Hello everyone. Im looking into making my own cable tracer using following description.

http://www.pic101.com/foxhound/index.html

I would like to add an 3.5 mm audio jack. And for the probe i was going to use telescopic pole off an old radio.

I wonder if someone could help me add 3.5 mm jack. Also how did the creator come up with various values for the resistor etc. In addition to that, how does this audio amplifier work. In the fox generator i can see flip - flop circuit , with what i think is Darwin pair transistors.Little confusing. As for hound there audio amplifier , FET?????. All bit strange. So love to know how it all works.

 

The LM386 is a very well known and widely used audio amp IC. Its datasheet provides a full explanation of how it works. In addition, this circuit has a JFET pre-amp, which is also widely used.

Where do you want an audio jack? If it's on the output, just put it instead of the speaker.

 

Fox generator? Pigtail leads? Hound probe? Has electronics sparked interest in the furry community or something?

As to your question, it's all basically answered in the description in the link you posted.

One part of the unit is clipped to a wire and ground or the other of the pair. When turned on, it puts a strong, rich two-tone square wave howl into the wire.

The other part of the device is a sensitive audio amplifier and speaker. Slowly waving this device over a group of wires will enable you to detect the correct cable run.

This device will not work as a quality audio amplifier, but will be enough for decent quality sound. The JFET allows capacitive detection of a signal because the gate has a very high impedance, so you will be able to hold the probe near a wire with a signal on it - without touching it - and the JFET will begin to amplify said signal.

The operation amplifier after that simply helps amplify the small signal generated by the JFET and drives a speaker.

If you want to build an audio amplifier, I suggest looking into AB-amplifiers (just google, there are loads of examples).

 

The LM386 is not an op-amp; it is an integrated circuit audio amplifier, and is perfectly suited for the OP's application. He has absolutely no need for a high end audio amplifier in a cable tracer.

As to the circuit he linked to, I have my doubts about the need for a JFET pre-amp. I have used JFET pre-amps with LM386 circuits and have not seen any advantage over the standard LM386 datasheet circuit especially when set up for a gain of 200 (46dB.)

 

The LM386 is not an op-amp; it is an integrated circuit audio amplifier, and is perfectly suited for the OP's application. He has absolutely no need for a high end audio amplifier in a cable tracer.

Yes you are correct.

 

I use a LM386 audio amp built into a PC speaker ( portable) to trace audio problems.
It's perfect.

But last week I blew 3 pcs. I kept touching the bias circuitry of a power transistor. Which I found out to be ±39VDC .

 

Thanks for your help. I understand the Probe bit makes sense. I notice allot of resistor capacitor value are the same. It all to do with gain getting gain of hound to be same as probe??? And then all the nor gates leading back to each other so is that bi stable flip flop with nor gates instead of nand gates??? If so it just because it easier. I had another theory that it could be to do with nor gates equalivant. Or am talking load of trash???

 

I use a LM386 audio amp built into a PC speaker ( portable) to trace audio problems.
It's perfect.

But last week I blew 3 pcs. I kept touching the bias circuitry of a power transistor. Which I found out to be ±39VDC .

If you can get hold of one of the least sophisticated Steinel voltage/continuity testers - they use a tubular thin-film PTC to limit the test current upto about 380V. One of those and a DC blocking capacitor at the front end of your test amplifier and a pair of clamp diodes - the LM386 would be pretty much bomb proof.

 

I am using it as a audio tracer. I built just the circuit per data sheet. It has a DC blocking cap but I bet the high DC is too much for the IC. I guess I need clamping the input to a safer value

 

I am using it as a audio tracer. I built just the circuit per data sheet. It has a DC blocking cap but I bet the high DC is too much for the IC. I guess I need clamping the input to a safer value

If you only clamp the input; you can still pump enough current through the clamp diodes to make them go S/C - don't forget to put some resistance in series with the input to limit the current.

It doesn't have to be enough to lose most of the signal - just enough to protect the diodes from too much applied voltage.