Printer and Photocopier Troubleshooting and Repair Collection : Toner sensors?                          
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Toner sensors?

I took one of those apart and found something that looked like a piezo element with 3 wire connections to concentric rings. I looks to me like some sort of resonator but I have not attempted to analyze the circuit.

My wild guess would be that the frequency changes with low toner or something like that.

(From: Morton Lee Cohen (cx163@FreeNet.Carleton.CA).)

Probably the disks stir the toner for even dispersion, and break a photogate to tell the controller PCB that there is toner there. Some Cannon laser printers/photocopiers use magnetic sensing of the toner level. Most Okidatas use the photogate sensors with magnetic toner agitators.