an alarm or a device of some sort.

PIC program and circuits by Phil Townshend 2008
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