Sometime we may need to turn off or turn on a device after some time. That's when timer circuits are useful. Here we're going to talk about simple MOSFET timer circuits.
You can determine the delay of this timer circuits by calculating the RC constant, but they're very unpredictable.
In fact RC time constant is not a constant at all, as capacitance can vary as high as ∓30%. So this timers are just for non-critical applications and educational purpose.
Why a MOSFET based timer circuit ?
First of all, these are mere experimental circuits to get a good overview of RC time delay.
I think they should not be used used for some time critical applications.
But what's the advantage of MOSFETs over transistors ? Well, MOSFET is a voltage controlled device where transistors are current controlled. You can turn a N-channel MOSFET fully on just by touching the gate and positive point of the power supply, and the reverse is true also.
So the load's power won't start fading over time, it's the biggest advantage.
Circuit 1: Turn on after delay with MOSFET
Let's have look at the first circuit, the load will be turned on after some delay after pressing the On switch.
This circuit has almost 10 to 12 seconds delay to turn on the load. You can use other MOSFET too, but that should match your requirements.
Another point to note that you could use DC supply with this timer circuit.
Circuit 2: Turn off after delay with MOSFET
Now another timer circuit, which will turn off the load after some delay.
But first you've to turn on the load by pushing the ON switch for a moment.
I'm getting around 20-22 seconds delay time with this circuit.
MOSFET based timer circuit with relay and AC load
You can couple any above timer circuit with a relay to operate high power AC operated devices like heaters, fans etc. etc.
Below an example circuit using a IRF510N MOSFET.
Obviously you have to take care while building this timer with relay, as it's connected to dangerous mains voltage.
Stay tuned, I'll soon publish another tutorial about how you can make an accurate timer circuit with AVR micro controllers.
So do you have any suggestion or question on this ? Just leave a comment, I'll be happy to talk about it.