Cut Phone Line Detector

[Earthsnail]

Hi,

First, some background info (feel free to skip)
I'm new to these forums, but I've been browsing this website for a while. I'm a member of the Georgia Technology Student Association (GATSA), and am a participant of the competitive event Electronics Research and Experimentation. I've worked diligently to find and fabricate a product with little success. My last attempts (including the one in question) have failed.

Now, the problem (the important part)

I finally chose the Cut Phone Line Detector as a final option.

Here's a link of the project: http://electronics-lab.com/projects/telephone/006/index.html

I put the components together and had made a PCB to make sure everything was correct. I followed the schematic perfectly and everything is in the right place. However, the project does not work.

The detector runs on the principal of voltage. when plugged in to the wall, a certain amount of voltage is running through the device, making the load (in this case, a buzzer) remain off. When the voltage isn't present anymore (i.e., when the line is cut or it's disconnected), the buzzer turns on and produces sound. In my case, as soon as I connect the battery, the buzzer turns on and remains on whether it's plugged into the phone line or not.

I am positive that I've connected everything correctly and that I followed the schematic exactly. Any help or suggestions is greatly appreciated. However, please remember that I am a high school student who knows only the basics of electronics, so please speak in layman's terms. Thank you again, and I appreciate your help.


-Earthsnail

[audioguru]

The phone line has polarity like a battery has. Maybe your phone line is connected with backwards polarity. Reverse the green and red phone line wires to the circuit's input.

Try connecting a 9V battery to the inputs to see if the buzzer stops when the circuit detects the battery voltage.

[Earthsnail]

I tried both of those things. The buzzer still sounds whether the inputs are to the wall jack/ 9v battery or not. No luck. Any more ideas or suggestions?

[audioguru]

Wall jack? Aren't you connecting this circuit to a telephone jack?
A positive voltage at the base of the 2N3904 NPN transistor turns it on. The turned on 2N3904 transistor turns on the 2N3906 PNP transistor that turns off the IRF510 N-channel Mosfet. When the Mosfet is turned off then the buzzer is quiet.

Maybe you have the pins of the transistors mixed up. Look on their datasheets for pics that show which pin is which.

[Earthsnail]

Whatever the thing in the wall is, that's what the device is being connected to, via some phone wire that I stripped to go into the PCB. I'm probably not explaining it very well, so I'll take a few pics so that you can kinda see what I'm talking about.


I'll check to make sure I don't have the pins mixed up and will report back later.

[audioguru]

Since the buzzer does not turn off with a 9V battery connected to the input of the circuit then the circuit has a problem.

Did you use the correct transistors?
Did you check the pins of the transistors from their pic in their datasheets so that the pins are not mixed up?

[Earthsnail]

I had the pins of the 3904 and 3906 transistors mixed up. I just need to flip them and it works fine now. Thank you for all of your help. I do have one question though while I'm at it. There is about a minute wait time before the buzzer sounds once it's unplugged from the phone line. I would like to shorten this wait time, and I imagine it primarily is because of the capacitor. I wonder, is it possible to shorten that wait time to about 30 seconds or so? If so, what size capacitor and other components would I need?

[audioguru]

Good, you fixed it.

The capacitor filters strong radio and TV transmissions from messing up the circuit.
It also keeps the circuit from buzzing if you are dialing with an old rotary dial phone.
1) Try removing the capacitor.
2) Try a capacitor with less capacitance like 100pF.

You want to hear the buzzer soon after the telephone line is cut so you can catch the bad guy who cut it before he gets away.