How to build a simple Cmos Timer.

COMMENTS SUGGESTIONS	Designing Cmos Timers - Section 4	MORE TIMER CIRCUITS

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It's assumed that you've read - and understood - the contents of the previous three sections. Here - we're going to add an automatic reset to Circuits 5 & 6. The act of switching the power off - will charge/recharge C1. So the timer is always ready to start - the moment Sw1 is moved to the "on" position.

Circuit No.7

This circuit is exactly the same as Circuit No.5 - except for the reset method.

When Sw1 is switched off - it connects the battery across C1.

And this charges the capacitor.

As before - R2 limits the peak charge current through the switch contacts.

When Sw1 is switched on - the relay will energize - and the timer will start to run.

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While Sw1 is in the off position - current flows through R1 & R4.

With two - 1 Meg resistors - this could have a significant effect on battery life.

But - in reality - the two resistors will have much higher values.

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Using a resistor to discharge a capacitor - is a reliable way of producing very long time delays.

Long delays require high value timing components.

High value resistors are smaller - and cheaper - than high value capacitors.

So R1 & R4 together - are likely add up to 10 Megs - at the very least. 


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Section 1 | Section 2 | Section 3 | You Are Here | Section 5

Circuit No.8

This circuit is exactly the same as Circuit No.6 - except for the reset method.

When Sw1 is switched off - it connects the battery across C1.

And this charges the capacitor.

As before - R2 limits the peak charge current through the switch contacts.

When Sw1 is switched on -  the timer will start to run.

And when the time delay expires - the relay will energize.

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While Sw1 is in the off position - current flows through R1 & R4.

With two - 1 Meg resistors - this could have a significant effect on battery life.

But - for the reasons given above - the two resistors are likely to total 10 Megs - or more. 


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Section 1 | Section 2 | Section 3 | You Are Here | Section 5

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Designing Cmos Timers - Section 4

Circuit No.7

Circuit No.8