Sensor on Aurora AS1018CD paper shredder doesn't work

[CapSnap123]

I'm getting frustrated troubleshooting a paper shredder. The way the shredder normally works is when you turn the power on, nothing happens until you insert a piece of paper. There is an IR shredder in the shredder and inserting the paper breaks the beam and starts the shredder motor. But now the motor runs constantly, even when no paper is inserted. I've replaced the photodiode and the IR emitter. Also the relay makes a nice clicking sound so I assume it is working too. I don't know what else to check. Here's a photo of the circuit board. The red and black wires marked IR - go to the IR sensor/photodiode. The red wire connects to the cathode. The red and black wires marked IR + go to the IR emitter. The red wire connects to the anode. I bought them both in the same batch so hopefully they're compatible. Any suggestions?

[CapLeaker]

I bet you getting tired of that thing. Problem is you identified a bunch of stuff, like the IR emitter, the receiver, the relay…
Question is did you actually check these components and the board for proper function? Are the both IR receiver and transmitter lined up? Is the IR transmitter emitting anything at all? Is the IR receiver doing something? Also just because a relay “klicks” does long not mean it is any good. Maybe a wire is backwards… How did the shredder got to this state first place?
My shredder has a similar board and if I put the switch to the left position, the shredder turns on (reverse). If I put the switch to the right position, a LED is on and it just sits there and waits for a paper to be inserted and the motor stays off until then.

[redwire]

The relay clicks but the motor stay running? Doesn't make any sense.
What is it doing. Motor off until paper in sensor, then it stays on?

There may also be a delay that keeps the motor running for a few seconds after the sensor is cleared. If that doesn't work the cap might be bad - but it should not keep the motor on forever.
What is the p/n on the transistor on the board.

[stj]

first is the actual path for the IR clear or blocked with dust?
second, i suspect the sensor drives the base of the transistor that then drives the relay and the cap next to the transistor may be on the base to hold it on a while.
so if the beam-path is clean i would look at that transistor and capacitor

[CapSnap123]

When working properly, with the switch on, the shredder sits quietly until a piece of paper is inserted. But currently it starts shredding as soon as the switch is turned on.
I previously cleared all the dust. I took the relay off the board today and tested it. It works. When the power is on, it completes the circuit and breaks it when the power goes off. I removed and connected the IR photodiode to my multimeter and it gave between 10 and 75mV when I shined a TV remote on it. In the dark it gave about 200mV (the values were very erratic). I looked at the IR LED with my iPhone camera and saw a purple glow so I think that's working too.

I'll look at the transister and cap when I have a chance.

[redwire]

Is it possible the wires got flipped between LED and sensor? IR- and IT+ are a bit confusing.
The IR LED should measure around 1.3V across it. Phototransistor should read open circuit in the dark. If the photo-transistor is disconnected, the relay should not activate. You could try disconnect a wire to it and see if it still runs. Then I would suspect the transistor on the PCB. A pic of the other side can help us follow the circuit.

[CapSnap123]

Finally had a chance to get back to this. I tested the transistor, the SCR, and all the caps. One of the caps had a high ESR so I replaced it but it didn't help. I get about 22V across the IR LED and open across the phototransister all the time (do you test it with the ohm-meter?). I've attached a photo of the back of the board.

[stj]

i think the ir led is fucked - it should read about 2v - if your seeing more then it's not drawing current.
to prove the point - short the phototransistor pins and see if the motor stops

[redwire]

You can test both the IR LED and photo-transistor using diode-test on a multimeter. Either it's wired wrong for polarity or swapped LED/transistor, or she's blown open-circuit.

[CapSnap123]

The receiver is a photodiode, not a phototransistor. Is that a problem? I didn't realize there was a difference when I bought the replacement and unfortunately don't have the original component anymore to check what it is - though it looked the same (black LED shaped component).

[stj]

no real difference as far as i know.
short the pins - if the motor stops the issue is with the optical stuff

[redwire]

A photodiode has pretty low sensitivity. What part did you get? It should be a phototransistor. Again, you can test it with a multimeter and power off.
The motor stops when light hits it, right? A photodiode I think would just not do much.

[CapSnap123]

I figured it out - it was a bad LED plus the wires on the IR photodiode were reversed (nice self-inflicted injury there). Thanks for all the suggestions.

[redwire]

So it works now? We can party?

[CapLeaker]

See my post #2 lol. It was close any, Glad it works!