Optical Doorbell Alarm

Introduction: Optical Doorbell Alarm

About: I like to play with electronics and other fun stuff, modify things to make them more useful or just more funny, putting things together that seem not to fit to make funny stuff. Sharing ideas make the world ...

Sometimes i meet a friend to practice DJing at home. Often it becomes a little too loud for my neighbors - and too loud to hear the doorbell. I needed a way to mention my doorbell also in the loud environment of a hobby-DJs home.

So i started thinking about an optical doorbell alarm.

In the apartment house i live we have an electric bell system. If someone rings the bell on the street, you can talk to him via the system. I think these systems are really common. So, i did not want to break the the device to make it fit to my needs.

Fortunatly the system has a nice feature: whenever someone rings the bell a little light on my device starts blinking.

Using my new knowledge about transistors from the book Make: Electronics, some thinking and research on the internet i found an easy and quick solution.

It's quiet easy to send the blinking signal from my corridor to another place. I just used a simple circuit consisting of an LED, a photoresistor, a transistor and two resistors. So i'll show you how to make it.

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Step 1: Parts, Tools and Skills


- 1 little peace of perfboard
- 1 superbright LED (LED1)
- 1 transistor BC547 (T1)
- 1 resistor (value depents on your LED) (R1)
- 1 10k resistor (R2)
- 1 9V block battery and battery clip
- two long pieces of wire (depends on the way from the doorbell to the place where it should blink)
- black electrical tape

Soldering iron and solder
Wire stripper
Hot glue gun
Diagonal Cutters

"Handy Hands" or "Third Hand" (if you like you could build your own )
Desoldering tool


- you should have basic soldering skills (Instructable How to solder )
- using your multimeter (Lady Adas Multimeter How to )
- some basic informations about LEDs (LEDs for beginners Instructable )
- reading circuit diagrams (see Instructable of aplauche)

Step 2: Building the Circuit

First I tried out the circuit on a breadboard (see image). You an skip this step, while knowing it works well, but you might test some LEDs and resistors for getting the best results.

So you can start soldering the components to the prefboard. Follow the circuit diagram should be easy. Dont get confused, I used european symbols for resistors in the schematic. Dont connect the photoresistor, because it goes to the bell device in the next step.

Test your connections using your multimeter.

If you've done everything well you can go ahead to the next step.

Step 3: Preparing the Photoresistor

First measure how long your wire has to be. It should be long enough to go from the place where the bell device is to the place where it should be shown off.

Solder one wire to one end of the photoresistor, and do the same with the other one. I used heat shrinking tube to isolate it, but you could also use electrical tape.

Attach it to your door bell device to the place where the light blinks after ringing. Use black electrical tape to protect the photoresistor from external light.

Run the cables / wires to the place where your LED should light up.

Step 4:

Solder the photoresistor-wires to your circuit and you're done.

Tell somebody to ring your doorbell and test it.

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