IR camera trigger design

Discussion in 'The Projects Forum' started by AdamM, Sep 23, 2009.

  1. AdamM

    Thread Starter Active Member

    Mar 7, 2009
    Hello folks,
    I'm building a simple automatic camera trigger using an infrared LED and photodiode to trigger the camera when an object moves into the target area. I have built a version of the device and it works well under test conditions, but becomes unreliable and finnicky when I actually try to put it to use.

    Here is a schematic for the receiver in its current configuration:


    R1 is used to adjust the sensitivity. The camera requires only a very small amount of current to trigger the shutter -- a 1 meg resistor across the camera terminals will trigger it. The basic design here came from a very brief article about typical photodiode circuits.

    The problem I'm having is that under real-world conditions (especially outdoors), I can't seem to get the sensitivity adjusted so that the trigger will fire only when the beam is broken. It likes to either fire randomly, or continuously, or not at all. With plenty of adjustment I can occasionally get it to trigger properly, but after being left for a while it goes out of whack with slight changes in ambient light level (or sometimes even when there shouldn't be any ambient light level changes, such as at night).

    Initially I used only one IR LED. I added two more LEDs, figuring that the signal level just wasn't strong enough, but it seemed to make very little difference -- one reason I suspect that my circuit needs some improvement.

    I have also had a piece of tubing around the photodiode to shade it from ambient light, but again without satisfaction.

    Essentially my question is what is the best way of using a continuously changing resistance (in this case the IR photodiode) to control an on/off switch. (An additional drawback of the design I've used here is that when ambient light levels are high, there is has to be a continuous current going through the resistor and photodiode, which seems like an unnecessary drain on the battery.)

    My knowledge of electronics is quite limited -- if my description or schematic contains errors or instances of newby nonsense, or there is critical information lacking, please let me know ;)

    Thanks in advance for your help,

    Probably the ideal set up would use a modulated IR light, which the receiver would recognize while ignoring the ambient, unmodulated IR. There is a product that uses this principle from Qkits ( ), but the trigger delay makes it unsuitable for photography. I have a 555 timer that I think I could use for generating the signal, but I'm not sure how I would be able to interpret it on the receiving end. If anyone knows of a system like that which I could buy, or where I could find out how to make one, that would be great.
    Last edited: Sep 23, 2009
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    how is your emitter/detector afixed, at what distance apart, and what power rating is the emitter?

    I use the pyrosensors with freznels for IR detection to fire camers.

    You may be able to do something with the rate of change, by employing a filter. Noise can often be filtered out for a more positive signal.
  3. AdamM

    Thread Starter Active Member

    Mar 7, 2009
    Unfortunately I don't know the actual power rating of the emitter. It is 3 diodes in series on a small board, salvaged from the controller of an IR controlled 'RC' helicopter, running on about 4.5 volts. The board is on the end of two flexible wires so it can be oriented to point towards the receiver.

    The detector diode is mounted through the side of a telephone jack box, which I'm using as the case for the receiver unit.

    Minimum satisfactory working distance between receiver and emitter would probably be about 5-6 feet. The larger the distance, the more versatile it is of course.

    Here's a photo of the whole set up:


    Receiver unit is in the center, with its battery pack hanging out to the left. Emitter is on the right.

    What type of pyrosensor do you use? And what type of emitter? Perhaps a more powerful emitter would solve the problem.

    Filtering for rate of change sounds like a good idea. What sort of a filter would you recommend using?

    Thanks very much for your help,
  4. HarveyH42

    Active Member

    Jul 22, 2007
    You should use modulation to reduce the noise. The receiver are cheap, free if you can grab an old VCR or something remote controlled off the curb, before the trash truck gets it. Use a 555 to generate a 38kHz frequency, transistor to drive an IRLED or two, at about 200 mA. Might need to invert the receiver signal, believe stays high until it receives the signal (been a couple years). Still might have trouble in sunlight, avoid it.
  5. AdamM

    Thread Starter Active Member

    Mar 7, 2009
    Thanks for that excellent suggestion HarveyH24. I'll keep an eye out for an old VCR -- in fact I think I might have the parts from one lying around. I'll probably be back for more advice once I get the parts rounded up.
    Direct sunlight won't be an issue as I can easily shade the receiver.
    Thanks for your help,