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.
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
Thanks Pete for sharing your Expertise, Information, and Assembly
Photos on the A15 Unit
(C) 2017 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 Zenner
Showing His 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.