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The ε24 power switch driver circuit


  • May 2008: ε24 blank circuit boards are available.


ε24 ("epsilon 24") is a simple circuit that lets you use some attractive momentary pushbutton switches (such as the Bulgin 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.

WARNING: 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.

Schematic diagram

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.

Parts list

The following is the parts list to populate the ε24 board.

QtyPartDescriptionVendor part numbers
MouserDigi-KeyFarnellRS Components
1PCBε24 printed circuit boardAMB audio shop
1R1470K 1/8W miniature resistor299-470K-RCS470KQCT-ND1700272-
1R22.2MΩ 1/8W miniature resistor299-2.2M-RCS2.2MQCT-ND1700281-
2R3, R42.2KΩ 1/8W miniature resistor299-2.2K-RCS2.2KQCT-ND1700241-
1R5330KΩ 1/8W miniature resistor299-330K-RCS330KQCT-ND1700270-
1R610KΩ 1/8W miniature resistor299-10K-RCS10KQCT-ND1700251-
1C1470µF 25V electrolytic capacitor (8mm diameter, 3.5mm lead pitch)647-UHE1E471MPD or UPW1E471MPD6P10274-ND or P12389-ND1219470628-3790
1C210µF 25V dipped tantalum capacitor (2.5mm lead pitch)80-T350E106K025AT399-3565-ND1457582538-1938
1C30.33µF 50V Z5U multilayer ceramic capacitor (2.5mm lead pitch)80-C320C334M5U399-4299-ND1457659211-5306
1D1W01G 1.5A 100V bridge rectifier625-W01G-E4W01G-E4/1GI-ND14975752509471313
2D2, D31N4148 small signal diode (DO-35)512-1N41481N4148FS-ND9843680670-8858
1Q12N3906 PNP transistor (TO-92)512-2N3906BU2N3906FS-ND1459017294-328
1Q22N3904 NPN transistor (TO-92)512-2N3904BU2N3904FS-ND9846743294-312
1Q3IRLZ24N N-channel MOSFET (TO-220)AMB audio shop
1U17812 voltage regulator (TO-220)512-LM7812CTMC7812ECT-ND7174047298-8542
1K112VDC DPDT miniature relay, Omron G6H-2-DC12 or Panasonic TQ2-12V653-G6H-2-DC12 or 769-TQ2-12VZ741-ND or 255-1002-5-ND1200949113-8831
1J1Molex KK 254 4P header 22-23-2041538-22-23-2041WM4202-ND1462920679-5596
1-Molex KK 254 4P crimp housing 22-01-3047538-22-01-3047WM2002-ND1462809679-5388
1J2Molex KK 254 6P header 22-23-2061538-22-23-2061WM4204-ND1462922679-5593
1-Molex KK 254 6P crimp housing 22-01-3067538-22-01-3067WM2004-ND1654529679-5391
10-Molex KK 254 crimp terminal 08-50-0114538-08-50-0114WM1114-ND1462641172-9178
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 moreTB1, TB2, etc.Thermal breaker (optional)(see note 4)

  1. Choose a 12VDC relay with the appropriate contact rating for your intended application. The TE Connectivity/Potter & Brumfield T9AP1D52-12 (Mouser 655-T9AP1D52-12, Digi-Key T9AP1D52-12-ND, Farnell 2708420), 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 (for 120V AC mains) or D2440 (for 240V AC mains), 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.

  2. 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 Talema 62002 toroid (Digi-Key 1295-1018-ND) or Amveco 62002 (Allied 70065651 or 70065722).

    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.

  3. 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 be used.

  4. 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.

  5. Clean both sides of the blank ε24 board with paper towel and isopropyl alcohol or electronics flux remover before soldering any parts on it.

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:

Additional information

  • 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 standoffs.
  • 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 Usenet group.
Copyright © Ti Kan
All rights reserved. Commercial use of this design is prohibited without prior permission.