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”:
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:
So keep that in mind if you try to build it.
Keep on Soldering!