What went wrong in my reverse engineering?

Question

I am reverse engineering a toaster.

I have the following part of the circuit that I don't understand.

Let me explain what I have at the moment:

We have a potential difference of around 20V DC coming from a divider bridge in the toaster followed by a rectifier diode and a smoothing cap.

Pressing the button lights the led up and activates something in the timing circuit. After releasing the button the function is kept in memory using the 4066 and the led still shines.

Now what does not make sense for me is that the divider \$R_{16} / R_{13}\$ gives for the transistor \$V_{be}\$ at around \$4V\$

Bonus questions :

• What is the point of \$R_{19}\$?
• Why not driving the LED by current?

I may have done some mistakes doing the schematics. I am pretty sure of resistors value and confident about the wiring because there is exactly the same circuit for an other switch button. I made them independently to double check.

I unfortunately don't have access anymore to the board.

Sorry, it looks more like a puzzle than a ESE question.

Answer 1

The resistor divider with R13 and R16 can't give 4V, or if it does then the transistor is damaged. Vbe will be about 0.6 or 0.7V, set by the transistor.

Point of R19 is unknown as it can't be seen what it connects to and how. Maybe it goes to a digital input which requires there to be either very near VSS or very near VDD, and without the resistor it would not reach very near VDD. Or maybe it is to dissipate energy so stray currents or charges don't leave it dimly lit for minutes.

The LED is driven by current, limited by R20. Or voltage, set by current through R20. Whichever is the easier way to think it, it's just a LED and a resistor as always, unless the parts not shown affects the LED driving.