GBPPR Silent Drill


A silent drill is a device for quietly drilling holes into walls, doors, floors, etc. for the insertion of a technical surveillance device.  For instance, say a $2600 meeting was taking place in a hotel room.  To protect your family and their privacy, you may wish to monitor this meeting.  First, you'd arrange to rent the hotel room next to the room where the meeting is taking place.  Then you would silently drill a hole through the wall in your room, and just before reaching the outside wall into the meeting room, you'd reduce the size of the drill bit so the final hole going into the meeting room is only the size of a pin-hole.  You could then place a microphone or even a video camera into the hole and capture any activity coming from the meeting room.

Stories & Background Information

Excerpt from Peter Wright's Spycatcher

... It actually began some months before I joined MI5, when Hugh Winterborn mounted an operation to bug the Russian Consulate on the Bayswater Road.  The opportunity arose when the building next door was refurbished in preparation for new occupants.  MI5 went in under cover as decorators and Winterborn fitted a new device called the probe microphone, which had been developed by John Taylor in the Dollis Hill Laboratory.

    The probe microphone was a large, high-sensitivity microphone, which was used to gain covert access through a party wall.  The device was lodged inside the wall about eighteen inches from the target side.  The eighteen inches between the probe microphone and the target room were drilled out by hand at a quarter-inch diameter in steps of half an inch.  Half an inch from the target side the quater-inch-diameter hole ended and a small pin-hole was drilled, again by hand, using a No. 60 size bit, so that the intrusion into the target side was almost invisible to the naked eye.  The eighteen-inch bore hole was then lined with a smooth perspex tube which was acoustically matched into the microphone.  The microphone fed out into the street and back along telephone wires to Leconfield House, where amplifiers boosted the captured sound into intelligible speech.


    The No. 60 drill bit had a special stop on it ensuring the bit turned so slowly that a flake of plaster or paint could not be pushed into the target room ...

GBPPR Silent Drill Construction

Overview on some of the pieces used in the GBPPR Silent Drill.  Various drill bits are shown on the left.  Get a good selection of sharp bits and extensions.  Large wood boring bits are also very useful.  Note the cork plug.  This is useful for plugging the hole after drilling.  The flexible drill shaft is along the top.  It's a Disston 40-inch long, 1/4-inch shaft.  The drill is an old cordless Makita.  Look for cordless drills at thrift stores and rummage sales.  The battery packs will be dead, but the drills will still work.

The new silent drill will be powered from an external +12 volt rechargable lead-acid battery pack.

You may wish to experiment with various types of sound absorbing materials to wrap the drill with.  The drill is fairly quiet as is, but there is alot of room for improvement.  Chopped up sponges work, but are not quite that dense for attenuating those low frequencies.  Also try using alot of those rubber "bouncy balls".  It may be worth a shot...

Pillows work well "in-the-field."

Close up view of the drill's power/speed trigger switch.  This will need to be modified slightly to allow the controls to be mounted externally.

Alternate view.  When the trigger switch is fully pulled, the speed control is bypassed and the full battery pack voltage is applied to the drill's motor.

Internal view of the drill.  The main components are the battery pack, power/speed trigger switch, reverse switch, motor, reduction gears, and the chuck.

Close up view of the motor and reduction gears.  A professional model would most likely have plastic or composite gears to reduce noise.

Alternate view of the reduction gears.

Testing setup.  To control the drill's motor speed, the trigger switch slides along a 2.2 kohm potentiometer.  You'll need to remove the slide potentiometer on the speed control and mount a new one externally.

Close up view of the cleaned gears.

Freshly greased gears.  The motor's power leads should be bypassed with a 0.1 µF ceramic capacitor and ferrite beads should be placed over both the positive and negative wires.  This will help reduce any Electromagnetic Interference (EMI) which may give away the operation of the drill (buzzing sounds on an A.M. radio).  Also, the plastic case of the drill should be painted with an EMI absorbing ferrite or metal flake paint.

Completed drill section.  The handle has been cutoff to reduce the size and prevent it from resonating any sound.  Any left over holes were filled with silicon caulk.

I don't know what to do from here.  A shock mount system is made from a block of wood, some nuts and bolts, sponges, and felt liners.

The drill is mounted to the wood block via three large zip ties.  A large sponge is wedged between the drill and the wood block.  The zip ties are epoxied into place.

Alternate view.

The mounting hardware is protected from vibrations with large rubber feet.

The drill and flexible shaft are mounted inside an old plastic tool box.

Alternate view.  The drill's mounting block doesn't touch the tool box except via the rubber feet.  The flex shaft leaves the tool box via a large rubber grommet.

The mounting bolts are isolated from the tool box with rubber grommets.  The large red things are rubber hose washers.

Motor speed control with the connections to an external panel-mounted 2.2 kohm potentiometer (wires going out the bottom).

Test setup.  Speed potentiometer resting on top of the drill, the large reverse relay is on the right.  Speed control and power distribution bus is on the bottom.

Close up of the reverse relay.  It is a large DPDT relay capable of handling at least 20 Amps.  This relay allows the voltage polarity going to the motor to be reversed, allowing the drill's motor to turn in both directions.  The drill includes a reverse polarity switch, but it's not rugged.  Be sure the motor is powered down before activating the reverse polarity relay.

Close up of the trigger speed control.  The "on/off" section of the trigger is bypassed.  Keep the pass transistor on its heatsink.

Completed drill internal view.  Any component which may cause vibration or sound is isolated from the tool box's walls via stand-offs.  Ideally, the tool box would next be filled with dense, sound absorbing foam (not shown).  Coating the inside of the plastic tool box with an EMI absorbing ferrite or metal flake paint would be very helpful.  Avoid heavy metal shielding or anything which could "rattle."

Behind the front panel.  +12 VDC input from the battery and the main on/off switch are on the lower left.  The panel-mounted speed potentiometer is above that.  A 10 Amp resettable fuse and a switch to active the reverse polarity relay are on the right.

Alternate view.  A large 1,000 µF capacitor smooths the DC voltage going to the drill's motor.

Front panel view.  Flexible shaft is on the left side.  The ammo box contains a large +12 VDC rechargable lead-acid battery.  The main power switch has one of those "missle launch" protective covers to prevent the drill from accidently being turned on.

Operational Block Diagram

Links & Notes

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