Picaxe Latching Relay Reset Circuit

Discussion in 'General Electronics Chat' started by crea2k, Sep 21, 2012.

  1. crea2k

    Thread Starter New Member

    Sep 6, 2012
    8
    0
    Hi, I am currently trying to build a trailcam for watching wildlife with using a Picaxe controller. I have written the program and everything works just fine. I initually tried switching the camera power / shutter using npn transistors but because of the weird way the camera buttons work, this didnt seem to work, and kept on sending the camera crazy. So I hooked them up to two 5v relays and this works just fine. The problem is when the shutter / power is going off all you hear is click click click click, so not really the stealthy setup I was looking for as I think it would scare the animals off, so I have been looking at alternatives.

    I have purchased off ebay some of these relays, they are really tiny an quiet, the problem is I didnt realise they were latching relays, so once the power is switched I cant get the damn thing to turn back off again without reversing the polarity to it, I know I can use an H bridge for this, but I dont want to use an extra output on the controller just for turning stuff off with.

    http://www.ebay.co.uk/itm/Solid-Sta...upplies_ET&hash=item27cad943b1#ht_1794wt_1398

    [​IMG]

    The seller has this solution on thier ebay page :

    [​IMG]
    [​IMG]

    I have built the came circuit on my breadboard, the theory I presume is that when the power is cut, the transistor discharges and flicks the switch back off again.

    I haven't tried it directly with my picaxe yet, but only by hooking up my psu @ 5v to it directly, with the continuity tester on my multimeter on the switching pins of the relay. I turn on the power and the relay switches and the multimeter bleeps, I turn off the power and.......the multimeter continues to bleep. If I turn the power back on and flip the polarity nothing happens either as the capacitor stops the flow I presume. If I connect the backwards polarity to the relay directly it then clicks back, so I swap the leads once more and click it goes again, then stays on when the power is turned off...........

    I have also put the multi meter on the capacitor and it is discharging / charging correctly, I have even tried a 100uf capacitor for a bit more grunt, but still nothing.

    The only things I can think of is by turning off the psu or pulling the + lead that this is a different way to how the picaxe would turn the power off.....totally lost now.....
     
  2. crea2k

    Thread Starter New Member

    Sep 6, 2012
    8
    0
  3. crutschow

    Expert

    Mar 14, 2008
    13,045
    3,244
    You need to rapidly discharge the capacitor by switching the relay directly from power to ground, not just cutting the power. That is the way the picaxe output operates, it drives the output both high and low.
     
  4. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    If you want silent you might consider using an optoisolator as opposed to a relay!

    The relay is latched in a particular state buy the direction that current is passed through it.
    the data sheet specifies 3.75v min into a 250 Ohm coil which means that you need at lease 15mA to latch or unlatch it.
    This will need to be maintained for about 5mS.
    You will need to experiment with cap size, unless anyone can give you a calculation, to produce a long enough pulse.

    I think the 20k is there to ensure that the cap will discharge if the IO pin is left floating but if you ensure that the pin is always active low, when its not high, you should be able to eliminate it.

    There is another option ... possibly more stable.
    Use two pins in a complimentary configuration. However that will consume power all the time. adding a series resistor of 50 Ohms would give you circa 16mA coil current in either state.

    Hope that helps
    Al
     
  5. crea2k

    Thread Starter New Member

    Sep 6, 2012
    8
    0
    Thanks crutschow, I did wonder if turning the power off was different to the way the picaxe operated, I shall try hooking it up and seeing what happens.

    Dyslexicbloke, I have ordered some optoisolators and are going to give them a go one they arrive, as the current through the push switch is tiny, so I think they should drive the switch ok alone.
     
  6. crea2k

    Thread Starter New Member

    Sep 6, 2012
    8
    0
    Where you put about the two pins in complimentary configuration, do you mean, having say pin1 to one side and pin 2 to the other, wouldn't this ground out the circuit though as you would need ground on both sides too which would be there all the time ?.
     
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