Darkness Detector Improvements

Discussion in 'The Projects Forum' started by JDR04, Jul 12, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    First of all thanks to all the forum members that helped me with my light detector circuit, it now works great.

    I now would appreciate some help with my darkness detector. The attached circuit is very similar to my light detector except I swopped the inverting and non-inverting connections around. Is this OK?

    The circuit runs off a standard 9V battery. I measured the output at pin 1. When the LDR is in light, pin 1 is at 6.4V. When the LDR is in the Dark the voltage at pin 1 drops to 1.8V.

    Would it be possible to connect my camera (DSLR)to pin 1 so that when the LDR detects darkness the camera is activated? I suppose I could use a relay but would prefer some sort of an optoisolater so that there is less chance of the camera being damaged in any way.

    Once again your help will be appreciated.- JDR04
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    That's going to depend on your camera, and what you mean by "activated". Just applying power would be relatively easy, but controlling the camera functions might be tough. Does the camera have any connections to allow this? Most consumer cameras have only a mechanical button to trigger a picture. A few support a remote, but still mechanical, trigger. So you may need to construct some sort of plunger to act like a robot finger.

    BTW, another way to switch the logic would've been to switch the relative positions of the sensor and the variable resistor. This would cause switching to occur at the same light level it used to. Switching the inputs as you did is fine too, but might require a small adjustment.
     
  3. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,132
    267
    Many modern DSLR cameras have a simple remote shutter input, it's usually pulled up through a relatively high impedance to 3.3Volts, all you would need is a switch to ground- an opto-coupler would work fine.
     
  4. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks wayneh, I'll try what you suggested and see how it works out. What exactly do you mean by "might require a small adjustment"?
     
  5. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks Sensacell, I'll try replace the Led with an optoisolater and see what happens. I dont need the autofocus at this stage so its only two wires that need to be closed to trigger the shutter. Any other suggestion will be appreciated. Thanks for your help- JDR04
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Just that a small change in the variable resistor might be needed to obtain switching at the same light level as before. No biggie.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    A MOSFET is a handy way to replace a switch. A regular MOSFET will probably work, but a logic level one would be more of a sure thing. The regular one needs about 10V to reach it's lowest resistance, whereas a logic level MOSFET can be fully on below 5V.

    Either way, the source pin goes to ground, shared by your circuit and your camera, and the gate goes to your switching signal. The two contacts for your camera are attached to drain and source. Turning on the gate voltage will "connect" the drain and source. You may need to use a multimeter to determine which contact is which n your camera.
     
  8. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Thanks wayneh, I'll give the mosfet a try.

    I've also removed the led and put a small optoisolater (SFH618-2)In its place and it triggers the camera as well. Can you or anybody else forsee any problems developing with this method? Thanks again!!!
     
    Last edited: Jul 13, 2012
  9. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Not knowing what's in the camera, I'd be a bit nervous to use 9V and only a 200Ω resistor (and the comparator) to limit current. Also, since the high sides are connected, be sure not to also connect your circuit and your camera grounds, as this could allow current to flow from one to the other.

    Oh nevermind, I thought IC2 was your camera! It's an isolator - perfect.

    Read the comparator datasheet for what to do with the un-used part. You want to prevent it from oscillating on its own.
     
  10. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    You've raised a point I forgot about......I was told the spare comparator input pins 5 and 6 should be tied together to prevent oscillations. Anyway, thanks so much for all your input and advice.....JDR04
     
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