How to ground a wire programatically with MDBT42Q?

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allObjects
From what I understand this is - mainly - going on: pulling KE to ground will shut of Transistor T3, turn on Transistor T2 and Thyristor TK1. Using output_pulldown on Espruino will do the trick on controlling NMOS FET or NPN Transistor. Connecting Gate or Base respectively with a 22k..47K helps to not have an open input in case Espruino is not powered (most darlingtons have already a built in resistor doing so). A plain "output" (or "open-drain") for pin mode is then good enough. Attached schema shows only relevant components.

Alternative, the dashed connections to a door open push button in the Unifon in conjunction w/ C7, R11, R12 and PK1 allow to 4-wire only vs. 5-wire Unifon operation and makes the Unifon Mic input line do double duty:
1. Unifon Mic signal adds AC component to MU (Mic Unifon) DC input signal. The AC part is coupled out over C7 to input of OpAmp which drives the Panel Speaker. Potentiometer PK1 (with R11 and R12) is used to adjust OpAmp input to the DC component (Mic button pressed, no speaking going on).
2. Pulling MU down - DC ground - also grounds 'input side' of R10, and with that also Base of T3, just like grounding Base of T3 directly by pulling KE to ground. R15 / 180R is then taking all the brunt of the 11V. With about 61 mA current it burns - momentarily - about 0.72 W power and is therefore the fat resistor, compared to the other ones, which are skinny. Quite a resourceful solution, because the Unifones can be run with just plain 4-wire phone cable... ;) - EDIT: I don't know about the Z1-Z2 Jumper which is open in the pictures. It could be that it has to be closed for option 2 to work.
For sure Espruino should not be connected directly to KE nor to MU - not on KE because voltage is way above 3.3V and you do also not want any noise on the pin from house wiring either. (@MaBe: sorry, this time no char graph schematics either); not on MU for same reason and could anyway not handle draining 61+mA. But what you can as added bonus: power Espruino MDBT42Q Bluetooth Breakout Board from the + (11V) of the control station... ;)

So, you have two options to make the unlock solenoid work: Ground either KE or MU... with a NMOS FET or a NPN Transistor.

Thanks for sharing this nice thought exercise.

P.S.: Colors help me think...

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user112057 in reply to @allObjects
Thank you @allObjects and @Robin for time you put into this :)

So indeed, all this time I thought that unlocking action is triggered by grounding MK. Analyzes of @allObjects shows that it should be KE. So I double-checked and you're right :) I was grounding KE and put wrong info in one of the first posts in this thread. To my defense this generator is installed in a place that is hard to get to, so I was relying on small mirror to read the labels.

In any case, it's definitely KE that is getting grounded.

I also did a bunch of measurements:

Is the voltmeter an analog needle type or a digital LCD type? Black ground and Red positive probes?

It's a digital LCD type, with black and red probes.
1. The voltage drop on on R18 by measuring between MK and MASA. The voltage drop is 9.6V.
2. The VAC between X1-1 and X1-2 (the black cables from the coil) and it's 13VAC.
3. The drop between MU and MASA is 14V.
4. The drop between KE and MASA is only 0.6V (this is what I need to pull-down to ground).
Does this information change anything in values of resistor or transistor type I should use ?

Today I got pulled into work and did not manage to do the shopping. Hopefully tomorrow I will go buy the missing parts :)

Thank you both for all your awesome help:)

How to ground a wire programatically with MDBT42Q?

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