push on/push off circuit

Discussion in 'The Projects Forum' started by transist0r, Dec 9, 2010.

  1. transist0r

    Thread Starter New Member

    Dec 9, 2010
    looking for some help with a small circuit workaround as the replacement switch available from the manufacturer is now momentary instead of latching. i dont want to lose the OEM look by using another sourced latching switch so im stuck trying to make the momentary work.

    the old switch was single pole latching normally open and operating it turned a feature on or off by creating an earth for an IC.
    the new switch is identical save for being momentary rather than latching so the circuit works but only while you keep the button pressed in with your finger. hardly ideal.

    has anyone got a simple and compact circuit i can use for this. power available is 12vdc. the current through the switch is almost nothing seeing as its just allowing the IC to earth.

    a miniature latching relay would have seemed the answer except that i cant find a way to have a single momentary switch set and reset it that makes sense.
    i had thought about using the circuit below, with a 6v DPDT relay. can anyone advise if it will work ok and what value cap i should be using since this is a 12v circuit design im needing.

    i really just want the simplest solution i can find so ideas are very welcome as im at a loss.
  2. transist0r

    Thread Starter New Member

    Dec 9, 2010
    theres also this circuit i found on here.
    it looks promising, if i use a double pole relay i can use one pole for the 12vdc to keep the coil energised and the other for the earth circuit that im switching off and on.
    the only question i have with this is that it looks like the only way to turn this off after it is activated by S1 is by killing the 12vdc coil feed coming through D1. am i right?
    is there a way to modify this circuit so that pressing S1 again will turn off the relay or does this just bring me back to the first circuit above.
    Last edited: Dec 9, 2010
  3. SgtWookie


    Jul 17, 2007
    The first circuit should work OK. Use the 1000uF cap specified.
    If you're using a 6v relay with 12v, then the values of R1 and R2 should be about the same as the coil resistance, or somewhat higher.

    As you figured out, the 2nd circuit stays latched until the 12v power is interrupted; and to change it to function like you want would require adding resistors and a cap as in the 1st schematic.
  4. transist0r

    Thread Starter New Member

    Dec 9, 2010
    thank you for the reply and info.
    i noticed the second circuit was your posting. im going to save this for future ref as i like it very much, it has elegance.

    regarding the first circuit,
    whats the theory behind changing the resistors if im using 12v feed with a 6v relay instead of the 24v feed, 12v relay as shown.
    im ok at the physical aspect of circuits but get lost with the whats going on when it moves above basics.
    if you have time id appreciate it as it might help me move on a little with my understanding and save me a question next time round.
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
    Last edited: Dec 9, 2010
  6. transist0r

    Thread Starter New Member

    Dec 9, 2010
    i just managed to get a look inside one of the new units that is utilising the momentary switch instead of the old toggle switch.
    they have added a small "relay" to the circuit to latch back and forth instead of the switch. i say relay as i couldnt see clearly what it was other than a small white unit the size of a postage stamp and about 10mm high. it makes a definate relay type click each time it activates.
    this switch is essentially providing an on or off to an earth pin on a chip.

    i will try and see if i can get a PN or ref from this new relay but in the meantime i'll describe how its wired and what its doing and maybe someone will know what it is and i can pick one up and be done. given its just 4 pins on the relay in a small package it would be a nice and easy solution if i could pick up one of these.

    earth connects to the 1st terminal on the relay and also to one side of the momentary switch. the other side of the momentary switch connects to a 2nd terminal on the relay which is the trigger for the switching. theres a 12v feed to the relay on a 3rd terminal and then a 4th terminal on the relay which is the "output" terminal

    when turned on the circuit between earth and the output is open. a press on the momentary switch creates a circuit with the earth and the output from the relay. each subsequent press of the relay alternates this state.

    it sounds a bit like a T flip flop but to say i'm fuzzy on logic would be putting it mildly. the fact that theres definately an audible relay type click each time the switch is pressed and the circuit switches would suggest its not to me.
    Last edited: Dec 9, 2010
  7. Wendy


    Mar 24, 2008