Initially I was surprised that they implemented the paper sense circuit without a step-down transformer and DC regulator. They just seemed to have a mess of diodes, some of which I now understand are part of a (capacitive) charge pump to produce the DC supply.

How stable a supply is this? Not very stable! I observed about 2 V peak-to-peak ripple with an average voltage of 23 V. It turns out that this works fine for this circuit.

When Q1 activates, it conducts current through relay K1, activating the relay which connects power to the motor.

Because the relay electromagnet is an inductive load, D4 is there to conduct transient current when Q1 abruptly shuts off.

That's the whole thing!

When paper is present, R4 pulls up to the DC supply, dumping current through D3 and charging up C2. When C2 is sufficiently charged, transistor Q1 activates, conducting current through D6 which provides reverse current protection.

When paper is not present, D3 stops conducting, and C2 discharges through R6. This arrangement makes the detector relatively slow to start when paper arrives and quick to stop when the paper is gone.

Paper is detected by a photointerrupter formed by IR LED D9 and IR phototransistor Q2. These two components are mounted individually on either side of the paper intake slot with wires extending to the PCB.

At first I wasn't sure if the detector was a photodiode or a phototransistor, but I think a photodiode would require more amplification.

When paper is not present, infrared light from D9 reaches Q2, causing Q2 to conduct and creating a voltage drop across R4.

Auto (forward) mode is much more complicated and is the reason for all of the remaining circuitry on the PCB.

DC for the paper sense circuit is supplied by a charge pump. The AC supply's neutral ("N") is used as DC ground. Current from the AC hot line ("L") traverses chonky through-hole parts C3 and R1 to drive the charge pump. When L swings low, current goes through D1. When L swings high, current goes through D2.

This charges up capacitor C1. Excess current is shunted through zener D7.

Reverse mode is simple. AC power is directed to a bundle of wires of various colors that extend to the motor assembly.

The blue wire passes through a thermal cutoff device which happens to be the part that failed on this shredder. It took me a while to find the thing strapped to the least accessible side of the motor, but I was able to pull it free and test it.

A brown wire, labeled "TH" on the PCB, connects the cutoff output to indicator LED D8 with reverse current protection provided by D5.

Infodump on this paper shredder control circuit since I reverse engineered it and think it's nifty:

This model is made for North American 120 V 60 Hz service. There is a safety switch (SW1) in line with the captive cable that connects power only if the unit is resting on its bin that catches the shredded paper. Fuse F1 is on the PCB.

A slide switch (SW2) allows the user to select "auto" (forward shredding only if paper is present), "reverse", or "off" (the center position).