I need to trigger a 12 relay. The positive rail varies from 10-15V and the negative rail is zero. I want to be able to adjust the trigger between 11 and 12.5 volts. I don't have a 5V supply or fixed voltage reference in the circuit.

I guess I could use a linear voltage regulator for the reference and a 741+BJT schmitt trigger for the switching, but that seems to require at least 3 IC's and a voltage divider and a trimpot. I don't have that much room in the board.

Is there a simple, small footprint way to do this? Maybe a single IC with a reference and schmitt combined. A diode, trimpot, and a BJT?

The threshold voltage does not need to be precise. Basically, I want to use the relay to 'wake up' a circuit when the battery charge reaches a certain level, and disconnect when it drops below a certain level, so I need a little hysteresis.

Thanks for any ideas.

 

OK, maybe I could clamp the reference on a schmitt with a 10V zener. Then I would just need an op-amp, a resistor and trimpot for the input voltage divider, and a BJT to switch the relay. Still to big of a footprint.

 

You could do it with a TL431 and a transistor and 3 resistors. I posted it somewhere here, I will need to do some research, don't have anything to draw here.

 

Have a look here:
http://forum.allaboutcircuits.com/showthread.php?t=63241

post #4

 

Thank you, praondevou. That is what I am looking for. I put together a sketch of an adjustable voltage switch. It looks like I can skip the transistor since the relay coil current is less than 20mA and the TL431 can sink 150mA.

What about stability, or hysteresis? I am only using this circuit to activate the relay ( adj @ 11.5V - 15V ). I don't need the circuit to turn off the relay if the voltage drops below a minimum, that is handled by another circuit. But I don't want it to chatter around the turn on voltage. In fact, I don't mind if it latches.
Would R4 work for this?