Access keypad to electronic doorbell wiring - Page 1
Forum section: electronics / projects

author: 5U4GB
author_rank: Super Contributor
timestamp: February 18, 2025, 12:33:08 pm
content: I'm playing with an electronic keypad that's supposed to be able to drive an electronic doorbell, but the details are a bit inscrutable. Here's the wiring diagram:I've figured out all the rest, but the doorbell part is a bit of a mystery, I was expecting a NO circuit that's then closed to trigger the bell, but what's actually happening is that an open circuit briefly drops to a few tens of MΩ when triggered and then goes back to open-circuit (this could be due to the response time of the meter, it might very briefly go to 0Ω before the meter can react).The bell I'm working with just has 12V and GND connectors, so it'd be a case of switching either 12V or GND through the BELL wires if that's possible. The device is potted but here's a photo of a similar one showing only one relay, used for the door strike:Is anyone familiar with either this sort of keypad or the electronic doorbell and can indicate what the switching element is? The fact that it's not polarised would seem to rule out a semiconductor switch, but there's no second relay present that I can detect, and I'm a bit worried about trial-and-error methods ending with a sudden splat followed by the keypad and/or bell going dead.

author: 5U4GB
author_rank: Super Contributor
timestamp: February 18, 2025, 01:19:12 pm
content: Some more data points, I ran a rare earth magnet over the entire surface several times with no effect on the output so it's not a case of there being some SMD relay stashed away somewhere, and hooking it up to a more cromulent meter showed an initial resistance of 50MΩ with a long-tail exponential decay to around 6MΩ after 20 minutes or so after which I stopped checking. Activating the bell connection reset the reading to 50MΩ with the same slow decay following it.

author: antenna
author_rank: Frequent Contributor
timestamp: February 20, 2025, 06:04:30 am
content: Are there voltages on those bell wire pins? Either across the two, or from each to ground?Did the kit you bought come with the pictured bell? I am not sure if you got it and simply want to use a different bell, or if you didn't get a bell with it. If you have the factory bell, you can temporarily short the two bell wires on the factory bell to see if that triggers it. Like you say, it may be too fast to see the pulse on your meter, but the bell can tell you (if you have it).I am wondering what the D0 and D1 wires do. Any idea? Communication for something else, perhaps programming?Above the two bell contacts, it looks like a discrete flip flop with the two bell pins being at the outputs, but I really can't see those traces well so I could be wrong. I bet it's a differential signal so there is no false triggering if the wires to the bell get run through a noisy environment. Hard to say without a scope or clear board picture.

author: ledtester
author_rank: Super Contributor
timestamp: February 20, 2025, 06:27:16 am
content: For what's worth, I found this doorbell on Aliexpress:https://www.aliexpress.us/item/3256806861794268.htmlMaybe the vendor can give you more detailed information or put you in touch with technical assistance from Lucking Door.Update: Here is a video of someone experimenting with a "B12" doorbell from Aliexpress:Wired Doorbell B12 from Aliexpress -- Tmxhttps://youtu.be/oGapPuY78a4From one of the comments there might be more information available on Amazon.Update 2: Here is an example product on Amazon:https://www.amazon.com/Doorbell-Access-Control-System-Fire-Resistant/dp/B0CSSYR6BMFrom the video I think that grounding one of the wires will trigger the bell sound. There might be two signal wires to support two different bell sounds -- e.g. front door vs back door.In the video one of the wires was permanently grounded which is probably why connecting the signal wires together triggered the sound but I think the real protocol is to ground the signal wire.

author: 5U4GB
author_rank: Super Contributor
timestamp: February 22, 2025, 08:14:59 am
content: Quote from: antenna on February 20, 2025, 06:04:30 amI've only got the bare keypad, the intent is to feed it into dry-contact inputs on a PLC for [long boring description here] where you don't want to have someone just flip a switch to enable things. So the keypad output (NO/COM/NC) is set/enable (PIN, authorised users only), the "bell" output is reset/disable (pushbutton, anyone can press it). It's just to demonstrate that some level of care was taken to prevent random people from switching things on when they shouldn't.In terms of voltages, good question, let me check.The D0/D1 wires are Wiegand output and not used in this case.I also possibly asked the wrong question, perhaps a better one would have been "which black-box switching circuit has the following properties?" and then the info in the second post.

author: 5U4GB
author_rank: Super Contributor
timestamp: February 22, 2025, 11:00:06 am
content: Quote from: antenna on February 20, 2025, 06:04:30 amThanks for this suggestion, I think this has probably solved the problem. To provide a bit more background, the reason for trying to figure out what's what is that there's a run of 4-core cable from some unknown previous use from where the PLCs are to where the keypad will be, of which two cores are +12V and GND, and no ability to run extra cable. The requirement was to run { NO + COM } from the keypad relay output and { BELL1, BELL2 } from the bell button over the remaining two wires. The relay wasn't a big deal, tie COM to GND and use one wire for the NO, the problem was running BELL1+BELL2 over the remaining single wire.Measuring voltages, there's no voltage across the bell wires, either with the button pressed or not pressed, but from either wire the voltage to Vcc and GND is -3.5V and 0.18V with the button not pressed and 0V and 4.7V when pressed.So it looks like I can send the bell output over the remaining wire with a relay at the other end to connect it to the dry-contact input on the PLC, depending on what sort of voltage drop or other changes occur at the other end of the cable run (its not very long, and I think I've got some spare optoisolated relay modules with a trigger voltage down to 4.5V). Tying either BELL1 or BELL2 to GND as with COM on the relay doesn't seem like a good idea given that there's a 4.5V swing between that and GND when the button is pressed.

author: antenna
author_rank: Frequent Contributor
timestamp: February 22, 2025, 09:00:11 pm
content: I agree, dont tie either bell wires to ground. Just use one and leave the other disconnected. Share the ground for the relay for the opto signal.

author: 5U4GB
author_rank: Super Contributor
timestamp: February 23, 2025, 04:18:01 am
content: Well, that worked perfectly. I was planning to connect a scope to get a better idea of what was going on but thought I'd hook up the relay module just in case it worked immediately, and it did.A brief touch on the bell button is expanded out into a 0.5s or so relay activation, see the yellow circled indicator LED. So it's possible to communicate machinery-on and machinery-off signals over one wire each.So if anyone else has to hook up one of these things, check the voltage on the bell output when not-pressed and pressed. Given the presence of 6.3V caps I'm guessing the keypad uses 5V internally, which would explain the ~0 -> ~5V swing on activation, in which case you can use the NO relay output for one output and the 0 -> 5V transition on the bell output for the other output.

author: 5U4GB
author_rank: Super Contributor
timestamp: March 09, 2025, 07:56:11 am
content: For anyone else having to deal with one of these things in the future, I've done amore detailed writeupthat goes over everything you'll need to set it up.