0
\$\begingroup\$

I am using a Murata IRA-S500ST01A01 PIR motion sensor. This is the datasheet. Based on a description of the pyro-electric effect from part 2 of these application notes for the IRA S210 (same family of sensors), the output voltage of the sensor should change in reaction to a change in temperature of the pyro-electric element.

I have built this circuit from this data sheet for the IRA S210:

enter image description here

The op-amp I am using is actually the LM324N. I have connected the OUT line to a 10K resistor and then to GND, and placed the nodes of a multi-meter on either side of the resistor to measure voltage. What I have observed is that the voltage is stable at about 850mV. When I wave my hand across the sensor I get no response, but when I cup my hands around it or wave a stove-heated spatula across it I do see an increase in voltage of about 10mV. Is this to be expected?

If so, how would it be able to detect a person walking in front of it?

\$\endgroup\$
0

1 Answer 1

1
\$\begingroup\$

how would it be able to detect a person walking in front of it?

I'd be looking at the other test circuit they show and recognize that to measure small changes in signal you need an AC amplifier as shown in the test circuit for white noise output: -

enter image description here

Everything to the right of the red arrow needn't be built. But, you really need an oscilloscope. What you may find is that the 2.1 MΩ resistor (feedback on the left op-amp) might be adequate or, it might need adjusting for your actual needs. You might also find that the 22 μF capacitor might need to be bigger to adequately detect slow moving changes in IR picked-up.

\$\endgroup\$
4
  • \$\begingroup\$ What resources are good to learn how this circuit works? \$\endgroup\$
    – cornelius
    Jan 14 at 6:55
  • \$\begingroup\$ @cornelius The best resource I can recommend is by asking a question on this site. Hint: the LM224 op-amp on the left amplifies the AC content of the signal from the sensor by about 200 times. If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$
    – Andy aka
    Jan 14 at 10:43
  • \$\begingroup\$ what would I expect to see on a voltmeter across a 10K resistor at the red arrow? It's just bouncing between 750 - 850 mV at rest. \$\endgroup\$
    – cornelius
    Jan 14 at 13:30
  • \$\begingroup\$ You really need to use an oscilloscope at this point. The signal is likely changing at a rate of a few hertz and this is not a good place to make any sense of it with a voltmeter either on AC range or a DC range @cornelius \$\endgroup\$
    – Andy aka
    Jan 14 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.