How to fix your blinking relay circuit

I really like the circuit that blinks a light with a relay.

Like this:

If you connect a battery to the electromagnet of a relay, through its normally closed contact, you will get a relay that switches on and off really fast.

If you add a big capacitor (like 1000 µF) over the electromagnet, the electromagnet will stay on longer.

Then, if you replace the connection from the normally closed contact to the electromagnet with a resistor, you will increase the time it takes to charge the capacitor.

That means the relay will stay off for longer also.

Connect a light to the normally open contact, and you have a blinking light!

This is the basic principle on blinking a relay.

I explain this with images in my eBook “Getting Started With Electronics”:

https://www.build-electronic-circuits.com/products/ebook-2nd-edition

BUT…

One problem that might arise is:

If the resistor you added is too big compared to the resistance of the relay coil – you won’t get enough voltage on the coil to make it turn on.

That’s because the resistor and the coil make up a voltage divider.

Which I’ve written about here:

https://www.build-electronic-circuits.com/voltage-divider

So keep that in mind if you try to build it.

Keep on Soldering!
Oyvind

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