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Lamp flasher with only 1 relay, 1 resistor & 1 capacitor

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Gasboss775

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Lamp flasher using only 1 relay 1 resistor and 1 capacitor.png


Note that the relay has a 1000 ohm coil and activates at 5 volts, with other relays the values of the resistor and capacitor might need adjusted.
 

MikeMl

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Ain't gonna work...

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...
 

MikeMl

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I have some reed relays with 1K coils but with a 100 ohm coil you could change R to 100 ohms and C to 1000 uF
With a 100Ω coil and a 100Ω timing resistor, the coil voltage will reach a maximum of 6V. How many 12V relays will pull-in with only 6V on the coil?

With a 100Ω timing resistor, how big would the capacitor have to be to produce a delay of say ~1/2s? About 5000uF...

18.png
 

JLNY

Active Member
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.
 

MikeMl

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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.

19.png
 

Gasboss775

Member
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! :cool:

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.
 
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Gasboss775

Member
Screenshot_2016-11-27-01-23-44.png

Version with Darlington emitter follower. The Blue trace is the capacitor voltage and the green trace is the relay voltage.
 

Gasboss775

Member
What simulation software are you using?
Love your cat's face.
Its called Everycircuit.

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.
 
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KMoffett

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One of our Physics professors uses this circuit as a simple electromechanical demo.

Ken
Relay RC Flasher.gif
 
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dr pepper

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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:
 
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