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Band Drummerís Microphone Switch  August 28, 2011

I received an email from guy who plays drums for a small band.  He wanted a simple way to turn on and off his microphone with just a tap of his drum stick onto a small box.  The circuit below performs this task with the aid of a piezoelectric device as a shock sensor and a small dual coil latching relay with two sets of contacts.  The contacts can be wired any number of ways to a microphone circuit.  I show one circuit which should work on both dynamic and condenser mics.  It should mute the mic without producing a pop sound.

The circuit is powered by a 9v battery.  It draws nearly zero current in standby mode. The mechanical shock of a drum stick hitting the box causes the piezoelectric wafer to produce a number of voltage pulses.  If the pulses reach the 4v peak mark, they are high enough to trigger the one shot circuit, which uses a CD4013 dual D flip/flop.  The one shot circuit lasts about a half second.  Its output is connected to the second flip/flop inside the 4013 package.  The resistor R2 defines the tap sensitivity.  The second circuit is configured as a classic on/off toggle.  Each time the one-shot fires, the second flip/flop changes state and stays in that state until the next strike of the drum stick to the box.  The Q and the inverted Q outputs of the flip/flop are connected to two pulse generator circuits, which are tied to gates of two FETs. 
Each time the flip/flop changes state, the two pulse circuits either latch or unlatch the relay.  It takes a pulse of only 25ms to latch or unlatch the relay.  I would suggest that a CD4013 from Texas Instruments be used.  Many 4013s from other manufacturers do not contain a Schmitt trigger function on their inputs.  A Serpac box from Mouser, part number 635-211-I-G is just right for this application.  It is about 4 inches long and 2.6 inches wide.  It has a nice compartment for a 9v battery.  The piezo wafer is glued to the bottom of the box, which also has the 8v battery compartment.  The box is mounted top side down.
Serpac Box

Click on Drawing Below to view PDF version of Schematic

Microphone Switch for Drums

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