Resistors: 330Ω ( 2), 10kΩ, 4.7kΩ
Capacitors: 100μF, 4.7μF
TSOP2238 infrared (IR) receiver
Analog R/C servomotor such as Super Tec S7.5
3x AA battery holder or 4x AA with 1 slot shorted
IC socket, 8-pin DIL for the microcontroller
Small toggle switch, SPST I used a push button.
Perf board, about 4cm×4cm
Male R/C servo plug
Small plastic or metal box
Small screws ( 2–4) for holding the servo
Sony-compatible TV remote
Hookup wire, duct tape, and velcro tape (10cm–15cm)
Picaxe Programming Editor from
Picaxe download cable
Resistors: 10kΩ, 22kΩ in circuit with download cable
Drill and small drill bits
Soldering iron and solder
consumption is tiny.
Servomotors vary in range of movement, so the
software defines 2 constants (TOP and BOT) for the
motor’s upper and lower limits of travel. If you use
the software with a servo other than the S7.5, you’ll
have to tweak those numbers.
Build the Device
With everything working, I transferred the circuit
to a piece of perf board, leaving out the Picaxe programming circuitry. I soldered in an 8-pin IC socket
to hold the Picaxe, and plugged it in with the final
version of the software already loaded (Figure A).
I selected the smallest project box where everything would fit, and then drilled 5 holes: 3 for the
servo axle and fixing screws in back, and 2 on the
front for the IR sensor opening and the power switch
(Figure B). Screw the servo to your enclosure firmly
and position the batteries as low as possible at the
other end. I wedged the battery holder between 2 of
the box’s screw posts, and used duct tape to attach
the circuit board to the inside of the box (Figure C).
I put the longest arm available onto the servo
motor axle, to maximize contact with the knob. Then
I covered the arm with the loops side (softer) of some
velcro tape, and affixed the hooks side (scratchier)
150 Make: Volume
Fig. A: Circuit on small piece of perf board.
Fig. B: Project box to hold circuit and servomotor.
Fig. C: Inside the project box. Fig. D: Ready for service.
to the front of the volume knob (Figure D).
When you first start the circuit, the servo travels
to its lowest setting. This lets you match knob and
servo positions by turning the volume knob to the
lowest setting and then joining the velcro. It also
prevents your amp from exploring the upper reaches
of its volume range. Your hearing will thank you.
Test the Up, Down, and Mute buttons. Check that
the movement range is adequate and, more important, that the amp’s knob isn’t hitting its off-stop on
the way down. If so, you might have to reposition the
controller slightly. Happy listening!
Improvements and Alternatives
» Replace the IR receiver for another one compatible
with your remote brand. Seven different variants
(TSOP22xx) cater to most brands out there.
» Include an auto power-off feature. This is desirable, but not easy to implement in a simple design.
For project software and a schematic, visit
Paulo Rebordão is a software practitioner but is considering moving into hardware for a firmer grasp on reality.