schmitt trigger into a inverting amp then NPN to drive coil GND?

Discussion in 'General Electronics Chat' started by elRey, May 2, 2011.

  1. elRey

    Thread Starter Active Member

    Feb 23, 2009


    V+ , Vin , Vref all = 12v+
    V- = GND

    Vin = 0-5v

    I'd like the circuit to go HIGH @ Vin > 2.5v and LOW @ Vin < 2.15v

    How do I complete the circuit to use Vout on a NPN that grounds a coil leg of a rely (60.4 Ohm across coil)?
    And what would be the resistor values needed. And needed diodes?

    I'm trying to use one LM358

    Last edited: May 2, 2011
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Assuming the circuit works

    Connect 1K resistor from Vout to base of NPN.
    Connect 10K from base of NPN to Ground.
    Connect Emmiter to ground
    Connector coil to collector.
    Leave the other leg of coil to Supply Vcc.
    Connect a 1N4007 Diode across the coil, the anode side of diode goes to the collector of the NPN. The cathode will go to Vcc.

    Now I dunno Ur Vcc Value and what the heck the coil is or does, so I cannot comment on it, the rest is the how to drive a transistor that coils switch anything from a relay to a Motor with the correct Supply Voltage
  3. ifixit

    Well-Known Member

    Nov 20, 2008
    Hi elRey,

    The output of the LM358 cannot drive all the way to -V (GND). Therefore the inverter output will always be the same, a few 100mV above GND.

    Change the design of the inverter to avoid this problem. Post your updated schematic when done.

    Is this homework, or a project? What is the relay voltage rating?

  4. elRey

    Thread Starter Active Member

    Feb 23, 2009
    Thanks for the replies. This is a hobby project. It's 12V DC relay. I've prototyped the schmitt trigger and have it working and controlling a NPN.

    And you're right, the inverting amp did not do what I wanted.

    So, how can I invert the .6v-9.6v square output of the schmitt to drive a NPN?

    I have the top signal, I want the bottom signal to drive a NPN:

    And I'd like to be able to do it with the other side of the lm358 or lm393.

    Can it be done with a op-amp or comparator? If it can, give me a little while to try and figure it out.

    Thanks again,
    Last edited: May 5, 2011
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Simply use PNP + LED to drive the NPN
  6. Wendy


    Mar 24, 2008
    A 555 makes a decent inverting Schmitt Trigger, and can drive a transistor nicely (that drives a relay).

    555 Schmitt Trigger


    Don't know if it is close enough to what you are after, but there it is.