Is there such a thing?

Discussion in 'The Projects Forum' started by Cold Turkey, Sep 15, 2009.

  1. Cold Turkey

    Thread Starter New Member

    Sep 15, 2009
    G'day everyone, Firstly please excuse my ignorance. I'm not very good with electronics but I am learning.... slowly.

    I am trying to take an air conditioning control unit from a modern car and use it to control the air conditioner compressor on an old custom truck I'm building. I have managed to use wiring diagrams to figure out all but one thing.

    The Problem: The old a/c control unit had an on/off push button switch, if you pressed it, it stayed depressed / on until you pressed it again (sorry, I can't remember the proper name for that type of switch.) The new a/c control unit has a momentary switch, when you press it, it makes a circuit.

    I need a circuit (as simple as possible) or a relay, that will close when the momentary switch is pushed and stay closed until it is pushed again. The added problem is that it needs to remain in its given state even if the power is cut. I don't want to have to press the a/c button every time I start the car :)

    Is there any such relay or an easy solution to this problem?

    Cheers for any and all help. The car is 12v by the way.
  2. Chris15

    Active Member

    Apr 15, 2009
    your problem here is if you use a relay and turn the car off it will flick back, if i understand correctly you want to have the AC constantly on when you put the key in and turn it? if so you need to connect the AC unit or whatever it is you want to power and connect it to the ignition power with the key on, using a test lamp is useful, so that when the car is running the unit is on and when car is off ac is off, and you could connect it to the cars AC switch so you can figure out the momentary thing.
    Hope it helped,
    Last edited by a moderator: Sep 15, 2009
  3. Wendy


    Mar 24, 2008
    Actually there is. You want a flip flop type circuit, there are many types out there. It does electronically what the other switch does mechanically. When I get a chance I'll look something up, if someone doesn't beat me to it. How much current do you want through this switch?
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    I have seen a specific automotive 'relay' module that does this.

    Rather than the coil directly operating a contact, it rotates a cam via a ratchet pawl.

    The cam position opens or closes the contact, so the on or off state is maintained after the coil is released.

    I cannot remember what make of vehicle I saw this in..
  5. Cold Turkey

    Thread Starter New Member

    Sep 15, 2009
    Hey and thanks for the quick replies.

    Chris15 - I don't want the a/c on permanently. I still want it to be able to be turned on and off via the switch, I just want it to remember the state it was in when the car was switched off. If you hard wire the a/c pump then you will be robbing power from your engine when you aren't using it.

    Bill Marsden - Yeah, I've seen the "flip flop" idea but it was presented to me in an overly complex circuit. Because it's going into a car I want a simple compact unit. As far as the amount of current.... I'm not sure really, I would say that it must be fairly low as the wiring diagram shows that the switch output is run through a transistor, and then to a relay I presume.

    rjenkins - You're right on the money with the idea. I haven't seen those cam relays before but I understand what you're saying. I did see somewhere that you can get a relay that is just like a typical single pole double throw but rather than having a native open side and a native closed side, it just stayed where it was set... No idea what it was called, where I could get it, if it would work for my application or how much it cost but the idea was right.

    Please let me know if you get any further.

  6. jonman24680

    New Member

    Sep 11, 2009
    Attached is a picture I drew in paint of an RS flipflop with a pushbutton triggering the set reset of the flip flop.
    A flip flop chip will reset when it looses power though. So you will need to make sure that it gets continuous power.
    Also make sure that both R and S are never 1. That is a bad state on the RS flipflop.
    I hope this helps let me know if you need more help.
    Oh and Q is your output, not Q'.
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    If you use an electronic bistable (flip-flop or 1-bit counter etc) you first need to clean up the switch input.

    It needs converting to the appropriate voltage for the logic in use and, most importantly, debouncing with something like an r-c filter plus a schmitt trigger to give a single, fast, clean transition to the logic.

    A mechanical switch often experiences contact bounce for a short time, and typically a 10mS time constant is used for it to settle before assuming it's in a stable condition.

    Without this, you will often get random results as the IC counter/bistable can switch at some megaherts and will faithfully follow every transition during the mechanical bounce period.

    Note also that switches intended for high current loads often do not work well at low currents, they may need some level of current load to 'wet' the contacts.
    Start with a fairly low value load resistor on the switch, eg. 470 Ohms, and add an extra load such as another resistor or a small filament bulb if the switch does not operate reliably.

    Thread here on contact wetting:
  8. someonesdad

    Senior Member

    Jul 7, 2009
    It's also called a latching relay.

    I suppose you want to use the switch in the new control unit, but if not, you could simply use a toggle switch to turn things on or off. Cheaper than any other solution and it gets you what you want. It may fit with the look you want, of course.
  9. Cold Turkey

    Thread Starter New Member

    Sep 15, 2009
    Ok, I've found what I'm looking for. It turns out that there is a company here in Australia who manufacture a relay that does exaxtly what I want. There is no name for it really, you just need to know the part number. It is exactly the same size as an ordinary relay and evverything but it isn't cheap.

    Thanks for everyones input though, I've learnt quite a bit from trying to work around this.