Yet another alarm (with a laser!)...

[howie69]

Yep, I'm new here. Hope I don't ask too many dumb questions, but you're all probably tired of these by now. Here goes...

There seems to be quite a few schematics and ideas for some simple proximity alarms that utilize a laser. I am working on a project right now (yes, for school), and I was thinking this would be a good one. I need to include a Wheatstone bridge and a voltage divider in the circuit.

My question is: is there a simple way to include the voltage divider, Wheatstone bridge, laser (5V, 474mA), red LED (2V, 20mA), green LED (2V, 20mA) in this alarm system? Basically, I plan on having the laser beam focused on a photocell. A green LED would glow, indicating the system is OK. Once the beam is interrupted, the green LED would turn off and the red LED would glow until the system is reset. Maybe include a buzzer/audio alarm?

Anyway, I had a simple bridge set up that didn't work too well. The resistance from the photocell kept changing as the laser was shining on it. I may have a better laser that I can use, same specs. I have basic carbon resistors at +/- 5% tolerance, ranging from 1 ohm to 1M ohm. I also have potentiometers, op amps, 25V/ 0.5A max power supply, and a few other components to work with.

Any help would be much appreciated.

 

[audioguru]

The photocell is supposed to turn on a transistor or opamp with its very small current, then the transistor or opamp turns on the LED through a current limiting resistor. It doesn't need a wheatstone bridge unless the LED must shine for miles to reach the photocell.

It sounds like you are overheating the photocell by having the fairly high current of the LED in it.

Why don't you upload the schematic?