Oscillating dual timer? X seconds on, Y seconds off

Discussion in 'The Projects Forum' started by GuySmily, Apr 26, 2012.

  1. GuySmily

    Thread Starter New Member

    Apr 24, 2012
    Hi all,

    I'm looking to build a semi-automatic climate control circuit for my car. When I turn the air conditioning on, it gets too cold. I don't like the idea of opening up the heater core to mix hot air to the cold AC, so I thought I'd try building an eco-mode to the AC.

    My old car (86 MR2) had an ECO button, which basically turned the AC on and off every few seconds instead of running it continuously. I'd like to mimic this in my new car (05 MR2).

    Unlike the old car which simply had a button, I'd like to have a couple of knobs to independently control the on and off durations. Minimum time would be around 5 seconds for both. Max on time should be infinite. Max off time should be maybe 30 seconds (not infinite, so that the driver is encouraged to click the AC button off to turn the AC off).

    Is this a terribly difficult thing to do? My biggest thoughts/concerns are:
    1. The AC button might be a momentary button, not a switch (I will have to test it or find a wiring diagram).
    2. If it's significantly more complex to allow independent on and off times (duty cycle?), then making them equal (or a fixed ratio) may be acceptable. Perhaps one knob determines cycle time, and the other determines duty cycle.
    3. I want to make sure this doesn't destroy the AC clutch. I already have to do it manually, but I figure this is a point worth mentioning.
    4. It might be interesting to do something thermoelectrically controlled. As far as I know, true climate control was never available. If I could at least automate the AC, that would be kinda neat, but I don't know whether to work that out now or if it'd be much easier to control the timers manually.


    So the reason I posted this is that I got an email from Jameco saying all IC's were on sale (well, free shipping). I figured I would need a 555 timer or two for the job so it was good timing.

    As for my skill level, I can handle soldering wires no problem, but I haven't learned about circuit design yet (I'm a computer engineering major). I figure I'd have to buy a breadboard or something if I want to build this on a board / in a box that I can hide inside the dash.


    edit- I guess my question is: How do I do it? I've read about setting up 555 timers in astable mode, but the actual circuit design and the math behind the resistances and such is beyond me.
  2. mlog


    Feb 11, 2012
  3. wayneh


    Sep 9, 2010
    That makes far more sense to me than a timer. You want to control temperature, you use a thermostat, not a clock.

    Look into a LM35 thermometer IC. You could compare its output (10mV per °C) to a reference voltage - set by a resistor divider, just like adjusting a thermostat - and have the comparator throw a MOSFET as the switch. The comparator has enough hysteresis - or you could add more - to prevent constant switching.