(Hopefully) easy question about relays

[matrixq]

Hello,

I found this site while desperately googling around (but not knowing proper terms to get myself some decent results!).

I want to build what I'm guessing (hoping) is a rudimentary circuit, and I"m hoping someone here will be kind enough to point me in the right direction.

The purpose is to take a switched 6v/12v input (i.e. either 6v or 12v goes to the same circuit) and turn that into a switch that activates two separate circuits (i.e. if 6v is coming through, send 12v to circuit A, if 12v is coming through, send 12v to circuit B).

I've read about single pole double throw relays which i think would work if I had 0v/12v switching voltages, but the dropoff is too low to reset the relay when going from 12v to 6v.

Can anyone point me in the correct direction?

One thing that may be important -- this is in a car and thus 12v doesn't really mean 12v from what I understand.

Thank you in advance for any help!

 

[chuckey]

A rough and ready way would be to put a diode (or two in series) with the coil. This would have the effect of dropping .8V for every diode. So a 12V relay that actually pulls in at 10.5V and drops out at 5.5V, would with one diode, pull in at 11.3V and drop out at 6.3V.
The next level of sophistication would be to use a transistor to drive the relay coil and use a zener diode in series with the base to drop the required voltage. So with a 8V zener diode, the transistor would pull the relay in at 8 + .8V and drop the relay out at less then 8V.
The next level would be to use a voltage comparator IC, you could then set the relay switching level to +- 1mV, which would be over the top for this sort of circuit.
Frank

 

[godfreyl]

Another easy option would be to use a relay with a 6V coil and put a 5.6V (or 6.2V) Zener in series with it.