Timer fired by PIR sensor

Discussion in 'The Projects Forum' started by allenpitts, Mar 9, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC forum,

    Looking for a circuit or a kit that will start a timer on a relay for ten to twenty seconds.
    The timer would be fired by a +3 volt signal from a PIR sensor (motion detector).
    https://www.adafruit.com/product/189

    Have found several kits
    http://www.kitsrus.com/pdf/k152.pdf
    http://www.canakit.com/mini-electronic-timer-kit-ck002-uk002.html
    Velleman has several kits but they are all 12v.
    Would prefer 5 to 9 volt since that is the voltage of
    load on the relay.

    Thanks.

    Allen in Dallas
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    The K152 with miner changes would work on 5 V, ie 5 V relay, some R values. Would be helpfull to know timing requirements.
     
  4. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello AAC,

    Here is the timing
    1. The PIR sensor sends out a 3.3 volt signal. (It lasts about two seconds)
    2. The relay closes for ten seconds
    3. The relay opens
    4. The system is ready to receive another signal

    The K152 was used a an example. Its is $19. Hoping to find
    something a little less expensive.

    I believe s 555 timer circuit in monostable mode would get it done. Example
    http://electronicsclub.info/555timer.htm
    But I have a lot of questions. What voltage does the trigger (pin 2)
    of the 555 timer want? The PIR sensor is sending out 3.3volts.
    Is a 10k resistor, as shown in the section of the page
    referenced above marked "555/556 Monostable"
    correct.
    The page at
    http://www.simple-electronics.com/2010/10/555-timer-calculator-monostable-and.html
    has a calculator. I think a 220k ohm resistor at R and a 40uf capacitor at C
    would yield a ten second pulse. If that is the case I will build the circuit myself.

    But what size relay is needed? I have 3 volt and 5 volt relays. Will
    the 555 latch these? I have poured over these pages but it never
    says what the output voltage is.

    Thanks.

    Allen in Dallas
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    The 555 needs a pulse from positive to approx 1/3 of the supply to trigger it, so if your working on 5v, thats less than 1.7v to trigger it,

    The pir goes positive when detecting, you will have to use a transistor to trigger the timer (in red,)


    555.png

    the output is whatever the 555 is being supplied from, again if your supply is 5v, then thats it output, the resistor and capacitor will give approx 9.6 seconds, its better to use a 5v supply and use a 5v relay with a back emf diode.
     
    Last edited: Mar 10, 2015
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    Hi Allen,
    The high going +3V pulse from the PIR can be used to switch ON a transistor [ via a series Base resistor].
    The Collector of the transistor can be connected to the same supply voltage as the 555 [ via a Collector load resistor].
    The low going pulse from the Collector, connected via a 10nF to the 555's Trigger input, will start the 555 Monostable delay.
    A 10K pull up resistor from the Trigger pin to to +V is required.

    E
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    On post #5 ckt., increase base to gnd. 1 k to 10 k, eliminate C & pin 2 pull-up R, connect pin 2 to collector. PIR pulse is shorter than desired pulse width so C coupling not required.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    I put the cap in to prevent the pir keeping the timer high in case of re-triggering, otherwise yes i agree.
     
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