Motion Sensor circuit

Discussion in 'The Projects Forum' started by SpiderSpartanju, Oct 11, 2009.

  1. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
    I need help with my circuit design for this motion sensor circuit. Independently the components all work fine, but when I put them together the relay never triggers. I suspect it has something to do with everything being connected to the same ground. I tried putting a 1K resistor between the relay and ground and that didn't work either. Can anyone help point me in the right direction here? Thanks.

  2. bertus


    Apr 5, 2008

    Do you have the datasheet of the pir sensor?
    Can it drive the relays you use directly?

  3. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
    The datasheet says this about the output function, "Connects to an I/O pin set to INPUT mode (or transistor/MOSFET)". So, I guess maybe it doesn't have enough amp output to trigger the relay? The output from the sensor is 3.3V, which should be fine since when I hook 3.3V to the relay from a power supply it triggers. So what can I do to overcome the output limitations? Thanks for the quick response.
    Last edited: Oct 11, 2009
  4. BMorse

    AAC Fanatic!

    Sep 26, 2009
    do as suggested and connect a transistor on the output of the sensor (or a MOSFET), before you burn out your PIR sensor by trying to actuate a relay with it....
  5. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
    Awesome! Thank you so much for your help. I'll try that circuit today and let you know the results.

    One question though. Do you see any problem with using a TIP120 in place of the BD675? Thanks again for the help.
    Last edited: Oct 12, 2009
  6. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
    Sorry for the delayed response. Thank you guys so much for your help. The circuit works perfectly, but now I'm running into a new problem. The cheap motion sensors appear to be triggering for no reason now. Anyone know where I could get a higher quality motion sensor without spending an arm and a leg? I'm thinking about maybe trying to cannibalize one of those motion sensor wall switches. You can get them at Home Depot for about $15. I'm in college and on a budget.
    Last edited: Oct 21, 2009
  7. hobbyist

    Distinguished Member

    Aug 10, 2008
    before you buy another sensor, try to work with what you got.

    It works independantely you said in first post.

    It may be that it is working too sensitive now with the transistor amplifier.

    Depending on the output voltage from the PIR. You may need to further bias with some neg. feedback your transistor amp.

    This is how I would try it.

    First this procedure will only work if the pull in voltage for the relay is NO smaller than 1.5v. when using a 5v. supply to run the circuit.

    If the relay can energize at a min. of 1.5v. than continue on if not... then this will not work with a 5v. supply..

    First measure the coil resistance.
    Than measure the pull in voltage (adjustable supply or potentiometer)
    calculate the pull in current from these measurements.

    Since the signal (PIR) volt out is 3.3v. then use the following equation below to solve for the emitter resistor.

    Take ........( 2.6 / calculated current) then use the closest standard value for the emitter resistor.

    If the cirucit is still too sensitive than begin increasisng a little at a time the emitter resistor, making sure it still small enough to supply the proper current for the relay.

    What this does is if it is too sensitive than by increasing the emitter resistor your in effect increasing the emitter voltage, so it would take more signal strength to turn the transistor on, it may be that the PIR is producing false miniscule voltage inputs and the amplifier is amplifying every little glitch.

    This is just one suggestion of many different ways this could be done.
    Last edited: Oct 21, 2009
  8. SpiderSpartanju

    Thread Starter Active Member

    Apr 10, 2009
    Thanks for the suggestion. I've got two PIR sensors, (the same model from radioshack). One is really glitchy and seems to trigger for no real reason, but the other one works a lot better. I'll see how that one works hooked up to my digital picture frame and take it from there. Thanks again for all the help guys.