pull down resistor in PIR circuit

Discussion in 'The Projects Forum' started by skyhorse33, May 26, 2015.

  1. skyhorse33

    Thread Starter New Member

    Jan 5, 2012
    15
    2
    I have an motion sensor circuit. The circuit calls for a voltage of 2-15 dc volts. On the PIR data sheet the source pins calls for a pull down resistor of 47K at 5 volts. My question is if I use 8 volts on the circuit do I need to recalculate the source pin resistor? I have searched this forum and others for the usage of higher or lower voltage, but cannot seem to find any examples. All example are made around the 5 volt.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Well if you go with the current needed, and use Ohms law, you need a 75K resistor for 8V.

    Is there a datasheet for it?
     
    Last edited: May 26, 2015
    skyhorse33 likes this.
  3. skyhorse33

    Thread Starter New Member

    Jan 5, 2012
    15
    2
    Thanks for replying and Your help. I do not have a data sheet on the circuit. I am using PIR325 on the circuit I have now and it calls for a 100R resistor and it seems to be rather slow. I am using 8 volts on it and I have the circuit tied into a PICAXE 08m2. I also have a regulator and a diode for the voltage on the picaxe. I am posting a picture of the schematic for the circuit without the picaxe. The picaxe is tied into the circuit a the large dot next to the led. I want to change PIR to a PIR LHI878. Here is the data on the PIR.
    http://www.excelitas.com/downloads/dts_lhi778_lhi878_pyd1388.pdf


    Hope this helps.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Looks like it works uto 12v at 47K load, so leave the resistor in and up the voltage.
     
    skyhorse33 likes this.
  5. skyhorse33

    Thread Starter New Member

    Jan 5, 2012
    15
    2
    I want to use the LHI878 so will change the resistor to 47K. Right now I have a 100K on the board because I am using the PIR325. I will upper the voltage to 12 volts and report my finding.
     
  6. skyhorse33

    Thread Starter New Member

    Jan 5, 2012
    15
    2
    Changed to 47K and 12 volts but the sensor would not settle down. Just stayed on the blink. Changed to 12 volts 75K and the same thing happened. Changed to 8 volts and 47k and the sensor worked well sensed up to 70 feet and was responsive. Changed to 8 volts and 75K and worked very well. sensed up the 90 feet and was very responsive. So will stay with 75K 8 volts.:) Thanks
     
    Dodgydave likes this.
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