- May 2008: ε24 blank circuit boards are
ε24 ("epsilon 24") is a simple circuit that lets you
use some attractive momentary pushbutton switches (such as the
vandal-resistant series) as an amplifier or other device'
main power switch. It provides debouncing for the momentary
switch to prevent false-triggering, and uses either a conventional
relay or a solid-state relay to perform the actual switching.
ε24 supports switches with built-in LED illumination,
or with discrete LEDs for power indication. Even dual-color
illuminated switches are supported (see below).
Mounting this circuit at the rear panel (near the power inlet)
has the benefit that AC mains wiring need not be run to the
front panel where the pushbutton is. The pushbutton only controls
a small DC signal and will not emit interference to sensitive
audio circuitry. Also, an appropriate choice of relay allows
this circuit to be used with a wide variety of devices, such as
a small DAC or a large, high-powered amplifier.
An optional sensing loop employing thermal breakers can be used
with the ε24, so that in the event of device overheat,
it will be automatically shut off. For switches with dual-color
illumination (or if separate discrete LEDs are used), the alternate
color is illuminated to show the fault status when attempting to
turn the device on again.
The following is a photo of the top and bottom sides of the
ε24 circuit board, a photo of an assembled board, and
the Bulgin MPI002 series ring-illuminated switches.
Building the ε24 will require that you work with AC line voltage,
which is a lethal shock hazard. If you don't know what you're doing,
please ask someone who is experienced to help you.
The following is the schematic diagram of the ε24 board.
Note that the 12V AC supply must be "always on" (i.e.,
constant power even when the device is turned off). The main
power switch relay is to be mounted off-board.
The following is the parts list to populate the ε24 board.
|Qty||Part||Description||Vendor part numbers|
|1||PCB||ε24 printed circuit board||AMB audio shop|
|1||R1||470K 1/8W miniature resistor||299-470K-RC||S470KQCT-ND||1700272||-|
|1||R2||2.2MΩ 1/8W miniature resistor||299-2.2M-RC||S2.2MQCT-ND||1700281||-|
|2||R3, R4||2.2KΩ 1/8W miniature resistor||299-2.2K-RC||S2.2KQCT-ND||1700241||-|
|1||R5||330KΩ 1/8W miniature resistor||299-330K-RC||S330KQCT-ND||1700270||-|
|1||R6||10KΩ 1/8W miniature resistor||299-10K-RC||S10KQCT-ND||1700251||-|
|1||C1||470µF 25V electrolytic capacitor (8mm diameter, 3.5mm lead pitch)||647-UHE1E471MPD or UPW1E471MPD6||P10274-ND or P12389-ND||1219470||628-3790|
|1||C2||10µF 25V dipped tantalum capacitor (2.5mm lead pitch)||80-T350E106K025AT||399-3565-ND||1457582||538-1938|
|1||C3||0.33µF 50V Z5U multilayer ceramic capacitor (2.5mm lead pitch)||80-C320C334M5U||399-4299-ND||1457659||211-5306|
|1||D1||W01G 1.5A 100V bridge rectifier||625-W01G-E4||W01G-E4/1GI-ND||1497575||2509471313|
|2||D2, D3||1N4148 small signal diode (DO-35)||512-1N4148||1N4148FS-ND||9843680||670-8858|
|1||Q1||2N3906 PNP transistor (TO-92)||512-2N3906BU||2N3906FS-ND||1459017||294-328|
|1||Q2||2N3904 NPN transistor (TO-92)||512-2N3904BU||2N3904FS-ND||9846743||294-312|
|1||Q3||IRLZ24N N-channel MOSFET (TO-220)||AMB audio shop|
|1||U1||7812 voltage regulator (TO-220)||512-LM7812CT||MC7812ECT-ND||7174047||298-8542|
|1||K1||12VDC DPDT miniature relay, Omron G6H-2-DC12 or Panasonic TQ2-12V||653-G6H-2-DC12 or 769-TQ2-12V||Z741-ND or 255-1002-5-ND||1200949||113-8831|
|1||J1||Molex KK 4P .100 header 22-23-2041||538-22-23-2041||WM4202-ND||1462920||679-5596|
|1||-||Molex KK 4P .100 plug housing 22-01-3047||538-22-01-3047||WM2002-ND||1462809||679-5388|
|1||J2||Molex KK 6P .100 header 22-23-2061||538-22-23-2061||WM4204-ND||1462922||679-5593|
|1||-||Molex KK 6P .100 plug housing 22-01-3067||538-22-01-3067||WM2004-ND||1654529||679-5391|
|10||-||Molex KK .100 crimp terminal 08-50-0114||538-08-50-0114||WM1114-ND||1462641||172-9178|
|1||-||12V power relay SPST-NO, chassis-mount||(see note 1)|
|1||-||Power transformer||(see note 2)|
|1||-||Pushbutton switch, SPST-NO momentary||(see note 3)|
|1 or more||TB1, TB2, etc.||Thermal breaker (optional)||(see note 4)|
- Choose a 12VDC relay with the appropriate contact rating for your
intended application. The Omron G8P-1A2T-F-DC12 relay
(Mouser 653-G8P-1A2T-F-DC12, Digi-Key Z215-ND), for example,
with 30A rated contacts is good for a medium to large power amplifier.
You can use a much lower-rated relay for low power circuits. If the
coil current exceeds 120mA, then you should add a heatsink to U1,
or use the
Murata 7812SR-C switching regulator for U1,
which runs much cooler (Mouser 580-7812SR-C, Digi-Key 811-1117-ND).
You may instead use a solid-state relay with DC control
For example, the Crydom D1240, with 40A rating, is good for a
high powered amplifier. Some power transformers may make a
buzzing noise when used with a solid-state relay. See Note 4 below
about the use of a solid-state relay and the thermal shutdown
feature of the ε24.
- Any small ~1.5VA-2VA power transformer with 12V secondary is usable.
For 12V+12V (dual secondary) transformers, you can parallel the two
secondaries, but be sure to do so in the proper phase. An example
of a good transformer is the Amveco TE62002 toroid (Digi-Key TE62002-ND).
If you're using the companion
σ24 transformer board, then you must use
an appropriate EI-30 form-factor 1.5VA PCB-mount transformer. See the
σ24 website for details.
- You may use an illuminated or non-illuminated normally-open
momentary switch. An example of an illuminated switch is the
Bulgin MPI002/28/BL with blue ring illumination, or the
MPI002/28/D4 with blue/red dual-color ring illumination.
If a non-illuminated switch is used, then discrete LEDs can be
added for status indication. Bulgin switches can be found
at various distributors such as Allied, Newark, Mouser and
Farnell. Similar switches from other manufacturers can also
- The Stancor STO-170 or Airpax 67L075 series are recommended
(Mouser 802-STO-170, Digi-key 723-1207-ND, RS 2508947626).
If you are using a solid-state relay for the main switching relay,
and you are using the thermal shutdown feature of the ε24,
then you should add a 220Ω 2W resistor across the
solid-state relay's control terminals to provide a suitable load
for the ε24. Without the resistor, the thermal shutdown
function may not operate correctly.
If no thermal-shutdown function is needed, then you may omit the
thermal breakers, K1, Q1, D2 and R4. Install a wire jumper across
T1 and T2 (J2C), and jumper the K1 relay pads (pins 2-3 and 8-9)
such that it's in the "unpowered" state.
The circuit board
The ε24 board is made of high quality FR-4 glass epoxy,
double-layer with plated-through holes as well as silkscreen and
soldermask. There is a ground plane on each side of the board for
lowered ground impedance.
The dimensions of the board is 2.0" x 1.2" (50.8mm x 30.5mm).
The thickness of the board is 0.062" (1.58mm). The following
image shows the board layout:
- Please see the
σ24 transformer board for an optional
companion power transformer mounting board, which lets you use
an inexpensive EI-30 miniature transformer for this circuit.
The σ24 board has the same dimensions and mounting screw hole
locations as the ε24, and could be stacked together using
- Please visit the
AMB DIY audio forum for support.
Portions of this circuit is an adaptation of work by Winfield Hill
(co-author of the book "The Art of Electronics"), originally
published on the sci.electronics.design Usenet group.
All rights reserved. Commercial use of this design is prohibited
without prior permission.