# Triggering a relay with a pre-set voltage? (Car related).

#### KeithMac

Joined Jul 23, 2008
4
Hello, Im trying to figure out the best way to make 2 circuits for my car.

The first circuit needs to pull a relay trigger coil to ground (50ma?) when the input is at 4v or higher (max 5v), the input is from the Throttle Position Sensor on the car.

I need to make the circuit transparent to the car, ie not load up the TPS signal wire at all. I have a circuit working using a small transistor but it draws some voltage from the input line affecting the signal to the cars ECU (reads 10% lower than it should), this needs sorting out.

The second circuit needs to pull a relay trigger coil to ground when the input voltage is 1.5v or lower, again it needs to be transparent to the Widband AFR controller its conected to.

I have a circuit working for this using 2 transistors and a pull up resistor but again the input is wired to the transistor base via a potentiometer and I dont really want to "load up" the signal wire for fear of wrecking my Wideband controller.

I have +12v supply and ground from the car, I could hi-jack a stable +5v supply from the cars ECU if it would make things simpler.

Any thoughts or suggestions on the right components to use are welcome.

Cheers, Keith.

#### SgtWookie

Joined Jul 17, 2007
22,210
a dual FET-input comparator would do the trick.

Even an LM393 or LM2903 might do the trick for you. Is 25nA current too much?

LMC6772, National Semiconductor. Dual CMOS comparator. Input current 2pA. That's low. Rail-rail inputs, open drain output can sink up to 30mA.

Last edited:

#### SgtWookie

Joined Jul 17, 2007
22,210
See the attached. I used an LM339 in the simulation because I don't have a better one loaded in the library at the moment.

It's just the basic circuit. The Zener ensures a pretty stable 4.3v feed to the pots. You can set the input reference level anywhere from 0v to 4.3v. C1 and C2 are for noise supression.

There is no hysterisis in this circuit. It may not be a bad idea to add a bit.

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#### KeithMac

Joined Jul 23, 2008
4
Thanks for that, Ill have a look at your circuit and get my head round it.

The LMC6772 looks a good bet at 2pa input, I think an inline resistor from the relay coil to limit the current to 30ma will still throw the relay properly.

Thanks for your time, much appreciated!

#### SgtWookie

Joined Jul 17, 2007
22,210
Instead of using a current limiting resistor, I suggest using a transistor such as a PN2222A or perhaps a small Darlington such as an MPSA13, MPSA14, or 2N6426. You'll need a current limiting resistor to the base input; a 1k would work fine.