Light-Pulse Operation of Sensor
This system uses a modulated Light beam in place of the steady beam used in previous systems.
The proposed method is a pulse waveform applied to D, and LED.
The LED flashes on and off as the pulse excursions pass. Although the Light beam flashes,.if the
flash rate is above the flicker frequency for human eyes (about 8-10 flashes/second), it appears to be constant.
If the output circuit of the light sensor is equipped with a capacitor coupling network (C1/R2), then only variations in tight Level are passed. That will prevent ambient Light modulated by such sources as ceiling fans or tree branches outside the window blowing the wind from also exciting the alarm.
It is a[so. possible to encode the light beam. lf a proper decoder is provided, then the alarm circuit will recognize on[y the correct code.