I hope you guys can bear with me on this one. I'm very new to building circuits but I'm trying to learn. Here's what I'm looking at.

If anyone is familiar with a Chassis Ear, it's a piece of equipment used by auto techs to track down noises. It has 6 leads with alligator clips to attach to the component in question and the leads go to a box with a selector knob to select which channel to listen to and a set of headphones plug into it.

Here's my situation, I have one of these that I'm pretty sure the box went bad on it and I have long since gotten rid of it. (wish I hadn't) Now what I'm trying to do is build another one so I can use this tool again.

I recently bought a Radioshack electronics learning lab and found a way to amplify the signal using a 272 OP-Amp and then further amplify that using a 386 audio amplifier. I have substituted the alligator clips from my Chassis Ear with this circuit and it works great but I'm concerned about the current this setup will use seeing as how it will be powered by a single 9 volt battery.

From what I remember of the old box, I don't remember seeing any IC's so I was thinking it might have just used a couple transistors to amplifiy the signal. Even if I have to use an IC to amplify the signal I've read that I could substitute a small transformer for the 386 amp to cut back on current.

So what do you guys think? Am I on the right track with this or do I just sound like a complete idiot Any thought or opinions would be greatly appreciated.

 

The piezoelectric transducers can be subjected to large vibrations, the signal generated can be harmful to any amplifier. Use diodes in counterparallel to the piezoelectric element to clip their amplitude, or to the amplifier input.
A microphone or general purpose amplifier from your experimenter kit should work.

The six alligator leads does not make sense unless they have the piezoelectric elements built-in and are mechanical attachment and not electrical hook-ups.
Miguel

 

The piezoelectric transducers can be subjected to large vibrations, the signal generated can be harmful to any amplifier. Use diodes in counterparallel to the piezoelectric element to clip their amplitude, or to the amplifier input.
A microphone or general purpose amplifier from your experimenter kit should work.

The six alligator leads does not make sense unless they have the piezoelectric elements built-in and are mechanical attachment and not electrical hook-ups.
Miguel

That is it exactly, the clips have piezoelectric material in them and the material is connected to electrical leads that end in a 2 pole 1/8" audio plug. Sorry I forgot to explain that bit as well.

I did take some measurements on the clips and the microphone in the kit. (The microphone is actually a ceramic earphone, it's just being used as a microphone for experimental reasons) The microphone generated @300mv AC and the material in the alligator clips generated @250mv AC so I figured they were close enough to be able to use the same circuit to amplify the signal.

Okay, now a couple more quick questions, you mentioned using diodes in counterparallel to clip the amplitude. I'm not too familiar with how to set this up, and I'm curious as to how this actually works.

Next up, I was trying to find a way to accurately measure current draw on the whole circuit and I think I might have got that nailed down. With just the OP-Amp driving the transformer and speaker I was getting a 1.8mA draw. With the OP-Amp driving the audio amp and speaker I was getting a 7mA draw. I was figuring a typical 9v battery has roughly 400mA hrs so that should give me roughly 57 hrs of use even with the audio amp. Cool, it looks like I'm good there.

I guess the only question I have now is if there would be anything better suited to this that I could use? So far the 272 is working well and I can play with different caps to get the sound I want, the volume of the 386 can be adjusted over a pretty good range, everything looks good, should I just build it or am I missing something big here?

 

... you mentioned using diodes in counterparallel to clip the amplitude. I'm not too familiar with how to set this up, and I'm curious as to how this actually works....

Connect the input of the amplifier to one diode anode, its cathode to ground.
Connect the input of the amplifier also to another diode cathode, its anode to ground.
Germanium diodes will limit the input signal to about 0.3V ; silicon diodes will limit the input to about 0.7 V.