PIR sensor circuit

Discussion in 'General Electronics Chat' started by LEDuser, Oct 11, 2013.

  1. LEDuser

    Thread Starter New Member

    Aug 29, 2008
    3
    0
    Hi,

    Recently I bought cheap Chinese mini PIR sensor module and after few tests I noticed really short ON time just around 20s after trigger. After some reverse engineering and measuring I found out that changing value of RC oscillator changes time. Probably SO8 IC uses external clock source (originally set to 12kHz C 1nF, R 150k) red mark on pic.

    Ok I thought to myself I solved problem but then I noticed that PIR module wasn't so sensitive anymore. I don't know why. I tried to tweak values of other R an C but with no significant differences.

    So I decided to make my own circuit with PIR sensor. I hooked PIRs GND to GND, "drain" to Vcc and "source" (out) to BC547 via R 1k and 100k to the ground. Then I was watching signal on oscilloscope but I don't know why I only got noise and with my hand hovering directly over PIR I got few mV difference which is not enough for ADC of AVR microcontroller.

    Now I have question if someone have any idea of simple PIR circuit without using op amp. I'm really tight on space on my PCB and I need simple but effective solution and because I don't have any experiences with PIR I'm asking for help.
     
  2. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    Welcome to AAC.

    Let's take a step back - what is it you want the PIR to do? What is the PIR hooked up to?

    I've played with these modules some, but I've never attempted to modify the PIR circuitry. While I know you don't want to add much more, I strongly suggest putting the original parts (or new parts with the original values) back onto the PIR PCB and adding a 555 monostable. Once the PIR triggers, the output of the 555 will go high for nearly as long as you want without the need for the PIR sensor to stay triggered - assuming this is your goal.

    I've made a few circuits using PIR sensors you may be interested in here and here.

    If you're really set on modifying the existing PIR sensor's circuitry, look at post #8 of the last link. JohnInTX provides a schematic of a PIR sensor circuitry that might help you.
     
  3. LEDuser

    Thread Starter New Member

    Aug 29, 2008
    3
    0
    I want to extend time of this module. I'm planing to put this sensor in to one device where I'm limited in space.

    Good idea is to add external circuit. I don't know why I didn't think about this before ... probably because I'm limited in space?

    I saw this circuit. Main thing again is again space limitation. I was planing to use one uC for time calculations and one amplifying circuit to amplify PIRs signal.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    If everything is hidden in a "black-blob" chip, it might just be easier to build an add on monostable that gives as much on time as you want.

    You could build as a standalone add on, or hack into the PIR circuit at the point it drives its relay.
     
  5. elec_mech

    Senior Member

    Nov 12, 2008
    1,513
    193
    I'll ask again, what are you hooking the PIR sensor to? How much space do you have? Can you fit a small board thru-hole with an 8-pin IC and a few components? If not, can you fit a surface-mount board with surface mount components?

    A uC is a microcontroller, a programmable IC. So are you saying you were planning to use an op-amp AND a uC? This is two ICs. Depending on what you're having the PIR trigger, one 555 could take care of both your need for longer time and the need to supply more power to your load. Most PIR sensors can only supply a few milliamps, but a 555 can supply up to 200mA. If not, one 555 hooked to a transistor will do the job.

    What is the voltage and current requirement of your load (the device you want the PIR to trigger)?

    Also, what voltage are you supplying to the PIR right now?
     
  6. LEDuser

    Thread Starter New Member

    Aug 29, 2008
    3
    0
    Sorry for late reply.

    I already have housing which is fraction bigger than this module in all directions so add-on board is no-go. Also I need few amps on output but one SOT23 FET is not so big. I will try with some more testing with amplifiers and uC. Few capacitors, few transistors and resistors and one uC should be smaller than chinese module.

    I just need to find one schematic with reliable transistor amplifier (or small op amp). Does anyone have any idea?
     
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