This circuit is used to detect objects by reflected infrared light. It can be built into a cylindrical enclosure just like an inductive proximity switch.
This is also useful as a level detector for colored liquids like oil. This has some immunity to ambient sunlight as it detects ac pulses.
The capacitor C6 blocks DC and only transfers AC pulses if any
to opamp amplifier U3A whose gain is set by R18, D9 rectifies
the pulses to DC and this DC voltage is used by opamp comparator
U3B which drives Q1 through Q2 for an open collector output for
relays. LED D7 turns on when relay Output is high.
The Optic switch can be used for both reflecting detection (retro reflective) or obstacle detection. The mechanical construction will decide this, for obstacle detection the diodes D1 and D2 could be put in two different tubes and can be kept far apart 2mts+ and both should be exactly opposite each other, any obstacle like a passing person will be detected. To make a retro reflective proximity switch this circuit is ideal, it can be housed in a cylindrical 30mm by 70mm metal unit with m30 threads and nuts for mounting, both D1 and D2 have to be fitted in the front of this tube on a plastic plug optically insulated from each other yet beside each other.
The types available are various and polarity hard to detect
even photo IR transistors can be used. The IR Led can be
tested in diode mode of a DMM (battery should be in good
condition) it should give around 1.1V drop in proper polarity.
An IR detector diode or photo diode can be tested in the same
way the drop will be 0.5V at 1 feet from a 60W lamp (no
sunlight), closing the IR photo diode with your hand will be
an over range on DMM this will happen on proper polarity. the
photo diode shows around 10k ohm resistance in daylight and in
Mega ohms when covered also the photo diode detects light on
reverse bias and used like that.
This is a Hand Crafted PCB Artwork done by a PCB Vendor years back, this method may be used even today.
PCB Design -
30th Aug 2020
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