I need a switch circuit please

[ahonda55]

Hi all , i am Ahmed from egypt...
First of all , GREAT work , really great forum.

I have a small project , and i need help from every one who can help me.....

What i need is a switch circuit drives a 12v relay at exactly 3 volts or more.
i mean a small simple circuit that closes the 12v relay when the input signal voltage of the circuit reaches 3 volt and opens the relay again if the signal runs below the 3v , and that circuit operates at 12v.

PLEASE this is my first request in this forum , so Plz help meeeee !!
any circuits , any links , any ideas anything

 

[furrycow]

this as a base circuit should work, you might just need to change a few values of components in there, if you wish to get the 3v/12v exact change

 

[furrycow]

it didnt show my diagram, ill try again

 

[furrycow]

i dont know why its not showing, ill put it on my website visit

www.atcy68.dsl.pipex.com

it may not show straight away, because itll take a few minutes

 

[ahonda55]

thanx , i had seen the circuit , but i can't understand anything .

The cicuit i need is not to switch between 3v and 12v , it runs on 12v , and the input signal is 3v max. so when the input reaches 3v or more the circuit output closes the relay.
Thank you very much

 

[furrycow]

i must have phrased mine wrong. unless i have missed something drastic in my diagram, i think that is exactly what youre looking for

 

[Sebi]

The cct switch on about 3.2-3.4V input. If You need higher precision, use a comparator.

 

[audioguru]

Hi Ahmed,
In your other post about a tachometer circuit driving a relay
https://www.electro-tech-online.com/threads/dc-motor-speed-controller.14649/#post-78754
I said that your tachometer IC already has a comparator and a relay driver transistor.
I have re-arranged the "Speed Switch" circuit in the datasheet for the LM2907-2917 from the 8-pin IC that is shown to the 14-pin IC that you have: