What is this V/UHF BJT circuit in a doorbell ? - Page 1
Forum section: electronics / repair

author: MathWizard
author_rank: Super Contributor
timestamp: April 28, 2025, 05:14:04 pm
content: I have a broken chime style doorbell, and I'm just having a look at the old button module, the button is corroded and the A23, 12V battery was dead. It had no charging circuits at all, and there was only 1 connection to the + battery terminal, just the doorbell SW, then a diode, then into the black-blob IC/MCU. I'm also not sure where the power-in wires attached, or if it was just battery operated, which is insane in this cold/wet place.Really the only external circuit, looks like some part of an oscillator circuit, using a V/UHF NPN MPSH10. I've seen 2 pinouts for this, so maybe I have the emitter and collector mixed up, they measure about the same 0.79V voltage drop, it's a pretty low current BJT. But I can't say the circuit makes sense to me. Maybe it's a class C amplifier.But then they also have what looks like a current shunt in a DMM, going from C2 to the middle BJT pin, which should be the emitter (or collector). To me that shouldn't be an antenna, and I can barely measure it's DC resistance, so it's a short. It would make more sense as Vcc in. IDK what else it's there for, they could have just used a trace, not a big thick steel looking wire.But then on the PCB, the label is "SL-6194-TX", and looking around, people say it is wireless. Nothing else looks like a antenna at all.I had a hard time measuring the pF caps, so some of them might be off. In LTSpice, w/o any IC or MCU attached, the only little bit of ringing I can get from the inductor, was only 1MHz, I doubt they are using AM radio frequency.If it was just an oscillator for a clock signal for an MCU, well why use a V/UHF BJT, maybe it's lower power, but then why would this even have an IC or MCU at all ??I haven't tried applying 12V yet, but I have to put the caps back 1st.So what does this circuit look like tho ?

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2025, 05:55:13 pm
content: C6, C7, and L2 appear to be part of a Colpitts oscillator, somewhere around 120 MHz, with the transistor.(That assumes your pF and nH values are correct, but the circuit is certainly a Colpitts.)This is just a low-power transmitter: the receiver is tuned to the same frequency and is more complex.It is not intended to clock anything.Please use wires in simple drawings instead of incorrect node indications: How do "MCU1", "MCU2", and "MCU5" connect to each other.

author: MathWizard
author_rank: Super Contributor
timestamp: April 28, 2025, 06:15:39 pm
content: Ok so a Colpitt's osc. Well the labels, just go into a black-blob, and the 1-2-3 is left to right. I'd have to solder on leads to check for sure what the BJT pinout is.But so is the shunt wire doing anything ? Is that really an FM inductor, and the 120nH is a blocking inductor ?

author: MathWizard
author_rank: Super Contributor
timestamp: April 28, 2025, 07:59:20 pm
content: Here's a picture of a similar doorbell, mine has SMD caps/resistors. The metal bar on mine, fits in 1 plane, it doesn't have the extra 90deg turn.

author: TimFox
author_rank: Super Contributor
timestamp: April 28, 2025, 08:08:12 pm
content: Quote from: MathWizard on April 28, 2025, 06:15:39 pmI see no 120 nH in your drawing. L2 = 150 nH is presumably the "tank" inductor for the Colpitts oscillator.Your explanation of labels is useless to the reader.