Laser alarm project

[fixit7]

I am working on this. I have the laser.

"https://blog.prayogindia.in/make-laser-light-security-alarm/"

I emailed the author but have no heard back.

"https://www.dropbox.com/s/cy7sy1itwzqcg7t/circuit_of_laser_security_alarm-engineeringprayog.com_-768x432.png?dl=0"

Is the 2n7000 the BC547 transistor?

And there is a 6.8K resistor that is not in his parts list?

Thanks for your help.

[ajb]

A 2N7000 is an N-channel MOSFET, a BC547 is an NPN bipolar transistor; they are not the same thing at all.  The information on that page is very confused; the schematic shows an NPN transistor but with MOSFET labeling (G,D,S instead of B,C,E).  Either one could be used in this circuit, but the value of the 6k8 resistor will need to be adapted depending on the resistance of the LDR.

[fixit7]

I agree that the page is confusing.

Would you be able to help me with the detector circuit?

[fixit7]

I found this.

https://makezine.com/projects/laser-tripwire-alarm/

Does this look like it may work?

It has fewer parts than the other project.

[fixit7]

Could use some help here?

Thanks.

[OM222O]

the easiest solution would be to use the LDR in a potential divider circuit. you can use 2 LDRs to negate the effects of the ambient light as well. then use a voltage comparator which allows you to have sensitivity adjustment as well. connect the output of the voltage comparator to a mosfet, which will drive the buzzer (make sure your buzzer works with DC, otherwise you need an oscillator circuit to drive it which is a bit more complicated)+ the LED 

[ajb]

What specifically do you need help with?

[fixit7]

The circuit diagram here is not real clear how to hook up the single pole double throw switch.

It shows 3 wires, one goes to the speaker, one to the 555 chip(but not which position), and not clear where the 3rd goes.

Since speaker has a positive and negative, does it matter which it goes to?

https://makezine.com/projects/laser-tripwire-alarm/

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. The system is now deactivated. To reactivate it, flip the switch back to the original position. The alarm will remain off until the next time that the light beam is interrupted.

[OM222O]

just use a buzzer that works with DC (makes a sound when you connect it to a battery). 555 is used to make oscillations for the buzzer like I mentioned before.
this circuit only needs 3 parts: 1 resistor, one LDR and one transistor (or mosfet, both will do just fine)

[fixit7]

just use a buzzer that works with DC (makes a sound when you connect it to a battery). 555 is used to make oscillations for the buzzer like I mentioned before.
this circuit only needs 3 parts: 1 resistor, one LDR and one transistor (or mosfet, both will do just fine)

That's good, but I will need some more details.

I made another circuit which was lacking in impt details and ended up with a smoking BC 547.

[fixit7]

I put this together on my breadboard. The author said that R1 was probably not needed, so I did not use it.

https://makezine.com/projects/laser-tripwire-alarm/

I do not have a potentiometer, but tried a 110 and 530 ohn resistor for R2.

With no power to the circuit, laser shining on the LDR, the reading was around 195 ohms.

I shined laser at the LDR and used the switch in both positions.

Piezo buzzer did not sound.

What can I try next?

[ebastler]

Could you post a photo of your breadboard setup? One picture showing the breadboard large enough to see all the wiring, and one showing the complete setup (including the power supply, switch, and speaker) would be best.

[fixit7]

https://www.dropbox.com/s/edy9xs8p5rfm15q/Project1.jpg?dl=0

https://www.dropbox.com/s/leak8ty4dsmw88e/Project2.jpg?dl=0

I found out that the power rail is broken up into 2 separate sections.

When I put 2 jumpers between the sections, the piezo buzzer started chirping.
It did so no matter if the laser hit the LDR or not.

I think the problem is in determining what to use for R2.

Specs for my LDR

Features:

    Maximum voltage rating @ 25°C: 350VDC
    Maximum allowable power dissipation @ 25°C: 400mW
    Resistance (Dark): 1MΩ
    Resistance (Light): 12kΩ
    Lead length: 1.44"
    Operating temperature: -30°C to +70°C
    Able to withstand soldering at 230°C for 3 seconds