What photocell I need in my project

Hi Donatas,

These are "Light-dependent-resistors" which are usually CdS (Cadiium Sulphide) "photocells"..  They are very cheap.. One that would work is

http://yourduino.com/product-details/Photoresistor-5516-datasheet.pdf   which is less than 10 cents US here:

http://arduino-direct.com/sunshop/index.php?l=product_detail&p=135

Radio shack usually has an equivalent...

DISCLAIMER: Mentioned my own shop!

Regards, Terry King

...In The Woods In Vermont

terry@yourduino.com

you can use any simple photo  cell reciever....

like some comes in pair  in IR-PCR pair....

you can use that one also....

LDR(light dependent resistance) can be a good choice.....

Several devices can be used as light detector. In the video a LDR is used. You could also use a photo-diode or a photo transistor.

The proposed design in the video has several weaknesses. It is a nice toy, but not a reliable burglar alarm.

This design can be improved several ways.

The schematic of the detector is too simple.

This way the alarm will only sound when the laser beam is interrupted. If the burglar jumps fast through the laser beam, you will probably hear nothing.

A latch is necessary to keep the alarm going after detection. There must also be a way to disable and enable the alarm if you want to enter the room yourself.

If a burglar is detected with this system, he can pull the detector from the wall and put it under his clothes or destroy is by stepping on it.

This way the alarm will sound, but only for a short time.

The detector should be installed in a hard to destroy box, firmly attached to the building.

Battery powering the detector is cheap. But you must replace the battery before it is empty.

After some time you might forget to do this. The light detector is most of the time illuminated and therefore conducting.

The switching transistor must be turned off when the alarm is not sounding. The only way to do this is to short the base drive of the switching transistor.

So a current of at least the required base drive for the switching transistor is flowing continuously.

Lower power consumption can be accomplished by using N-channel and P-channel enhancement transistors.

Mains powering the laser will also sound the alarm during short power outages. The best way is to power the laser and the detector from a rechargeable battery, continuously connected to the charger.

Burglars sometimes disconnect the power line before entering a building.

To avoid disturbance from other light sources, like the sun, the detector must be placed at the end of a black painted tube, not at the surface of the box.

You can increase the sensitivity of systems like this by modulating the light and detect the modulation at the receiver side.

For instance you can operate the laser from a 1 kHz square wave. The detector then must detect a 1 kHz signal and will not respond to other sources of light.

A sensitivity pot might not be necessary this way. A LM567 tone detector or a tuned audio amplifier can be used to detect the 1 kHz signal.

The battery powered siren in the video will make a lot of noise in the room where it is installed. But if it is in the living room and you are in the bedroom, it will not wake you up.

A louder and more power consuming siren is recommended. It is not a bad idea also to install a loud siren near valuable objects.

Some burglars know it will take 10 minutes before the police will arrive. They continue their job, even when an alarm is sounding.

Burglars don’t like loud noise, because they cannot hear people approaching.

So if there is a lot of noise, they might stop trying to open your safe or cutting your paintings from their frames and leave.

You could start with the design from the video and improve it step by step until you have a reliable burglar alarm. Then I will recommend to use a not too loud siren, until the system is reliable and not frequently giving false alarms.

Johan.