Tecknet TK-WD004 doorbell 230V AC to USB conversion (WHL #98)

A while back, the landlords decided to toss the old doorbell system. It was fine for ringing, but the audio part was troublesome (50Hz hum of some transformer supply), plus the physical call buttons were mashed multiple times by some brutes and beyond repair. I never used the phone system, it’s just two meters away from the actual front door. The landlords also didn’t bother, because opening the actual door (plus their apartment door) can’t be done electrically, so someone still needs to dash down. It’s a bit of a granny flat situation, so live on ground level and they take up the rest of the building.

Since we’re all used to the old setup, they probably checked pricing on some 1:1 replacement – and instantly chose the cheapo option from like… Amazon: A 433MHz wireless solution. One with batteries in the ring button, and an AC-based receiver for each tenant. It’s a “Tecknet” brand, model TK-WD004. Somewhere in the 20€ ballpark (per sender-receiver pair), orders of magnitude away from a new “proper” installation.

Yes, colors are a bit weird, those images were taken in an intermediary state of the current lightbox ;)

Straight-forward design: Plug in, pair once, and it just works

While I appreciate the simplicity, I have two issues with that:
a) Power consumption and electrical safety
b) Plugboard space

Power consumption is less than 1W as mandated by law, so it costs less than 3€ per year to operate. Clearly more than “nothing”, but also not excessive. Safety however is a different story, as I didn’t fully trust the unit. It does work, it is sold here in Germany so if shit hits the fan there’s someone to sue, but it clearly could be better. Unit looks like this on the inside:

Pried out of the case:

And from the back side:

AC comes in, runs through a 100R/1W fusible resistor, gets rectified and smoothed (2x 2.2µF 400V wet electrolytics) and then some DC-DC chip converts to more manageable voltages. Actually it is more like a current source, since the BPA8504 is a Chinese non-isolated CC LED source driver, but whatever…probably penny pinching, and they found a way to make it work for them.

Since there are over 9000 ring tones on that device, it’s clear that this is digital audio and no 50Hz AC reference is needed for tone generation. Sooo…why not cut out the AC-DC part and just run it from some always-on USB port instead?

While only one of the other chips yields results on Google (NY9A001AS8 is a 2.5W audio amplifier), measuring the AC-DC output voltage is easy, and 5V is exactly what I expected. There is a 3.3V converter present, but I don’t really have any 3.3V sources readily available – USB however is common as mud. So I’ll keep one of the converter stages and sacrifice efficiency for easy of use (and placement), plus maybe the amp is running on 5V directly, so it could be needed anyway (datasheet says 1.8V to 6.8V operation, though).

Removing the AC bits is easy, and so is patching in some spare USB cable. I used the 230V pin holes to route the cable, so drilling a separate hole was not necessary. And since I looped the cable twice, that’s even working as a strain relief.
Patched unit looks like this:

Keep the center clear as pieces of the upper shell physically contact the PCB, especially for the LEDs.

Final power consumption is now 0.030W at 5.04V (0.006A), clearly pushing resolution/noise floor of the measurement device. But it’s down about an order of magnitude, even considering the device that supplies power to that USB also has some meh AC conversion efficiency. 0.03W x 8760h per year is a quarter of a kWh, so it costs about 10 cents annually to operate. Fine by me!

Space occupation in some power strip (up to three ports) also no longer is an issue, since the device just bounces around with half a meter of USB lead on its rear end. That’ll fit anywhere.

I quite like the new system. It just needs a small software patch to automatically lock when Jehovah’s Witnesses are roaming the area… :D

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