Help with Laser Alarm Circuit

[PlagueHuntR]

Hey all I'm new to both the forums and electronics. Recently I chanced upon a Laser Alarm System and decided to give it a go.
Here's the link: http://www.ehow.com/how_4605253_laser-alarm.html
Ok so I followed the instructions and have just made the circuit board just as is presented in the diagram.
However I have encountered 2 problems.
1: Firstly whether I have the photocell in the light or dark the alarm continues going off.
2: Secondly the 9v battery gets extremely hot when it is hooked up and over heats.
So I would just like to ask if any1 has any ideas, suggestions as to how I can fix these problems and check that the circuit board should work and that the schematics arn't dodgy.
Thanks Plague

 

[TBennettcc]

Of course the alarm will continue to go off. With the schematic you linked to, the potentiometer and photocell have no impact on the circuit, because the wiper and one end of the pot are shorted together (right below the depiction of the 9V battery).

They probably intended for the schematic to look like the one I have attached; unfortunately, I am not experienced enough to be able to tell you whether it should work correctly or not. I also lack the requisite parts in front of me to able to play around with the circuit to try different things.

I believe the idea for the circuit is as follows: The photocell has a high resistance when it is in darkness, and a low resistance when it has light shining on it. When the photocell is being lit, the current goes through the two resistors, and back to the battery. However, when the photocell is dark, the resistance is high, and the current goes through the transistor, pulling current through the siren.

It's possible you've fried your transistor, as the schematic you linked to doesn't show any resistance between the base and the emitter of your transistor to limit current. It would be a good idea to measure the resistance of your photocell when the laser is shining on it. You need to be careful, between the pot and the photocell, that you don't drop the resistance to zero between the battery and the base of the transistor. Might put an additional small resistor between the top of the pot and the base of the transistor, just in case.

What are you using for the siren? Is it supposed to be powered by 9 volts?

Those could be reasons why your battery is getting hot.

 

[poor mystic]

For those who haven't followed the link, the recommended circuit is reproduced below. I thought myself a fool, I know now that not even that have I achieved to distinction.

PS I'm sorry you got such a bad steer into your first project, and I'll see what I can find in the way of a reliable circuit for you to build.
In the meantime, do you have any equipment please? You need a multimeter, mostly.

 

[PlagueHuntR]

Hmmm shame, thx for the quick replies and sorry it took me a while to respond .
Yea I have a multimeter and a soldering iron but thats about all at the moment.
Would be open to any ideas for a begginer in electronics. Found a couple of good ones to keep me occupied in the mean time
Also would still be interested if anyone finds a working version of this circuit.

Thanks for the help!
Plague

 

[jackorocko]

Plague, I was just like you when I first started out. I had even looked at this schematic, but as poor mystic has stated. It is a poor example of a schematic if I have ever seen one.

This is where I started. Note: the link in the first post of this thread used to point to a video with the same schematic as you posted but has since been switched(as is typical with poor designs, bury them quickly). So don't view the link!!
https://www.electronicspoint.com/help-understanding-math-behind-schematic-t219655.html


This is where I ended up with my laser trip alarm circuit. Nothing difficult here, the main trip circuit is the comparator sub-circuit which when activated will sound a alarm for a predefined time and then reset itself. That is what the 4060 binary counter and flip-flop does.
https://www.electronicspoint.com/experienced-eyes-needed-finalizing-my-project-t221907.html

Have some fun, I know I learned so much from that simple stupid schematic we both stumbled upon.

 

[BobK]

For those who haven't followed the link, the recommended circuit is reproduced below. I thought myself a fool, I know now that not even that have I achieved to distinction.

PS I'm sorry you got such a bad steer into your first project, and I'll see what I can find in the way of a reliable circuit for you to build.
In the meantime, do you have any equipment please? You need a multimeter, mostly.

Actually, there is nothing wrong with that circuit (at least that I can see) once you realize that the large circle with the +9V is intended to mean connect all three wires coming out of it to the positive side of the 9V battery, with the negative side being connected to ground.

So, to the OP: Is that how you interpreted it?

Bob

 

[jackorocko]

Actually, there is nothing wrong with that circuit (at least that I can see) once you realize that the large circle with the +9V is intended to mean connect all three wires coming out of it to the positive side of the 9V battery, with the negative side being connected to ground.

So, to the OP: Is that how you interpreted it?

Bob

Well I suppose if you discount the lack of convention used, there is nothing wrong with it. But that is why we have convention and why people who know, use convention to avoid confusion. Something a beginner should not be exposed too.

 

[BobK]

Well I suppose if you discount the lack of convention used, there is nothing wrong with it. But that is why we have convention and why people who know, use convention to avoid confusion. Something a beginner should not be exposed too.

Oh, I agree that it is crappy. I was just saying that if you interpret it correctly the circuit might actually work. I suspect the OP wired it wrong due to the bad schematic.

Bob

 

[PlagueHuntR]

Thx for the links

Thx for the links Jackorocko.
Haha certainly seems like I threw myself in the deep-end with this dodgy project. Never the less at least I have gotton something from it! With your first link in particular I can now see where I need to fill in the gaps in my knowledge. Once I understand this a tad clearer I look forward to completing this circuit with the nessacary adjustments.

Btw Bobk the way I read the schematic I connected the positive 9v to the 3 wires shown and grounded the negative. I am assuming that is how the schematic is meant to be read?

Thx for the help guys!

Plague

 

[Sparks_21]

I made a circuit that does exactly the same thing you are looking for i think...

I used an op amp set up as a comparator, you set up a voltage divider in the inputs of the op amp, and when the resistance of a LDR (light dependent resister) changes when the laser beam is broken the output is triggered.

here is the circuit -




when the Laser beam is pointed at the LDR the relay remains off, when the beam is broken the relay turns on.