2PDT Module

The board is organised as four small functional blocks: a regulated 5 V supply, an NE555-based bistable flip-flop, a single-transistor relay driver, and the relay itself with its associated indicator LED.

Power supply

The 9–12 V input enters through the +9V pad and is regulated down to a clean 5 V by IC1, an LM1117-5.0 LDO. Bulk smoothing is provided by C2 (10 µF electrolytic) and high-frequency bypass by C3 (100 nF ceramic). The regulated 5 V rail is labelled VD internally and supplies the NE555, the relay coil, and the LED branch.

NE555 bistable flip-flop

The toggle action is implemented by U1, an NE555 timer wired as a bistable (not as the more familiar astable or monostable). The trick is to tie pins 2 (TRIG) and 6 (THR) together at a common node, then bias that node halfway between the chip's two internal threshold voltages — V_TRIG = ⅓ V_CC = 1.67 V and V_THR = ⅔ V_CC = 3.33 V.

Resistors R6 (10 kΩ from VD) and R7 (10 kΩ to GND) form the divider that holds the trigger node at 2.5 V — exactly between the two thresholds, where the 555's internal flip-flop happily stays in whichever state it is in. C4 (220 nF) and R8 (220 kΩ) form the toggle network: C4 charges through R8 toward whatever state OUT (pin 3) is currently in, and provides the brief voltage push needed to flip the 555 the next time the user closes the momentary switch.

When the external momentary switch is pressed, it shorts the SW pad (one side of C4) to the SW2 pad (the trigger node). This briefly couples the charged side of C4 onto the trigger node, forcing it across whichever threshold is appropriate to flip the 555's state. C4 then discharges through R8 to the new OUT level, ready for the next press.

Relay driver and indicator

The 555 OUT pin drives the base of Q1 (BC847 NPN BJT) through current-limit resistor R3 (10 kΩ). When OUT is high, Q1 saturates and pulls the relay coil and LED branch to ground; when OUT is low, Q1 is off and the relay drops out. Flyback diode D1 (1N4151) clamps the inductive kick from the relay coil when it de-energises.

The LED is wired in series with the relay coil through the user-selected CLR resistor — so the LED follows the relay state directly: lit when the relay is energised, dark when it is not.

Relay contacts

The relay K2 is an Omron G6K-2F-Y, a sealed signal relay with two independent changeover poles brought out to six pads at the edge of the board. In each pole the common terminal connects to NC when the coil is de-energised and switches to NO when the coil is energised.

K2 (G6K-2F-Y) — DPDT contact arrangement. Pole 1: NC1 / COM1 / NO1 on pads marked 7 / 6 / 5. Pole 2: NC2 / COM2 / NO2 on pads marked 2 / 3 / 4. Coil is on pins 1 and 8 (driven internally).

The toggle behaviour, indicator current, and relay drive of the module are all set by a small handful of components — most of which are surface-mount and already populated. The values below are calculated directly from the schematic and should give you a feel for how the module behaves.

Toggle network timing

The RC network around the trigger node sets how quickly the 555's input recovers between presses. C4 charges through R8 toward the new OUT level after each toggle:

This time constant determines the minimum interval between two clean toggles. Press faster than roughly 50–100 ms apart and the 555 may not have settled to its new steady state — pressing about twice per second or slower is well within the design margin and feels natural in use. The R6/R7 divider holds the trigger node at:

This sits squarely between the 555's two thresholds (1.67 V and 3.33 V), which is what gives the bistable its rock-solid two-state behaviour.

LED indicator current

The CLR resistor is the only through-hole part the builder fits. It sits in series with the LED in the relay-coil branch, so it sets both the LED brightness and a small contribution to the relay-coil current. With Q1 saturated, the loop voltage available for the LED branch is roughly V_CC − V_LED − V_CE(sat) ≈ 2.8 V (assuming a 2 V red/yellow LED).

CLR options for different LEDs

The module ships with a 2.7 kΩ resistor for CLR which gives roughly 1 mA through a standard red or yellow LED — comfortably visible on a darkened pedalboard but not glaring. If your LED is dim, use a smaller value; if it is too bright (or you are using a high-efficiency modern LED), use a larger value. Any 1.5 kΩ–10 kΩ ¼ W metal film resistor will work.

The 2PDT module is shipped fully assembled — the regulator, NE555, transistor, relay, flyback diode, all SMD passives and the bulk capacitors are already on the board. The list below shows only the parts you fit yourself and the parts you wire to the module. Everything else is on board and tested at the factory.

Builder-fitted parts

External parts (wired to pads)

On-board (already fitted, for reference)

Because the module ships preassembled, the build is genuinely a single soldering step plus the external wiring. Follow the steps below in order — wire the switch, LED and signal jacks last so you can route the leads cleanly inside your enclosure.

All connections to the module are made through labelled pads along the edges of the PCB. The full pad list is below — match each external part to its corresponding pad.

Pad function

Contact behaviour

The two poles operate together — they always switch from NC to NO at the same instant. This makes the module a true 2PDT replacement: you have full freedom to use either or both poles, with whatever signal-routing scheme suits your project.

Two complete wiring examples are shown below — both use the module as a 2PDT A/B switch with a single input and two switchable outputs. The difference is what happens to the inactive output (grounded vs. left open) and how the indicator LED is driven.

Example 1 — A/B switch with inactive OUT grounded

The simplest A/B configuration: input goes to COM1, the two outputs come off NC1 (output A — selected when relay is OFF) and NO1 (output B — selected when relay is ON). The second pole is used to ground the inactive output, which prevents any signal bleed and gives you absolute silence on the output that isn't selected. A single LED indicates that B is active.

Example 1 — A/B switch with inactive OUT grounded. Single indicator LED lights when output B is active. Pole 1 routes signal; pole 2 grounds the unused output.

Example 2 — A/B switch with inactive OUT open, dual LEDs

The same routing but instead of grounding the unused output, both poles are used to drive a pair of LEDs — one for each side of the A/B selection. This is ideal for an A/B box where you want clear visual feedback of which output is active without any audible interaction between them.

Example 2 — A/B switch with dual LEDs (one per output). LED A indicates that output A is active; LED B indicates that output B is active. The inactive output is left open.

The 2PDT Module sold by TH Custom Effects is intended for DIY and small-scale builds. This module may be used in commercial builds as long as the on-board logo and silkscreen are clearly visible and have not been removed, sanded off, or otherwise obfuscated. Redistribution of the bare PCB or of the artwork from this document is not permitted.

You may use these instructions and the module to build and sell your own product based on modules ordered from TH Custom Effects.

© TH Custom Effects 2024–2026. Build documentation V1.0.