#Hot lava laser alarm tag series

When attaching the photoresistor, I mounted it with the leads sticking out about one inch from the board.

#Hot lava laser alarm tag series

The three battery holders are soldered together in series and the end leads are soldered to the circuit board. The batteries are mounted in individual AA battery holders. This makes it easy to change out the IC but it is not necessary. I also used an IC socket to attach the IC. These are really convenient for circuits that are built around small ICs like the 555 timer. The board that I used is a general purpose IC board. If everything is working properly, solder it all together on a printed circuit board. But when you move the laser away, the alarm should go off again. As long as the laser is centered on the photoresistor, the alarm shouldn't sound. Now shine the laser pointer on the photoresistor and flip the switch one more time to reactivate it. Flipping the switch the other way should turn off the alarm. Without the laser shining on the photoresistor, the alarm should sound. Any buzzer can work as long as it is rated to operate at the appropriate voltage.įirst assemble the circuit on a breadboard to test it. The alarm that I am using is a piezo buzzer. In many cases it can be left off without causing any problems. It helps to prevent false triggering from static electricity. The resistor R1 acts as a pull-up resistor for pin 2. This gives you the option of powering the laser pointer with the same battery pack as the alarm circuit. I chose to use 4.5V (three AA batteries) because this is the same voltage that is used by the laser pointer. The supply voltage can be anything from 4.5V to 18V. The alarm will remain off until the next time that the light beam is interrupted. To reactivate it, flip the switch back to the original position. To turn off the alarm and reset the system, a (single pole double throw) switch disconnects the speaker and sends the LOW signal from the output pin 3 to the trigger pin 2. This causes the output pin 3 to go LOW and activates the alarm. As a result, the voltage at pin 6 also increases and goes above the reference threshold. When the light beam is interrupted, the resistance of the photoresistor increases dramatically. So I used a 100 ohm fixed resistor for R2. In my case, its resistance was about 100 ohms.

So connect the photoresistor to the multimeter and shine the laser pointer directly at it. Because the output characteristics of photoresistors varies considerably from one to the next, you need to measure it with a multimeter. The value of R2 should be approximately the same as the resistance of the photoresistor when you are shining the laser pointer directly at the light sensitive face. These two resistors form a voltage divider that is used to activate the IC. This is wired in series with standard fixed resistor (R2). The light sensor that detects the laser is a CdS photoresistor (R3).

This alarm circuit is yet another way to use a 555 timer IC.