TECH VIEWS
R.G. KEEN
Make a Mic Muter
Enough theory for a while, let’s go build
something! I always had a problem on stage
because I didn’t have enough hands to play
guitar and flip mic-muting switches, too. A
microphone that sensed when I stepped up
to it and activated it would’ve been perfect
for me. I finally came up with an add-on that
does exactly that.
This is based on burglar-alarm pressure mats:
plastic mats with built-in switches that connect when you step on them. All
you need to do is put the mat in
front of the mic stand and wire up
your microphone so that stepping
on the mat un-mutes the mic and
stepping off the mat mutes it. There
is a minor problem in that the mat
switches are open when you step
off and closed when you step on
them. If we used the mat switch
directly, the mic would mute when
we stepped on it. So we need to
make the mat control a switch that
works the opposite direction—on
when the mat switch is off.
A 1/4” phone plug from the normally-open
switch mat comes into the box at the jack,
where the sleeve of the mono plug connects
the battery to the rest of the circuit, saving
battery life when the plug is not there. The
battery provides 9V to a 47K resistor to the
base of the 2N3904 transistor. The transistor
is therefore fully on whenever the switch mat
is open. When you step on the switch mat, it
shunts the base current away from the transistor and the transistor turns off.
I’ve shown the setup for low impedance
balanced mics. If you use high impedance
unbalanced mics, wire the H11F1M to shunt
the signal to ground. The connectors will be
different, of course, to match the mic cables.
Some low impedance mics will have a hot
enough signal to bleed through even when
the circuit is trying to mute them; however,
they should have a very much reduced signal
level even if they’re not fully muted.
Suitable pressure switches are available in most large cities from suppliers of burglar-alarm equipment. If you
can’t find them where you live, here
are some web addresses for suppliers
of suitable mats.
We also need a switch that will not
impair sound quality by adding
noise, distortion or hum. Relays
might work fine, but they eat a lot
of power, too much for such a simple device. Luckily, there is a suitable solid-state device: the Fairchild
H11F1M. This LED-to-PhotoFET optical
switch is almost ideal, having no effect on
the audio when it’s open, and muting the
signal when it’s closed so that any non-ideal
characteristics cannot come through.
http://www.spytown.com/usp-usp-
pressure-mats.html
http://www.alarmsuperstore.com/con-tacts/ uspcomplete.htm
http://www.trossenrobotics.com/store/
p/ 3508-Switch-Floor-Mat-14-x-30-.aspx
http://supersoda.com/detail.
php?id=00000000036 (Make your
own switch mat)
http://www.switchmat.com/
OperatorControlMats.aspx
http://www.electronicplus.com/con-tent/ ProductPage.asp?maincat=SW&
subcat=sfm
This is the kind of project that is most useful to an experienced hardware hacker, so
I’m not going to provide “solder wire C to
contact 5” kind of instructions. You advanced
hackers won’t need details except where to
find an H11F1M. If you look at the schematic
diagram, you’ll see how to hook it up. The
box is primarily a pass-through box for your
mic cable, and probably ought to sit right at
the base of the mic stand. Ground the box to
the signal ground on the mic cables.
The battery also provides current through the
750 ohm resistor to the anode of the LED in
the H11F1M. The cathode of the LED is tied
to the collector of the transistor, so when
the transistor is on, the LED emits light and
causes a low resistance between pins 4 and
6 of the H11F1M. The H11F1M then shunts
the two signal lines together, effectively muting the mic whenever the LED current is on.
When you step on the mat, the mat turns off
the transistor and LED, letting the signal lines
pass signal freely.
Here’s the H11F1 datasheet online:
http://www.fairchildsemi.com/ds/H1/
H11F2M.pdf
There are actually three parts that may be
used interchangeably in this circuit: the
H11F1M, H11F2M, and H11F3M. They
differ in certain specifications, which don’t
matter for this application. As of this writing, Mouser Electronics has the H11F1M
for $1.88, the H11F2M for $2.00 and the
H11F3M for $1.75.
This is probably best built on a 1”x1” bit of
perfboard. It’s too simple to design up a PCB
for, unless you’re making thousands of them.
The resistors can be 1/4W or 1/2W, or even
5W if you want. Carbon composition mojo
resistors will make absolutely no difference
whatsoever here.
Once you get used to stepping away from the
mic to mute, I think you’ll find it’s really handy!
Use an insulated 1/4” phone jack for the
switch mat to plug into so that it cannot
induce clicks and pops in the signal lines.
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
