Project 2: Laser Perimeter Alarm

[Ocalon]

Hmm, okay lets start with making sure your transistor leads are correctly identified. Some companies create their transistors differently so we need to identify which is the base, emitter and collector pins. You can use the circuit diagram below:

Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10kohm resistor in the base connection or you will destroy the transistor as you test it!

If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released.

[Doc Oc]

First, try disconnecting the transistor and shorting the two leads (Emitter and Collector) together to make sure that the circuit does in fact go off. If not then that means that your buzzer may not have a low enough rating to go off. So then try and just apply the voltage directly to the buzzer to see if it will sound.

If all that checks out and as Ocalon's suggestion up above works, then it might be a problem with getting enough voltage on the transistor switch (Base terminal) to turn the transistor on.

Side note: As Ocalon suggested, sometimes transistors do have the leads swapped so you can't be absolutely certain about their orientation. That means you'd need to check and find which pin is the ase and which is the Collector/Emitter.

Generally in the US the middle pin is the base of the 2N3904 transistor but abroad it sometimes the rightmost pin.

[Jasper.tm]

Oke i've tried 6 (yeah SIX times) building this alarm. I tried the one from Kipkay, but that was no succes, so i moved on to this one.

I used the schematic where all the parts were laying on, and just the one to print. I just can't get it to work. Buzzer works, 10K Trimpotmeter...

I'll upload photo's.

[TLOZ Link5]

haha welcome to the world of electronics! where nothing ever works right the first time, nothing works as its supposed to, and above all - the solution is almost always soo simple you'll hate yourself afterwords for not catching it sooner...

['stache]

Use the schematic and not the one with the overlaid parts (that one was an earlier one that had an error). The base of the transistor (2N3904) should connect between the trimpot and the CdS photocell.

[Ocalon]

With that said, yeah post some pics of your circuit so we can see if anything stands out. Second mention exactly what the problem is whether its not turning on/starting up or its acting weird or just running behind you with a butcher's knife!

Other than that, since you tried a buncha times (I'm assuming you changed it around to see see if anything else - configuration wise - would work) it seems that it might be a compatability problem with the parts. For instance, some buzzers won't go off with a simple DC voltage applied to it - they need the incoming voltage to rise and drop at a given rate to make it ring. It depends.

Hope sum of this helps

[dirtbiketrailsrider]

Hey guys, I am having a problem with my circuit. I am using the 10k trim pot and it is cranked all the way up and I still have about 2-3 volts powering the buzzer even with the laser hitting the photo cell. Any suggestions on why this might be happening?

Also I have a 9v power adapter that I would like to use to power my alarm. I would have a 9v battery as a backup in case someone unplugged it. Would this be possible or would I run into conflicts? I could also make it so there is only 5 volts coming from the power adapter (this might fix my problem with the buzzer not shutting off when the laser is hitting it). That way if someone unplugs it there wouldn't be enough resistance even with the laser on so the buzzer would sound. That would be an added safety feature.

Let me know, thanks.

[Ocalon]

Originally Posted by dirtbiketrailsrider

Hey guys, I am having a problem with my circuit. I am using the 10k trim pot and it is cranked all the way up and I still have about 2-3 volts powering the buzzer even with the laser hitting the photo cell. Any suggestions on why this might be happening?

Also I have a 9v power adapter that I would like to use to power my alarm. I would have a 9v battery as a backup in case someone unplugged it. Would this be possible or would I run into conflicts? I could also make it so there is only 5 volts coming from the power adapter (this might fix my problem with the buzzer not shutting off when the laser is hitting it). That way if someone unplugs it there wouldn't be enough resistance even with the laser on so the buzzer would sound. That would be an added safety feature.

Let me know, thanks.

So that means your also gonna need a higher value potentiometer, like a 20k. As for the power adapter that should work well (is this to be hooked up with the laser or reciever module?). Just make sure the voltage is outputs is less than 9v.

I would say having the adaptor outputing 5v would be better than 9 volts. That way you should be able to keep the 5k pot.

[vommits09]

hi
have you tried to disable the ultrasonic sensors on the alarm system ?
there may be a fault in one of the sensors.

[MegaLord]

Hi!

I recently found this amazing tutorial in "instructable.com", from where I found this forum.

I bougth all the needed parts and tested the shematic on my breadboard... but it does'nt work

I checked, double.checked... triple-checked everything, step by step from the tutorial, but it still does'nt work.

In step of a buzer, I´m using a small 12v lamp, that give a bright light wen conected to a 9v batery. Wen conect to the circuit, in sted of being turned off wen the laser hits the cell, the lamp is constantly ON (with or without the laser), with a very week light, so week I can actualy look at it and see the blasing filament.

Any sugestions?