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"
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.
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
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
Thanks Pete for sharing your Expertise, Information, and Assembly Photos on the A15 Unit
(C) 2014 By: Bugplan.com
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.