# Turning a doorbell into a light

#### OfficialDom

Joined Feb 7, 2015
3
I have a remote operated doorbell which delivers 0.06V to the speaker when the switch is pressed. Obviously the voltage is only created very briefly. My plan is to replace the speaker with a relay switch (?) which then connects the three 1.2V AA batteries across an LED (forward current 24mA, Power dissipation 80mW, forward voltage 3.3V, residual voltage 0.3V, 20mA current requires 15 ohm resistor). Obviously as the voltage is only delivered briefly the switch would need to stay on when the voltage stops and then switch off again when the voltage is reapplied. Can relay switches work like that?

Now my problem is that I haven't a clue how to select the right switch for the job, or how it should be connected to the circuit. I would be most grateful if you would point me to somewhere that can explain how to design a circuit indicated above in words of one syllable. I am a complete novice and my total knowledge about circuits was exhausted calculating the size of resistor above! I tried looking it up in a tutorial, but I was swamped with discussion about NPN transistors, PNP transistors, MOSFET something or others.

I was very proud of my string of LEDs I wired up for Christmas, but it's not exactly rocket science. I would love to learn more and hope you can help me out here.

Many thanks

Dom

#### absf

Joined Dec 29, 2010
1,935
Dont think the relay can hold until the next bell is rung. You need a flip flop of some kind and a level shifter to convert the 0.6V to logic level high. So it can be identified by the logic circuit.

Allen

#### spinnaker

Joined Oct 29, 2009
7,815
You might get by with a super cap. Doorbell button gets pressed. Charges supercap. Supercap discharges through LEDs. If you have a large electrolytic hanging around you can experiment with that.

Problem with supercaps is they are usually low voltage so you will a voltage divider or regulator.

#### crutschow

Joined Mar 14, 2008
23,158
.06V (AC?) is a low voltage.
How did you measure that?

#### spinnaker

Joined Oct 29, 2009
7,815
.06V (AC?) is a low voltage.
How did you measure that?

Yeah I did not notice that. Door bells are typically 24V correct? That is right too they are 24 vac.

Reading the OP again, the super cap idea might not work. Looks like it is some kind of electronic doorbell. I'd be curious if there was not another voltage applied to the "bell box" that could be used to power the lights.

#### Dodgydave

Joined Jun 22, 2012
8,295
if its a rf door bell, the receiver will have an output that goes high or low to switch the tune on, can you tap off that to work a relay?

#### OfficialDom

Joined Feb 7, 2015
3
Thanks for your input everyone. I will need to do a little research to understand exactly what you mean but I really appreciate the sign posts. Just to clarify, the bell is an electronic device which plays a tone, rather than ringing a bell. I'm of a generation that thinks in terms of door bells whether or not an actual bell is involved! The "bell" has three AA batteries, which, if rechargeable, gives 3.6V. The circuit board converts that voltage to an output which is fed into the speaker. I put a voltmeter across the speaker terminals and measured the voltage when the door bell button was pressed, and measured 0.06V. I don't know what current was generated.

Dodgydave - if I understand your comment correctly, that's exactly what I had in mind.

Allen - I was afraid that a relay would work as you suggest. I'm not sure what a flip flop might be technically, although its name certainly suggests exactly what I need. I have the 3.6V on hand from the three AA batteries, but I need the relay to operate the "flip flop" which would close the circuit involving the three batteries a resistor and the LED. A second burst of voltage across the relay would operate the "flip flop" and open the circuit switching off the LED. The question then is what technical specs would I need for the relay and the "flip flop" and would they need other components to make them work?

Dom

#### crutschow

Joined Mar 14, 2008
23,158
Was the voltmeter set to measure AC or DC?

#### OfficialDom

Joined Feb 7, 2015
3
DC. Do you think that the circuitry could be converting the DC from the batteries into AC? It never occurred to me that this could happen?

#### spinnaker

Joined Oct 29, 2009
7,815
You measured at the speaker. Typically that is going to be an AC voltage / audio.

Not sure why you would want to convert DC to AC.

#### Dodgydave

Joined Jun 22, 2012
8,295
Have you got the number of the IC receiver or a photo ?