====== Proximity Sensor with PLL-IC ======

By using the PLL IC NE567, only light of the frequency and phase sent is detected:


{{:elektronik:wechsellichtschranke.svg?736|}}


R2 determines the sensitivity;
if too large, strong stray light may overdrive the sensor.

R3 and C4 determine the frequency and need only be changed if stray light of the same frequency is very strong.

Q1 is the LED driver and delivers the signal in the right phase; R4 determines the current through the LED.

The output at pin 8 is an open collector transistor switched on if the PLL locks, i.e. reflecting surface or not disrupted.
It can drive 100mA (on) and withstand 15V (off); the resistor R1 is shown only for convenience.