Hello there, i'm quite new to electronics but i already did some cool projects, now i am bussy making a laser triggerd alarm, and here is the schematic for the alarm, the laser will light up the photocell and when you go through the beam, the alarm should work. But first i need some help with this schematic.

First question: Is this schematic correct?
Second question: What do i have to do with the components that have a
ground symbol?

 

Hello,

There are several tings not ok in your schematic.
The potmeter is shorted.
The transistor has no current limiting at the base.

Here is a link to a light activated relays circuit.
The relays can drive the siren.

http://www.sentex.ca/~mec1995/circ/actrelay.htm

Greetings,
Bertus

 

Ok thanks for the reply but the guy that made the schematic, his name is kipkay, shows that it works! but i think there is still something wrong in the schematic. Could you plz help with it. I already tried to make it once and if I turned the potmeter the buzzer would buzz louder, and even if it was light or dark the buzzer buzzez.

 

Hello,

I have altered your schematic a bit.

The transistor is protected and the output of the potmeter is free.

Greetings,
Bertus

 

Can you post the schematic you want to follow?

The red is my guess at two errors. Presumably you mean that the photocell is a light dependent resistor (LDR). Its resistance will determine the bias on the base of the transistor. As you have it shown, the transistor will be on continuously and possibly over driven. Thus, as bertus suggested, I have also added a base resistor with a question mark. I suspect it won't be needed, based on what you find to be the working value for the variable resistor.

If the photocell actually produces voltage, then the circuit will need to be changed in a different way.

John

Edit: My change and bertus' are very similar. We were posting at the same time. I recommend following bertus' design.

 

Bertus, i have one last question, will a 1k resistor be enough, and will the buzzer go of if the beam is broken?

 

Hello,

You can adjust the sensitivity with the potmeter.

Greetings,
Bertus

 

are you sure this is gonna work?

 

Hello,

Pretty, but take also a look at the schematics in the link, those are tested.
You did not state what kind of siren you use (the transistor can handle a limited current).

Greetings,
Bertus

 

well actually its just a 12volt buzzer

 

Hello,

If it is a small buzzer it can be placed in stead of the relays in the schematic on the link page.

Greetings,
Bertus