Laser alarm system not working correctly..

[neba3939]

Hi guys, i am a first year college student and this is my first project to build by myself.. and i need help.
I build this laser alarm system and it works good. the Alarm sounds when the laser is broken and it stops when the laser is not interrupted. But the problem is when the laser is disturbed the alarm sounds but as soon as the laser touches the photocell again the alarm stops..
i was thinking, is there any way to make the alarm sound constantly non stop ones the laser is broken, Is it possible to make the alarm sound continuously without non stop unless it is stopped manually?
Please look at the schematics and help me..
thank you

 

[Reloadron]

Think about using a SCR rather than a transistor. Once gated on it will remain on until the circuit is reset.

Ron

 

[Joe G]

you could even use a latching relay, like those found in those portable 12v car air compressors, when the pressure hits the set mark, it turns itself off, and resets when you flip the switch off/on.

 

[alec_t]

Shouldn't the 1k and the photocell be swapped over in that circuit?

 

[Reloadron]

Shouldn't the 1k and the photocell be swapped over in that circuit?

I was also wondering about that? I would think so.

Ron

 

[neba3939]

I went to radio shack to buy the scr and the told me they don't have it in store and I can order it online and when i went online there are alot of options, can someone tell me which to buy and some people told me I could use two transistors to make the current flow continuesly if so where do I add the second transistor? Thank you very much

 

[ericgibbs]

I went to radio shack to buy the scr and the told me they don't have it in store and I can order it online and when i went online there are alot of options, can someone tell me which to buy and some people told me I could use two transistors to make the current flow continuesly if so where do I add the second transistor? Thank you very much

hi,
Look here ,
http://ecelab.com/circuit-transistor-scr.htm

I Googled. transistor scr circuit there are man circuits on line

http://www.google.co.uk/search?q=tr...rg.mozilla:en-GB:official&client=firefox-beta

 

[Reloadron]

You may want to give this a read. Using a SCR was merely one suggestion. You could also use a latching relay type configuration. As to a SCR, maybe a 100V 2A version or in that range. You don't need much. I'll try and see what comes up in Radio Shack if I have time today.

<EDIT> I see Eric is about and thanks Eric. </EDIT>

Ron

 

[alec_t]

some people told me I could use two transistors to make the current flow continuesly if so where do I add the second transistor?

Like this perhaps?

 

[neba3939]

Help with Problem Circuit

I was working on this project and i connected everything according to the schematics, but i am facing one problem. when i supply power to the circuit(with no laser on the photocell) nothing is happening when it was suppose to sound, and with out changing anything to the connections when i apply power to the circuit again(everything the same) the alarm sound.... one time it sounds and one time it doesnt.. I am guessing there is a problem with the resistors and their amount, but i am not sure.. can you please help me find the problem and tell me how to correct it. Thank you
I attached the schematics...

 

[DerStrom8]

I was working on this project and i connected everything according to the schematics, but i am facing one problem. when i supply power to the circuit(with no laser on the photocell) nothing is happening when it was suppose to sound, and with out changing anything to the connections when i apply power to the circuit again(everything the same) the alarm sound.... one time it sounds and one time it doesnt.. I am guessing there is a problem with the resistors and their amount, but i am not sure.. can you please help me find the problem and tell me how to correct it. Thank you
I attached the schematics...

Hmm, the circuit is not very well drawn. What are you using for the alarm?

 

[neba3939]

i am using "Audio Buzzer Piezo 12VDC 250mA Wire Lead 6VDC 1500Hz to 3500Hz "
you can see the details here
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_207993_-1

 

[Reloadron]

You began this in this thread and neglected to maintain the thread. The circuit was given to you or suggested to you by a forum member who could likely continue to assist you with it. Why start another thread? You should keep this all in the same thread for the best help.

Ron

 

[neba3939]

I was working on the schematics you gave me and i connected everything according to the schematics, but i am facing one problem. when i supply power to the circuit(with no laser on the photocell) nothing is happening when it was suppose to sound, and with out changing anything to the connections when i apply power to the circuit again(everything the same) the alarm sound.... one time it sounds and one time it doesnt.. I am guessing there is a problem with the resistors and their amount, but i am not sure.. can you please help me find the problem and tell me how to correct it. Thank you

 

[neba3939]

oh, i posted the same question, thank you for the information..

 

[DerStrom8]

Thanks for pointing that out Ron. Maybe the mods can move this thread back over there?

Anyway, neba--your problem is that you're using the wrong buzzer. The circuit calls for an 80 ohm speaker, which is about the impedance of a pair of headphones. A piezo buzzer WILL NOT WORK, since they are set up to auto-oscillate, and have a different impedance.

Honestly, I'm not sure why it asks for an 80Ω alarm. That is one of the parts that makes this circuit confusing. Why would you put headphones on a laser tripwire alarm?

This circuit might be a better bet. It's simpler, but will work with your buzzer. Simply connect the alarm you have now to the relay contacts and to a power supply, and it should work fine.

Did you by any chance get your circuit from colin55?

 

[neba3939]

thank you for your help, but i am new to this so i dont exactly know how to connect a relay in a circuit and which pins to connect and can you also suggest which relay to use, and the other thing is that i tried bunch of simple circuits like the one you gave me, and they work fine, but the problem is once the laser light touches the photocell the alarm stops it doesnt continue sounding...

is there any way that i can use the buzzer on the last schematics, i am by changing the value of the capacitor or something...
and i didnt get the circuit from colin55

 

[DerStrom8]

thank you for your help, but i am new to this so i dont exactly know how to connect a relay in a circuit and which pins to connect and can you also suggest which relay to use, and the other thing is that i tried bunch of simple circuits like the one you gave me, and they work fine, but the problem is once the laser light touches the photocell the alarm stops it doesnt continue sounding...

is there any way that i can use the buzzer on the last schematics, i am by changing the value of the capacitor or something...
and i didnt get the circuit from colin55

Any small relay should work, as long as the coil voltage matches the voltage you're switching with, and the contact voltage is greater than what you're switching. In a relay, there are 4 main pins that you need to worry about. There are two connected to the coil--if you put a voltage between them, it will switch the relay. The second pair are connected to the NO (normally-open) contacts, so that when you apply power to the coil, they connect. You need to connect one of the contact pins to the (-) side of your battery, connect the (+) side of your battery to your buzzer, and connect the other wire from your buzzer back to the second contact pin. That will make a complete circuit when the relay is on.

There are ways to latch transistors using nothing but transistors. Take something like this:

View attachment 62498

You can see where the "new" base, collector, and emitter go. Just replace the single transistor that connects your relay with this "double transistor". It will remain on until the "new" base is pulled low (connected to (-) with a pushbutton switch). That's really all you need, I think.


By the way, many thanks to the moderators for moving this back here

 

[Reloadron]

Alec provided the circuit the OP is currently working with as well as the .asc file for it.

The correct alarm device is likely a cheap buzzer or similar. Neba, your original approach was actually not a bad one. Given some time you will learn there are several ways to go about this. Do you have any data at all on the photo cell you are using? That would be a good starting point.

Hopefully yes, a mod will come along and merge things.

Ron

 

[neba3939]

than you for your help and one quick question does the double transistor work if i use it in this schematics?(look at the pic)

and Reloadron i am using this photocell (https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_120299_-1)

thank you for the help