contact microphone circuit
piezoelectric circuits
spy circuits
A15 Wall, Door, Window Contact Microphone  - Assembly Photos 
Built By Pete Zenner, Sioux Falls, SD. - March 2014
I made a piezo pickup. The disk element was taken from a small burglar alarm (from our local Dollar Store). The two wire were clipped short and soldered to the center conductor and ground of a piece of RG174 coax. The leads were covered with heat shrink tubing. Then I added epoxy to make the connection a little more rigid. The other end is simply a 1/8" mono plug. This one works very well without a great deal of hum. Only the "Super Sensitive #9210" gave me no hum at all. The #9210 comes with a really really long cord and I have to change the plug from 1/4" to 1/8".
John, I was flipping through my book of circuits and I don't remember building your A15 Contact Microphone Circuit. I laid out the PCB a few days ago, made the first diagram. I first laid it out for a LM389 (the IC chip with 3 transistors and an integrated LM386). It didn't save any space, plus I don't have one yet. I then laid it out using regular parts. The board was certainly smaller, but soldering became difficult. I made two mistakes on my prototype layout. I managed to fix both without having to scrap the board. Here's the end product. Very nice. Works excellent.

The input/piezo jack is on the left. The 3 transistors on the top half. The top switch is the voice filter. The gain pot next to it. Red/Black wire is power. On bottom are the headphone jack and the recording output.

contact microphone circuit in enclosure
contact microphone amplifier circuit
homebrew piezoelectric pickup circuit
contact microphone circuit enclosure
piezo  pickup circuit
Footnote:    When I originally manufactured the A15 Electronic Contact Microphone, IE - Wall, Door, Wall Listening Amplifier, I mounted it's Piezo directly to the enclosure. The agent or operative simply pressed the device up to the surface. Zenner was extremely creative in the way he put this unit together and especially the addition of his homebrew pickup. Regarding the hum he has encountered, I suspect it is from the cable itself. Given my dithers on how to solve that problem, I would measure the frequency of the hum encountered and then solder a bypass capacitor across the input jack that would dump the hum frequency to ground before it can enter the circuit. I would choose a capacitor that has a figure of around "0" Xc at the hum frequency.
Thanks Pete for sharing your Expertise, Information, and Assembly Photos on the A15 Unit
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For the back of the PCB board, I cut a thin piece of rubber and used epoxy to adhere it to the board. Once dry, I used epoxy to stick the entire board to the inside of the tin. The power indicator is a red LED from Radio Shack (#276-209) -- which is for 9-12V and isn't bright enough to blind you. I used a regular LED first, thought the brightness too annoying and changed to this one.
Fully Assembled A15 Contact Microphone Circuit By Pete Jenner - March 2014 Showing Assembled Circuit Board
I used an Altoids tin painted black. The earphone and recording jack on top on the lid. I added a power indicator light. The knob is the 10K ohm potentiometer gain control. The input for the piezo is on top side. Immediately I noticed the difference between piezo inputs. Some have a definite hum. The quietest was the Super Sensitive SS8210. If the piezo has a two wire input, twisting them together knocked down the hum a good deal, but not entirely. I suspect that only coax cable as an input will eliminate the hum.