Old Junk Given New Life

PIR Motion Sensor Rebuild Project

This article is a breakout from a longer article: Long-Range Wireless Gate Annunciator.

A microwave motion sensor (see) proved ultimately to be a better choice for the application,
but you might find this information useful in your application.

Max Carter


I had 3 defunct motion sensors, removed from motion-operated floodlights, all Heath/Zenith units, a model that seems to be particularly unreliable. Given the poor reliability of these sensors and the need to troubleshot a bad unit before even attempting to adapt it to my purpose (here), I decided not to try and rehabilitate the sensor electronics and returned instead to first principles. I would retain the enclosure and mount and build the sensor electronics up from scratch.


Heath/Zenith Motion Sensor



Original electronics and Fresnel lens removed - outer case and clear plastic horn retained.



Brittle and crumbling after years in the sun, the Fresnel lens could not be reused, so I ordered a new lens.
($1.20 from 3Dlens.com, #5058, note #5059 would likely work with some trimming.)



Also, not wanting to trust the PIR sensor on the original Heath/Zenith electronics board, ordered in a new PIR sensor.
($1.90 from Futurlec, #PIR_D203S)



PIR Sensor Interface Electronics

A pretty good explanation of how a Passive (or Pyroelectric) InfraRed (PIR) sensor works can be found on Wikipedia. The main takeaway is that the useful output of the PIR sensor is a tiny AC signal superimposed on a DC voltage. The detector circuit reacts to abrupt changes in the sensor's output, in other words, to the AC component produced by moving objects. The (usually relatively large) DC component is produced by steady or slowly varying background IR radiation from objects and terrain, fixed artificial heat sources or very slowly moving objects. From information in the Wikipedia article, the data sheet for the PIR sensor, snippets of circuitry found on several web pages, and some experimenting, I was able to piece together and build the amplifier/differentiator circuit shown in Figure 1.


Figure 1

The output of the PIR sensor appears on pin S and is directly coupled to the first amplifier stage (U1a). The first stage amplifies the desired AC signal (~2-20 Hz) with a voltage gain of about 20. The non-varying and slowly varying (<1/10 Hz) background level is not amplified. The first stage is coupled to the second (U1b) through the 22 µF capacitor. Only the AC component of the signal is passed by the capacitor. The voltage gain of the second stage is variable via the SENS control from 125 to about 350, for a total gain of 2500-7,000 (68-77 dB).

The circuit was developed and tested on the bench, but some issues could not be known until the sensor was in service.*


*The Partly Cloudy Problem

After the rehabilitaded motion sensor was first installed outdoors, the sensor would occasionally trip when no moving objects were visible in it's field of view. It always occurred during windy daylight hours under partly cloudy skies. After scratching my head over this phenomenon for months, I finally reasoned that clouds moving across the sky were the cause. If you look at an atmospheric IR transmittance chart you will see a large, deep notch appearing in the water vapor (H2O) portion of the IR spectrum (5-8µm). Being composed of water vapor, I supposed that clouds must contrast with the background sky. Clouds must be either warmer, because IR re-radiated from the ground is being absorbed, or colder, because solar radiation from above is being blocked. Either way the result must be IR shadows moving across the ground, in and out of the sensor's field of view. Since the edges of clouds are feathered (without sharp edges), I reasoned that the sensor amplifier/differentiator circuit was too sensitive to slowly-changing input signals. Lowering the value of the capacitor connected to pin 2 of the opamp (through the 47k resistor) from its original value of 47 µF (suggested by the PIR sensor data sheet) to 1 µF raised the sensor's low-end response from about 1/14 Hz to about 3 Hz and eliminated the problem. The capacitor value shown in Figure 1 is the corrected value.

First Amplifier Stage, Frequency Response

BEFORE Component Change


AFTER Component Change


The output of U1b is directly coupled to U1c and U1d, which, with two diodes, form a voltage comparator and full-wave rectifier. When the output of the second stage amplifier reaches a level of about 1 volt peak-to-peak, one or both of the diodes conduct, charging the 150 µF capacitor and turning on the NPN transistor (2N2222) and the PNP transistor (2N2907). The charge on the capacitor maintains the output in the ON state for a few seconds after the moving object has passed. **The output is HIGH [12V @ 500 mA] when motion is sensed. The output can be used to directly power devices [LEDs, audible alarms, etc.] or to drive a relay.


Optional Sensor Circuit Output

If the output of the sensor will be used to drive 5-volt logic [LS, CMOS, microcontroller, etc.] the output circuit can be simplified as shown below. The output is pulled LOW when motion is sensed.

Note: For 3.3-volt logic, replace 4.7 volt zener diode with a 3.3-volt unit.


The new interface circuit was built on a piece of perforated circuit board with the same dimensions as the original detector board, with the PIR sensor placed at the same physical location at the center of the board.


PIR Interface Board

The PIR sensor is the only newly purchased device on the board. The balance of the circuit was built with parts on hand, mostly unused old stock. Some of the capacitors had been cannibalized from equipment.


The new board was installed in the original case in place of the original detector board. The original plastic horn was reinstalled, along with the new Fresnel lens, and the case sealed up with RTV. A hole in the bottom of the case allows access to the SENS control.


I assume the function of the horn is to help focus IR radiation arriving from the front onto the sensor and maybe to shield the sensor from the tiny amount of heat generated by the electronics board.


Completed Motion Sensor




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