Transistor Switch?

[Calem]

Hello guys,
I've just built this LED blinking circuit and it works wonderfully. What I'm looking to do is instead of connecting an LED I instead have the output hooked up to a relay coil. The problem comes because this circuit outputs a blinking 9v signal and the relay coil requires 12v. I'm hoping that there is some way to hook up a transistor to the blinking circuit running on 9v to then switch on and off the relay circuit running off 12 volts, with two seperate power supplies. Would this be possible?

Thanks,

 

[Harald Kapp]

Welcome to our forum.
We have a ressource on this topic.

 

[Calem]

Thank you for your welcome
I will read the resource and try it out now. Thanks again!

 

[Colin Mitchell]

Connect the relay to pin 3 and it will work

 

[Calem]

Connect the relay to pin 3 and it will work

I have tried this however unfortunately while it can power an LED it cannot power my relay.

 

[Harald Kapp]

Connect the relay to pin 3 and it will work

I have to disagree: it won't work. The 555 has a push-pull output. Therefore you cannot drive a 12V relay when the 555 is powered by 9V. A transistor as level shifter is a simple and good solution. Don't forget the backwards oriented diode across the relay coil (freewheling diode, or flyback diode) to protect the transistor from voltage transients when turning the relay off.

 

[Colin Mitchell]

That's the fault of your battery not the chip.

 

[Harald Kapp]

No, it isn't.
The battery does what it is supposed to do: deliver 9V.
The chip, too, does what it is supposed to do: deliver either 0V or 9V (give or take a few mV for the voltage drop across the output transistors).
The relay does what it is supposed to do: turn on with 12V, turn off with 0V.

The dilemma is the mismatch between the relay's coil voltage and the chip's output voltage. You match these by an additional transistor stage.
Alternatively you could operate the circuit from 12V directly, then the voltages match and the function of the relay is only a matter of required coil current which has to be lower than the 555s max. output current. Plus you'll need the diode across the coil in either circuit.

 

[Calem]

No, it isn't.
The battery does what it is supposed to do: deliver 9V.
The chip, too, does what it is supposed to do: deliver either 0V or 9V (give or take a few mV for the voltage drop across the output transistors).
The relay does what it is supposed to do: turn on with 12V, turn off with 0V.

The dilemma is the mismatch between the relay's coil voltage and the chip's output voltage. You match these by an additional transistor stage.
Alternatively you could operate the circuit from 12V directly, then the voltages match and the function of the relay is only a matter of required coil current which has to be lower than the 555s max. output current. Plus you'll need the diode across the coil in either circuit.

Thanks for your help, I have been able to make my circuit work