Electronic Design

PIR Light Controller Relies On A Missing-Pulse Detector

Figure 1 shows a way to control a light (or other 120-V ac appliance) with a passive-infrared (PIR) alarm sensor. The original concept was to turn a closet or shop light on whenever someone went into the room and turn the light off when no one was in the area.

In this application, the PIR sensor sampled the area every 12 seconds. When it sensed a person in the area, it flashed an LED. After another 12 seconds, it flashed the LED again if the person was still detected. This original unit was powered by a 9-V battery.

Using a flip-flop or latch circuit on the PIR sensor will provide a convenient way to turn the light on and hold it, but it doesn't provide an easy way to turn the light off after the area is vacated. Therefore, this circuit uses a missing-pulse detector to detect the PIR pulses.

Normally, when the area is empty, no pulses are detected and the output goes low. When pulses are detected, the output goes high. Power (12 to 14 V dc) is supplied through the stereo phone jack along with the output signal, and it is then regulated to +10 V. This circuit was placed inside the PIR sensor, and the phone jack was installed in the lower part of the case. Resistor R4 is attached to the PIR LED driver. Common and +10 V is attached to the appropriate locations in the PIR.

The missing-pulse detector is a well-known "555" circuit. The 555 IC (U2) is set up as a triggered one-shot multivibrator. The 555 timer is set so that its on time is just longer than the incoming pulse rate—in this case, about 15 seconds. As long as a pulse arrives before the timeout, the timer will be triggered and start a new time sequence. Thus, the output will stay high as long as a person is detected in the area. If the trigger pulse doesn't arrive in time (less than 15 seconds or is missing altogether), the 555 will time out and reset its output, turning the light off. Missing pulses indicate no detected person in the area.

Figure 2 shows a triac-controlled ac switch that also supplies an unregulated 12 to 14 V dc for powering the PIR. The triac is driven by the output pulse from the Figure 1 circuit through a matching phone jack. A standard stereo audio cable connects the two circuits. A convenient source of male and female ac connectors is a standard two-conductor extension cord, cut in the middle and attached to the switch box with strain-relief connectors.

To use the device, place the modified PIR sensor in a convenient location and attach the stereo cable. Put the switch box near a wall outlet and connect the stereo cable to it. Then, connect the lamp or appliance to the female ac connector. Switch S1 turns the unit on.

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