LED Doorbell Mod

Okay, I was wondering if I could get some better color into the $4.99 doorbell my house came with, so decided to give this a shot for fun. LEDs are more efficient than regular bulbs and offer better color variety.

The original pushbotton disassembled.

Testing resistance of the original incandescent bulb. When the bulb heats up this would change, but it's a good place to start.

Since Volts = Current (amps) * Resistance (ohms) we could say that if fed 16VDC, this bulb would flow about 0.200 amps. Since Power (watts) = Current (amps) * Volts, this bulb is probably about a 3 watt.

I always wondered how if the doorbell pushbutton is lit how the circuit isn't completed and the bell isn't always actuated, but it looks like the small current running through the bulb isn't enough to trigger the bell. Once you press the button though, the resistance goes to zero (copper springs make contact, see below) and you get the noise inside where the bell is. Cool, we can work with that.

I picked up some cool blue small LED from Radio Shack. Each drops 3-3.5V and likes 20mA current.

The original setup in the background, and the new one constructed from a cheapo Radio Shack project board, two blue LEDs and a 4700ohm 1/4Watt resistor (later replaced w/ 1kohm for brighter light). Since LEDs only light up when fed a positive voltage, I hooked up two, one for each direction of the AC coming from the source in the garage. In this setup, each light pulses 60 times per second, but only one at a time.

Oooh, that soldering job is nasty!

The small transformer in the garage steps the 120VAC down to about 20VAC for the doorbell circuit, and this is what I hooked into to test the light before installing outside.

Test probes connected

Whoa, it works!

I spent some time nerding up a spreadsheet for this one. I was concerned about how LEDs like DC power and I was replacing a single low resistance bulb with two of these on an AC circuit.

With the 20VAC through 4700 ohm resistor the light wasn't very bright. Using the spreadsheet, I dialed in to a 1000ohm resistor and estimated a max current of about 20mA (light blue line), which is about right.

(click image for Excel spreadsheet)

Installed outside before "upgrading" the resistor to 1000ohm for more light.

The 1000ohm resistor installed in the circuit now peaks at about 375mW power consumption (see below), at an average of less than 0.25 watts. This is way less than the original bulb, so in theory the house won't burn down as a result of this mod.

Anyway, that's it! For less than $10 and a bit of soldering you can make a doorbell that will make your wife say "Wow, that's great, now do something productive."


2009 03 14