12v automotive relay, schmitt trigger, hysteresis, lm358, adjustable 1-4v threshold?

Discussion in 'General Electronics Chat' started by elRey, Apr 30, 2011.

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  1. elRey

    Thread Starter Well-Known Member

    Feb 23, 2009
    Hello all,

    Would someone mind giving me advice on a circuit to switch on/off a 12v automotive relay based on a 1v-4v threshold from a 0v-5v signal.

    The relay was switched to ground by ECU at one point and has a resistor in the relay itself. (note: I'm reusing an extra relay, NOT altering the function of an existing relay) I measure 60.4 Ohm across the coil leads.

    I want to switch on the relay (either to ground or 12v+ whichever is better for the final circuit, BUT switching to GND is preferred) when the MAP sensor reads ~5psi above atm. But I don't want to switch the relay off until MAP reads below ~1-2psi (hysteresis). I'd also like both thresholds to be adjustable.

    I'd like the circuit to use either a lm393 or lm358 which I have on hand.
    And if possible not require a regulated 5v+ or 12V+. The car and relay will be running ~13.2-14v off the alternator.

    The thresholds can be allowed to fluctuate with alternator voltage to simplify the circuit.

    Relay will drive a secondary fuel pump.

    Thanks in advance!
    Last edited: Apr 30, 2011
  2. Jaguarjoe

    Active Member

    Apr 7, 2010
    Let me get this straight-
    You're turning on the 2nd fuel pump when the boost is equivalent to 4 volts rising and turning it back off at 1 volt falling.
    Use the 393 comparators to feed a set/reset flip flop made from 1/2 of a quad NOR gate. The "ON" comparator will go high at 4 volts (or whatever) and set the FF high. The "OFF" comp will go high below 1 volt (or whatever) resetting the FF to low.
    Do NOT run this, or any circuit direct from the 12 volt electrical system. The spikes and surges will kill any solid state devices. Use an LM2937 or 2940 regulator.
    elRey likes this.
  3. elRey

    Thread Starter Well-Known Member

    Feb 23, 2009
    And how would you suggest I drive the relay of the FF? NPN?
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    Use a CD4001 to build the FF. You can use an NPN like a 2N2222 to drive the relay that will drive the pump, or you can skip the junk in the middle and have the FF drive a logic level MOSFET like an IRL530 and run the pump directly from that. Make sure the MOSFET current rating can handle the starting current of the pump.
  5. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

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    Last edited: Jun 21, 2011
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