Laser alarm system not working correctly..

[DerStrom8]

The correct alarm device is likely a cheap buzzer or similar.

Ahh, you're right. For some reason I was thinking the circuit acted as an oscillator for a speaker. That was my mistake.

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

A mod already did. We're talking in the original thread now

 

[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

I am waiting for your answer DerStrom8

 

[DerStrom8]

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)

Yes, the dual-transistor setup should act as a latch in that circuit as well, if it replaces the transistor shown there now. You will want to connect a push-button between the base and ground to shut off the alarm.

 

[neba3939]

thank you DerStrom8, i really appreciate it
I will let you know the result
thank you again

 

[Reloadron]

I looked at the data sheet for your photo cell. Something you may want to try is hopefully you have a meter. Measure the resistance of the photo cell under light and dark conditions. The data sheet gives the foot candle illumination ratings but it would be nice to see the numbers for your specific application.

Ron

 

[neba3939]

Yes, the dual-transistor setup should act as a latch in that circuit as well, if it replaces the transistor shown there now. You will want to connect a push-button between the base and ground to shut off the alarm.

I tried the circuit by replacing the transistor with the dual-transistor setup, and the result is: when i connect it to power, the alarm starts to sound immediately, and when i put the laser light on the photocell there is no change, and i tried it this way, fiirst i applied the laser onto the photocell and then connect the batteries but the alarms sounds, this is what happened after i set it up with the dual transistor, but when i use only one transistor the laser light stops the alarm(but still it wont keep on sounding when the laser is broken like other security alarms)

 

[DerStrom8]

I tried the circuit by replacing the transistor with the dual-transistor setup, and the result is: when i connect it to power, the alarm starts to sound immediately, and when i put the laser light on the photocell there is no change, and i tried it this way, fiirst i applied the laser onto the photocell and then connect the batteries but the alarms sounds, this is what happened after i set it up with the dual transistor, but when i use only one transistor the laser light stops the alarm(but still it wont keep on sounding when the laser is broken like other security alarms)

I forgot that that setup is very sensetive, so you may want to add a pull-down resistor to the base. However, I am not very good with analog circuits such as this. I'm sure some of the other guys here will be able to help you more than I can.

I hope you can figure it out soon

Regards

 

[neba3939]

I forgot that that setup is very sensetive, so you may want to add a pull-down resistor to the base.

where should I add the resistor, On which base, on the PNP or the NPN? and would a 10k resistor work? Thank you again

 

[neba3939]

I looked at the data sheet for your photo cell. Something you may want to try is hopefully you have a meter. Measure the resistance of the photo cell under light and dark conditions. The data sheet gives the foot candle illumination ratings but it would be nice to see the numbers for your specific application.

Ron

I measured the resitance value and when the photo cell is under light the resostance value is 0.5 k and in the dark it is 10k-12k

 

[DerStrom8]

where should I add the resistor, On which base, on the PNP or the NPN? and would a 10k resistor work? Thank you again

If I'm actually right about a pull-down resistor actually helping, it would be on the "base" of the "combined" transistor setup. And 10k is probably way too high. I'd recommend more like a 2.2k to 4.7k. I know there are actually some calculations that you can make to figure it out, but I can't think of them right now. That's why I recommend you ask someone besides me. I'm afraid I can't help much more...

 

[Reloadron]

If you scroll down this page about half way you will find a variety of photo cell circuits. They use transistor as well as SCR latching designs.

Circuits like this are commonly called "Light Activated" or "Dark Activated" photo cell circuits. You mentioned that your LDR had a resistance of about 500 Ohms with light striking it. Your LDR along with another resistor in series with it will form a voltage divider circuit. Generally you want the series resistor to be about 3X the value of the LDR light resistance. I would look to place a small 2K Ohm pot (ten turn preferred) in series with the LDR. For circuits like this the placement of the LDR determines if the circuit is light or dark activated. LDR on top of divider "Light Activated", LDR on bottom "Dark Activated". The link shows light activated and you want dark activated. Also, in the SCR examples you want a small normally closed push button switch in series with the load (your buzzer, bell, relay or whatever). That is the reset breaking the load connection. Once a SCR is fired ON it remains ON so to turn it off we use a PB N/C switch.

If the circuit(s) start in an ON or set state they simply require a reset.

Ron

 

[neba3939]

If you scroll down this page about half way you will find a variety of photo cell circuits. They use transistor as well as SCR latching designs.

Circuits like this are commonly called "Light Activated" or "Dark Activated" photo cell circuits. You mentioned that your LDR had a resistance of about 500 Ohms with light striking it. Your LDR along with another resistor in series with it will form a voltage divider circuit. Generally you want the series resistor to be about 3X the value of the LDR light resistance. I would look to place a small 2K Ohm pot (ten turn preferred) in series with the LDR. For circuits like this the placement of the LDR determines if the circuit is light or dark activated. LDR on top of divider "Light Activated", LDR on bottom "Dark Activated". The link shows light activated and you want dark activated. Also, in the SCR examples you want a small normally closed push button switch in series with the load (your buzzer, bell, relay or whatever). That is the reset breaking the load connection. Once a SCR is fired ON it remains ON so to turn it off we use a PB N/C switch.

If the circuit(s) start in an ON or set state they simply require a reset.

Ron

Thank you very much for the help, but I have one question, can you please tell me where I am suppose to put the SCREEN in the circuit? Please...
Thank you

 

[neba3939]

Where am I suppose to put the SCR in the circuit to make it sound continuesly?

 

[DerStrom8]

Thank you very much for the help, but I have one question, can you please tell me where I am suppose to put the SCREEN in the circuit? Please...
Thank you

Who said anything about a screen??

 

[neba3939]

Who said anything about a screen??

i meant to say SCR and my auto correct changed it to the word "screen"

 

[Reloadron]

Where am I suppose to put the SCR in the circuit to make it sound continuesly?

The attached is an example of similar to the link. The normally closed push button switch is included. A N/O push button switch can be had about anywhere including Radio Shack as I believe you mentioned RS earlier. My version shows the SCR turning on a relay but it can be your buzzer or any suitable load. Again note the positions of the photo cell and series resistor as I mentioned earlier. They can be changed for Light Activated or Dark Activated as mentioned earlier.

The SCR can be one of many that would work in a circuit like this, as long as it can handle the forward current for the load. There are also improved versions in the link.

Make Sense?

Ron