RTB 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. The two relay poles are wired internally to form a complete true-bypass network — the builder only sees the four signal pads (IN, OUT, TO, FRM) and does not have to think about the underlying contact arrangement.

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 tantalum) and high-frequency bypass by C3 (100 nF ceramic). The regulated 5 V rail 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). Pins 2 (TRIG) and 6 (THR) are tied together at a common trigger node and biased 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 the OUT line 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 (effect engaged), dark when it is not (bypass).

Internal true-bypass network

The two changeover poles of K2 are wired together internally to form the standard true-bypass topology. With four signal pads on the edge of the board:

In bypass (relay de-energised, NC contacts active): pole 1 connects IN → OUT, so the audio passes straight through. Pole 2 connects TO → GND, muting the effect input to prevent any chance of the (still-powered) effect bleeding signal or picking up noise on its input.

When engaged (relay energised, NO contacts active): pole 2 connects IN → TO, sending the input audio into the effect. Pole 1 connects FRM → OUT, sending the effect's output to the output jack. The signal now passes through the effect, exactly as on a mechanical 3PDT.

The toggle behaviour, indicator current, and relay drive 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.

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. 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 RTB 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 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

Switching behaviour

The two relay poles always switch together at the same instant. In the bypass state the input jack drives the output jack directly through pole 1, while pole 2 holds the effect input at ground potential — the effect circuit cannot inject any noise into the signal path because its input is shorted. In the engaged state pole 2 sends the input audio to the effect, and pole 1 returns the effect's output to the output jack.

Below is the complete wiring diagram for using the RTB module as a standard true-bypass pedal switch. The input jack tip goes to IN; the output jack tip comes from OUT; the effect circuit's input is wired to TO and its output is wired back to FRM. All sleeves and the effect's GND go to a common ground (any GND pad on the module is electrically the same point).

Standard true-bypass wiring: blue = input signal (IN), green = output signal (OUT), magenta = signal to/from the effect (TO and FRM), red = +9 V, black = GND, orange = momentary footswitch, with the indicator LED above the module.

The RTB 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 V1.1.