The resistance of the relay coil is like 100Ω. That forms a voltage divider with the 1K timing resistor, so the capacitor charges to a maximum of 100/(100+1000)*12 = 1.09V, which will never pull-in the relay...
Hmm, what is the intended application here? The issue I see with using a mechanical relay for flashing a light is that, depending on the rate of the flashing, it will put tens of thousands of cycles on the relay in a matter of hours. If the intended device is going to be in continuous use, the contacts may start to wear out quickly, even when using a reed-type relay.
Hmm, what is the intended application here? The issue I see with using a mechanical relay for flashing a light is that, depending on the rate of the flashing, it will put tens of thousands of cycles on the relay in a matter of hours. If the intended device is going to be in continuous use, the contacts may start to wear out quickly, even when using a reed-type relay.
Not so much...Car turn signal flashers use contacts similar to those in a relay, and last for the life of the car in that application...
Here is a more practical circuit which relies on feedback from the relay contacts. You can also do this with an electronic (two transistor or 555) multivibrator circuit driving the relay coil (or not) without using feedback that comes from relay contacts.
Hmm, what is the intended application here? The issue I see with using a mechanical relay for flashing a light is that, depending on the rate of the flashing, it will put tens of thousands of cycles on the relay in a matter of hours. If the intended device is going to be in continuous use, the contacts may start to wear out quickly, even when using a reed-type relay.
TBH this was just a "fun" circuit demonstrating that something could be done with a minimum of components. I wouldn't recommend it for a serious application!
I do think MikeMI's addition of an emitter follower to drive the relay is an excellent augmentation, a low power MOSFET like a 2n7000 used as a source follower could permit even bigger resistors and hence longer delays with large capacitors or permit much smaller timing capacitors to be used.
It is free and available for PC's ( runs in Chrome browser ), iOS and Android. I use it on my Android tablet, Whilst its not as good as something like LTspice it can be more convenient to have on a mobile device.
Thanks for the compliment about the cat picture, she is no longer with us, but I think that was the best picture ever taken of her.
It can be done.
I have seen old weld sets that have a burn back timer of a second or so using a similar method allbeit with high impedance relays.
This isnt a 'good' circuit but it proves a point: