This type of motion detector uses the same basic concept as the active infrared motion detector. An interruption in a 5 kHz modulated pulsating beam that is transmitted by an infrared diode and received by an infrared transistor sets off the alarm.
For our project, we decided to construct an active infrared motion detector. Originally, we wanted to build both an IR and an ultrasonic detector, but we decided that an ultrasonic detector would require too much time for a three week project.
The circuit on the left is the transmitter circuit that establishes a 5 kHz modulated infrared beam. As you can see from the schematic of the receiver circuit, a resonance-tuned narrowband amplifier reduces the detector`s sensitivity to stray light. C1 and L1 in IC2A`s feedback loop cause the op amp to pass only those frequencies at or near the LED`s 5 kHz modulation rate. IC2B`s output increases when the received signal is sufficient to drop the negative voltage across C2 below the reference set by R2. The output of this circuit is then attached to some load resistance, which can be an alarm or, for demonstration purposes, an LED.
However, we also decided that just building an IR motion detector would probably be a trivial exercise. So, we decided to expand on the concept by building an ambient light ignoring motion detector.
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