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This simple circuit is an IR tester able to test and help troubleshoot any infrared control. The circuit is using an UA741 OPAMP and a BPW41 IR detector to produce a visible light on red LED when infrared signal is detected from BPW41.
Simple IR tester - [Link]
Most 741 opamps do not work when their input voltages are within a few volts from the pin 4 negative supply voltage which is ground in this circuit because the inputs use NPN common-emitter transistors and the emitters have current-mirrors to the pin 4 voltage. The datasheet shows that the allowed input common-mode voltage range has a minimum that is 2V to 3V more positive than the pin 4 voltage.
But this circuit has its pin 3 input connected directly to pin 4 so the pin 3 input does not work and it has the pin 2 input connected to the photo-diode which has an output that is very close to the pin 4 voltage so this input also does not work.
The circuit will work if pin 4 is disconnected from ground and connects to a -9V supply. The positive of the -9V supply connects to ground.
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