Instructables

Laser Beam Alarm System with Rechargeable Battery for Laser

Step 4: Schematics Part 2, the LDR and Alarm Circuit.

This schematic is much larger and contains a lot more components than the first one. What Im going to do is break it down into two halves and explain how each works. If you have experience with putting together schematics feel free to skip ahead to the image of the final schematic where you can get right to assembling.

For those that want more help continue to the next section where I will explain the first part of the schematic, the LDR part. For those that just want to start assembling, a schematic of the final product is in the image bellow.
 
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luudvig4 years ago

Yes, by putting a electrolytic capacitor in paralell with the buzzer

Kryptonite5 years ago
Would it be possible to make it so that the buzzer bit only goes for around a second?
Going to hook it up to a nerf dart shooter for an Indiana Jones style walk way.
revhead (author)  Kryptonite5 years ago
That sounds cool!
what I suggest is you do is scrap everything after the relay from my circuit and use the relay as your switch, just replace the nerf dart gun switch with the relay. If your nerf gun doesn’t have a power supply then you would need to connect a power supply in series with the relay and the nerf gun. Hope this helps. If you need any help or get confused just message me and I’ll try to help. 
cheers!
thanks mate I understand now. If it weren't for you I'd fail, my electronics teacher is useless.
I'm doing this for a school project in electronics and i really don't understand the reed relay part and the variable resister part (it has 3 leads and u only seem to use two) could u please show me somehow how these two things work?
revhead (author)  III ScYThE III5 years ago
ok first of all, the variable resistor. With most variable resistors there are three leads. For example a 10k ohm variable resistor has two leads on the end and one in the middle. if u don’t want it to be variable you connect the two outside leads to your circuit, but if you want it to be variable you need to hook up the middle lead and one of the end leads, so basically you need to hook up the middle lead to one side of the circuit and any one of the outside leads to the other side (only 1 of the outside leads, not both, u will end up with one lead on the outside not being used) The reed relay is built into a nice small package but is actual two parts, a little glass tube with bits of wire inside (like a tiny switch) and wrapped around the glass tube is alot of wire. The relay has four leads, one on one side and three on the other. The one on the end is one side of the glass tube and the other side of the glass tube is in the middle of the three leads. So, on the alarm side of the circuit you connect the one lead on its own and the middle lead of the three leads. On the LDR side you need to connect the two outside leads left over. The reed relay you buy will probably be different so u need to check the code on the side of it. Google this code and you will surly find a web site that has a picture of what each lead does. This is a lot of info and will probably be confusing. Just let me know if you need any help. Happy to help you out, cheers
jezznar5 years ago
hmmm I'm wondering if you can change the 3rd module(alarm/buzzer part) into something like a motor circuit. I wanted to try and interface it into a door lock.