Author Topic: The most over-engineered doorbell..... in the world!  (Read 1646 times)

Offline AdeV

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The most over-engineered doorbell..... in the world!
« on: February 03, 2019, 11:37:47 AM »
So, apologies for not posting a proper project log on this one - frankly, I didn't think I'd need it! It's only a doorbell for goodness sake! Anyway, here we are, 2 years later, finally it's ready to go on the front of my house...  :palm:

The back story

I bought a £9 (about US$12) battery powered doorbell from B&Q, to replace my broken previous one. It worked fine, for a few months, then the switch started corroding, so you needed to really mash it to make it work. Then the batteries went flat, and who keeps 4x "D" cells around the house these days? So... sod this for a game of soldiers, I needed to do something about it. The rules were:
  • Mains powered, so it didn't ever run out.
  • Illuminated button, so you knew it was working...
  • ...with visual feedback so the person pressing the button knew it had worked
  • No mechanical switches
  • No wireless button, because I hate batteries.

Rather than go looking for one, I figured I'd make it. After all.... how hard could it be?

Adapting what you've got

First - and most obvious - decision was to use the existing bell box and bell unit. It works fine, has a decent "ding-dong", I can hear it throughout the house, so why try harder? The touch switch, on the other hand... a bit more tricky. In the end, I settled on an ATTiny-85, a diminutive 8-pin Arduino-like computer chip, which will run with minimal external components (a tiny power-smoothing capacitor, that's it!

The final off-the-shelf BOM includes: Aforementioned ATTiny-85, 9v PSU, 5v regulator, a small logic level power MOSFET to drive the solenoid, a handful of various resistors, couple of diodes, some stripboard. The firmware I got from t'interwebs, and modified to suit my needs. The capacitive switch circuit is dead easy with an Arduino, just one pin, a resistor, and something to touch. As room in the button box was limited, I split the circuit into the 9v solenoid section, and the 5v computer section.

The computer is currently inside the bell push box, which is not ideal, but I wasn't sure what effect having a long wire between the bell "push" and the sensor would have on it's reliability. I may experiment in the future. The advantage of splitting is that the 5v section goes nowhere near the 9v section, so no chance of frying the chip accidentally.

Tools to make tools...

Having got the basics up and running, including a pair of diodes to light appropriately, and the circuit made up onto stripboard, I ran into an issue or three. First was getting the "light pipe" working (see this thread for that little sub-adventure). Got it to work well enough, in the end.

The next problem was - the stripboard was just too damn wide for my desired button size. Either the button push would have to be ridiculously tall, or it'd sit there all squat like on the side of my house. Not Good... So, a £150 CNC drilling machine, maybe £50 of chemicals, and a tenners worth of PCB board later... I had a much slimmer device  :loco:

Putting it all together

So last (but not least), it all needs to go in a box. This was relatively straightforward; a nice aluminium block, drill/bore a 15mm hole, then counterbore 20.4mm diameter, 3mm deep, for the plastic insert. Machine up a button out of aluminium, with a threaded shank to accept a small nut; this is the actual switch. Hollow out the inside of the box; then make a backing plate which takes a small machine screw to hold the front to the back.

Fiddle around getting everything to go inside... then tweak the software to work again (now it's inside a metal box, the capacitance is much higher, so it was triggering constantly)... finally solder some crappy old bellwire into it, and put it all together! The final step will be to screw it into place where the original went, and wait for a visitor!!

Cockups and other miscellany

It didn't all go well... when hollowing out the box, I must have used THE bluntest end mill in existence! Somehow, I managed to set the depth too deep; you can see the "chain drilling" I had to resort to to try to make the damn end mill cut properly. I gave up in the end and used an 8mm roughing tool to finish off.

Next error - I needed a pocket for the chip to sit in; which I made too big & cut into the plastic insert (doh!) It still works OK though, so I carried on.

The back plate is a marvel of engineering prowess (erm, ego old chap!  :poke:) Actually, no, it's not - but it was a proper fiddly little thing to make. It actually works quite well, but due to the position of the screw holes on the wall, and the position in the backplate the wires come out, I can only attach it to my door with one screw, instead of two. Hey ho. Better luck next time... should have measured THEN cut, not the other way around  :scratch:

Lastly - as I went to remove the temporary wires from the bell push unit, to install the real wires, I managed to lift a trace! Damn!!! Fortunately, I've managed to bridge it with solder, but I'll have to be careful not to break that joint. There's just not enough room in there for strain relief, unfortunately. A better solution would be some kind of plug; but, again, there's sod all room... A future version (!) might have another hole in the PCB, so the wire can be soldered in from the top, then passed through the hole; this would, at least, give a workable strain relief. Still not ideal, but there ya go.

So... finally... if you read all that bumf... the bit you've been waiting for, some pics! I'm afraid I didn't take any useful pictures as I went along, and I accidentally deleted the only ones I took midway through the project, so you'll just have to put up with the finished article. Also, I put a little YT video up so you can see it working in the semi-dark of my office. Be interesting to see how many people are sufficiently interested to actually watch a video of someone pressing a doorbell!  :lol:

Th...th..thth...that's all, folks! Thanks for the assistance along the way from various contributors, all of whom helped me overcome some of the self-inflicted hurdles I made for myself.
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline AdeV

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Re: The most over-engineered doorbell..... in the world!
« Reply #1 on: February 03, 2019, 11:38:25 AM »
Link to the truly awesomely produced video  :lol:

Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
Skype: adev73

Offline howsitwork?

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Re: The most over-engineered doorbell..... in the world!
« Reply #2 on: February 03, 2019, 04:58:27 PM »
well at least i’ve found out what you’ve been doing for the past , say year ??? LMAO but impressed
Ian

Offline tom osselton

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Re: The most over-engineered doorbell..... in the world!
« Reply #3 on: February 03, 2019, 07:23:14 PM »
Turned out alright in the end. For my garage I have a store bought one the sensor goes on the back door and the chime is in the garage so while in there I can leave the door unlocked while out there, this type of system could be used if you are home and some “ Waste of skin “ is interested in your garage tools.
Never give up never surrender!

Offline Brent Crude

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Re: The most over-engineered doorbell..... in the world!
« Reply #4 on: March 26, 2019, 07:08:18 PM »
Nice bell! I like the delay to foil the’aggressive pusher’ 👍