Gosh I didn't know there were toaster chips but it seems so. One here 8 pin package with pin 3 being the bagel pin. https://www.diodes.com/assets/Datasheets/PT8A2514A.pdf
> As input, requires a positive pulse to active "Bagel" function.
> As output, gives a "Bagel" function indicator, active high.
Unfortunately I still have more questions after reading the schematic.
SW4 (BAGEL) allows current flow through R9/LED3 (BAGEL) which raises Vbe on Q2, forward biasing it, allowing current flow through Q2, dropping Vcs.
Meanwhile, power has been flowing through the main switch, with RL3/RL4 making a voltage divider. That main power flows to D1, forward biasing it, and then hits the cathode of D4, reverse biasing it, and allows Relay 1 to hold main power closed. The cathode of D4 also tees off to inputs on Relay 2, and the cathode of D5 (whose anode is fed by both another input on Relay 2, and the source of Q2).
So, when Q2’s Vcs drops due to it turning on, D5 presumably reverse biases, so the only input felt on Relay 2-2 is from D1. And Relay 2 does… something. If anyone else can explain, I’d be grateful.
Source: I'm not married to toaster moguls but I do peer into operating toasters to see which wires turn red. Feel free to replicate my research and post your observations.
Looking closely during operation, the elements facing out from the center didn’t appear to change color with the bagel button depressed. It toasts the bagel just fine (as well as bread), though.
I should get a wattmeter and see if power draw changes.
Relay 2, activated when the "bagel" light is lit through the gate of Q2, disconnects neutral from heating element 2 (the one in between slot 1 and 2) and 1 (the one on the outside of slot 2).
This means that there is only power to the outside heating element of slot 1, allowing you to toast only one side of a bread or bagel.
The power to the circuit flows from in between those heating elements, through a single rectifier (D1), through a 1W resister (R1), before being clamped to 5.1V by a zener (VZ1). The use of the heating element as a voltage divider is just to get closer to the target voltage (<15V considering the rating of C1), using the fact that a heating element is just a resistor with an enormous power rating. No one cares about the power efficiency of a toaster control circuit.
However, the schematic is missing a connection to VCC from the 5V rail it created - it only connects to the high side of the mains relay/toast magnet as well as the bagel button. It was supposed to connect to VCC, which in that schematic is only connected as to the clock circuit.
(I was not using the RL numbers when I numbered the heating elements in my previous comment originally - my bad. I updated the comment.)
I have certainly misinterpreted iconography completely before...
[0] https://www.cuisinart.ca/on/demandware.static/-/Sites-ca-cui...
https://www.overclock.net/threads/worried-about-13900k-tempe...
Now I want to buy a couple... 0.856 € unit price for 1pcs, available in stock.
New evil plan: corner the market on toaster controller ICs and build up a strategic reserve to resell to manufacturers at inflated prices.