Comment on Building DIY a smart doorbell but would appreciate some help with the power and wiring
ch00f@lemmy.world 1 month ago
So a doorbell transformer steps 120VAC down to a lower AC voltage. That’s what you’re measuring (surprised you can’t tell if it’s AC. Your multimeter should have an AC/DC setting).
The circuit puts the transformer, the button, and the bell (classically an electronagnet that accelerates the hammer) in series. When you close the switch (push the button) the hammer strikes the bell.
An electromagnet is just a coil of wire, and it is possible to pass a small amount of current through it without activating the hammer. If you’ve ever seen a doorbell with a light up button, the light bulb is placed in parallel with the button. That way it’s always on drawing a small amount of current. It also explains why the light switches off when you press the button. You’re shorting across the bulb, so it’s voltage is zero.
If you want to use this power for an electronics project, you’ll need to find a way to draw just a small amount of power from the transformer. It has to be very small or you’ll activate the hammer.
Drop a full bridge rectifier in parallel with the button, then a bunch of caps to give you a steady-ish voltage and then some kind of voltage regulator to make it useable.
Just remember that you can’t draw too much power, and you’ll lose that power whenever the button is pressed.
Jimbabwe@lemmy.world 1 month ago
I couldn’t remember if it was AC or DC, but you’re almost certainly right that it’s stepped-down AC. I just went and checked and there’s no light on the button, so unfortunately no bonus parallel always-on power source to tap into. This is my dumb guess about how this all could work, but there are some question marks. Does this seem kinda correct though? Image
ch00f@lemmy.world 1 month ago
Your drawing is a little confusing. Here’s what you need:
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There’s nothing special about a setup with a light in the button. It’s literally just an incandescent bulb across the button. If you connect your circuit across the button terminals, it’ll work fine.
The 4 diodes in a loop is a “full bridge rectifier” that gives you DC from AC. The 5V regulator could be something like an LM7805.
The “large capacitor” is to keep power applied while the button is pressed, though you might be better off in that case with some small onboard battery. You just have to make sure that your battery charging circuit doesn’t draw too much power.
Limonene@lemmy.world 1 month ago
Remember that voltage is measured across a pair of wires, so you can’t power the chime with only a single wire. That’s part of what makes this difficult – these doorbell systems only have the bare minimum of wiring in them. Powering the camera and the chime in series with each other is quite difficult. I think a lot of these things just accept the short circuit, and use a battery to power the camera while the button is being held.
Here is what I was proposing, and I think what ch00f was also proposing. Replace “5VDC power supply” in this diagram with “a full bridge rectifier and a bunch of caps” in their description, and also note that your camera probably requires a well regulated 5VDC supply.
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Jimbabwe@lemmy.world 1 month ago
Awesome, thank you both. This is more complicated than I thought, so on the plus side I don’t feel as dumb as before.
Maybe i’ll just ditch the dinky chime I have and setup some alert automations or something. In that case, I could just connect the two chime wires to create a closed loop, then replace the transformer with a 120vac outlet plug, plug in a generic 120va -> 5vdc power supply, chop the two output wires from the power supply and connect them to the existing doorbell wires. And violá, 5vdc delivered right to the front door? Is this plan insane?
Limonene@lemmy.world 1 month ago
Actually, I think that’s a really good idea. Just be aware of possible inductance in the long wires from the transformer area going to the camera. Putting a large electrolytic capacitor next to the camera should fix that.
As for your bell, there are dead simple systems that use a battery-powered stick-on button with a transmitter, and a chime/receiver that plugs into a wall. If you want to be fancy, you could find a way to power the transmitter with that 5VDC, in parallel with the camera… or you could just keep it simple and use the battery that comes with the button. They last near forever.