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I´m playing around with some PIR-Sensors (I don´t know the exact type) and I don´t have any experience in designing circuits for this type of sensor. I have googled some basic circuits and I took a look into the D203B datasheet to get some ideas how I have to design a circuit for this sensor. I want to test this one:

enter image description here

But the transistor doesn´t pull down the signal line. I have measured the output voltage of the sensor and my oscilloscope displays a voltage of 520 mV, but this voltage doesn´t change when I move my finger in front of the sensor. So I´m pretty unsure if this sensor is broken or if my circuit is "too cheap".

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I worked on PIR systems for 10 years.

The output signal of those sensors is 0.2 mV.

Amplify that signal by 3000 and then your scope will see something.

There are tons of circuits around that use operational amplifiers.

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Here’s an ST appnote showing signal conditioning for PIR sensors: https://www.st.com/resource/en/application_note/dm00096551-signal-conditioning-for-pyroelectric-passive-infrared-pir-sensors-stmicroelectronics.pdf

Summary: it uses a quad op-amp, with two stages designed to provide gain and filter out noise, and a pair making a window comparator. Together these provide a reliable detection of the PIR signal.

Note that the appnote has a high-gain non-inverting amp as the first stage. You'll need this to get any kind of a useful signal out of the PIR sensor.

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  • \$\begingroup\$ High-gain follower is nonsense. \$\endgroup\$ – user263983 Mar 24 at 19:02
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As others have stated you have totally inadequate gain in your design.

Page 4 of the datasheet you link to has a recommended circuit. You can feed the output of D1 and D2 into the transistor in your circuit to do what you want.

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Output of sensor is miliVolts, not enough to control MOSFET. If you mean to apply the amplifier circuit in that document it is good point to start. But you should keep in mind that sensor does not work without lenses. The sensor is differenial types and IR radiation should be different to each half. Lenses create this pattern and transfer radiation only nessesary band. Quality of lenses affect sensitivity

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This circuit will not work.

The sensor source is a JFET buffered not a logic level.

Try the circuit provided .

Rin = 47k to 0V
Av= 3600
Filter shaped as provided with DC biased output to 40% of Vcc.
Window comparator for signal <= 1/3 of Vcc or >= 2/3 Vcc

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  • \$\begingroup\$ Sorry, but what's the meaning of "cct"? And do you have an idea for a replacement? \$\endgroup\$ – Kampi Mar 24 at 17:16
  • \$\begingroup\$ circuit=cct. Try the circuit provided with high-gain, low-noise , low pass filter and window comparator. \$\endgroup\$ – Tony Stewart EE75 Mar 24 at 17:42

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