Son of Operating a relay with a PIR sensor

Discussion in 'The Projects Forum' started by allenpitts, Mar 15, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello All About Circuits Form,
    Have learned a lot about the PIR sensor/relay combination.
    I have gone from

    I have bread boarded this and done it on a PCB and it works great.
    (Reload Ron lamented the demise of Radio Shack as a source for small
    prototyping boards. Have found a supplier for the Archer 276-1591B at
    and board that in some ways is an improvement:
    Working on getting this PIR relay combination to work an LED display
    The schematic shows a momentary switch SW1:
    It is a switch that connects two different traces at the same time. It seems they both must
    be closed at exactly the same time because experimentation with using jumpers to close them resulted
    unpredictable results.
    So it was reasoned that a DPDT relay would get the job done.
    Have tried two different DPDT relays:
    CIT WJ0982Cs9VDC
    and a
    Kest KS2E-M-DC12
    Here's the problem: Have breadboarded the PIR to Relay Schematic just like that shown herewith
    above. I can get the either relay to work if I take out the LED and R1. But when I put the LED and the R1 the relay
    won't close. The resistance of the relay was measured at about 130 ohms so it was tried with
    100 ohms for R1. Same result. So it was tried with no R1 resistor between LED1 and Vcc. Still no relay
    So it looks like the circuit will work with SPST relay but not a DPDT relay. That doesn't make sense.
    What am I missing?


    Allen in Dallas
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    Not surprising. The LED was severely reducing the voltage across the coil.
    T1 is probably having difficulty providing enough current to drive both the relay and LED. Leave R1 at 220 Ohms, but try reducing the value of R2 to, say, 330 Ohms (providing that won't draw excessive current from the PIR output). What is the PIR output rating? What is the supply voltage?
  3. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello Forum,
    Before any body spends too much time on this.
    I found the mistake. It was in the breadboard

    Allen in Dallas
  4. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
  5. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello Alec_t,

    Thanks for your reply.
    I have got it to work.
    But to answer your question.
    The output of the PIR is 3.3v
    The supply is a 9 volt battery.
    The next step is to go from breadboard to PCB.
    Then connect to the Velleman MK152
    Will advise.

    Thanks again

    Allen in Dallas
    Last edited: Mar 16, 2015
  6. JohnInTX


    Jun 26, 2012
    FWIW Allen,
    You can get proto boards like that and much more at Tanner's in Carrollton, TX. Valwood Pkwy a block west of I35.
    Altex off Midway just south of Trinity Mills (near Addison airport) is a great source as well.
  7. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    Hello Forum,

    Before I went to PCB I decided to use the working breadboard of the PIR relay circuit, which
    looks like Pencil's circuit above, to connect to and operate the LED display

    I hooked it up like this:

    It worked, sort of. In this configuration the LEDs spin around
    the wheel and stop when the signal from PIR stops. Always on
    the same LED. In normal operate the wheel spins and then begins
    to slow down until stops on a random LED.

    So I played with it awhile and tried in several configurations
    and finally hit on this which included reinserting the momentary
    switch that came with LED display kit.

    It worked!
    Ok I just figured it out. I can move the bottom right SW1 connection
    to P3 and the bottom left SW1 connection to P4 and eliminate the switch.
    This is one of those cases where, by thinking logically about the thing
    I didn't understand long enough to write it down and explain it
    to some else, I figured out the answer.

    So I didn't need DPDT after all. SPST throw will get it done.
    Will PCB the PIR/relay circuit, hook it up to the LED
    display and post a video of the whole thing in this thread.

    I think I could have just used a transistor instead of a


    Allen in Dallas