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PIR Sensor output is only 1.65V! — Parallax Forums

PIR Sensor output is only 1.65V!

SeBsZSeBsZ Posts: 6
edited 2011-02-24 14:21 in Accessories
Hello there,

Simple explanation of my problem:

The output of the Parallax PIR sensor is only about 1.65V when high, 0V when low. Supply voltage is 3.3V. This means that my microcontroller doesn't trigger, as it needs at least 2V to detect a high logic state. I have measured the PIR sensor's output both when connected to the micro, and when disconnected (floating). Both times it is only 1.65.

Finally, I have about 6 of these sensors, and have so far tried two different ones and they both do this. What am I doing wrong, and how do I solve it?

Regards,

Sebastian

Comments

  • kf4ixmkf4ixm Posts: 529
    edited 2011-02-22 08:07
    try powering the sensor with 5vdc. i do this with mine and have no problems with them. i once measured the voltage on the output using 5vdc to power it, but can't remember what the reading was at this time.
  • SeBsZSeBsZ Posts: 6
    edited 2011-02-22 11:06
    Hi,

    When powering the sensor with 5VDC, the output voltage is about 3.23V. This is hardly acceptable, are all my PIRs broken?
    My application will be powered at 3VDC, so I require the output voltage to be at least 2V, preferably close to 3V. Anyone else experiencing this, or just me?
  • Mike GreenMike Green Posts: 23,101
    edited 2011-02-22 12:30
    The PIR sensor has a logic level output. A voltage of 1.65V with a power supply of 3.3V is perfectly acceptable. Most logic level inputs have a threshold well below that. With a 5V power supply, an output voltage of 3.23V is well above the logic high threshold.

    If you need an output voltage near the power supply voltage, you will need some kind of buffer. Although it will invert the output sense, a simple bipolar switching transistor and two resistors would work fine. If you can't handle an inverted signal, use two transistors or a non-inverting buffer. TI makes a nice dual buffer in an SOT package.
  • SeBsZSeBsZ Posts: 6
    edited 2011-02-22 12:39
    Well it seems its not good for say atmega8, the datasheet mentions vcc*0.8 which is about 2 v for a 3.3v supply for active high minimum
  • ercoerco Posts: 20,256
    edited 2011-02-22 13:13
    0.8 of 3.3V is actually 2.64 volts. That's curiously high. Have you actually tried it yet? Your atmega8 spec may be wrong or misinterpreted.

    BTW, our beloved 5V Parallax Stamps trigger at a low 1.4V. Come on in, the water's fine!
  • SeBsZSeBsZ Posts: 6
    edited 2011-02-22 13:22
    Hi, in the hurry i made some mistakes. Okay, datasheet says MINIMUM input HIGH voltage is 0.6VCC, this is 1.98Volts.

    My PIR output is 1.65 V, which is why I believe my interrupt routine isn't firing, which is triggering on a low to high edge. Yes, the interrupt fires when I use a wire to connect the interrupt pin to VCC (from ground).

    EDIT:

    Perhaps a simple transistor level shifter will do the job, but I had rather solved this without additional components, as the PCB is already produced... A device with a supply voltage of 3.3V giving an output signal of 1.65V, surely this is not TTL compatible?
  • Mike GreenMike Green Posts: 23,101
    edited 2011-02-22 15:48
    Actually, TTL compatible uses a switching threshold where 0.4V is the maximum low voltage and 2.4V is the minimum high voltage with a supply voltage of 5V. There really is not a "TTL compatible" spec for a 3.3V supply.
  • ercoerco Posts: 20,256
    edited 2011-02-22 16:04
    There were a few posts several months back about people who thought their new PIR sensors were defective. Not sure if they were correct about that or not. It's possible there may have been a bad batch.
  • SeBsZSeBsZ Posts: 6
    edited 2011-02-23 01:01
    Hello,

    That's what I'd like to know, does everyone get the output Im mentioning?

    Regards
  • SeBsZSeBsZ Posts: 6
    edited 2011-02-24 14:21
    In addition, it seems my PIR sensors consume about 3mA at 3V, substantially more than the 100uA the datasheet claims!
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