Miniature RTB

Full schematic — Miniature RTB V1.0 / Digital Bypass Switch (TH Custom Effects, 03/2014). The V1.1 silkscreen ships with the same circuit; minor layout refinements only.

Architecture

The board has three functional blocks: a 5 V power supply (REG1, C2, C3); the microcontroller (IC2 — PIC10F202 in DIP-8); and the relay drive (Q1, R3, K1, D1, plus the LED on R2). The audio signal itself only ever touches the relay contacts — it never goes anywhere near the digital side.

Switching sequence

Each press of the momentary footswitch (S1) sends a short pulse to the PIC's GP0 pin. Firmware:

1. Debounces the switch input — the firmware ignores any signal bounce within a few ms after the press is registered. This protects against false toggling from mechanical contact bounce.
2. Toggles the internal state variable — engaged or bypassed.
3. Drives Q1's base via R3 from GP2/T0CLK. When Q1 saturates it pulls K1's coil low, energising the relay; when Q1 is off the relay coil is at +5 V on both ends and the relay de-energises.
4. Updates the LED — Q1's collector also drives the status LED via R2 (the LED is in series with R2 and lights when Q1 conducts).

The relay: K1 (Fujitsu FTR-B4C)

This is a subminiature DPDT signal relay with a 4.5 V coil. One pole switches the input side, the other switches the output side — this gives proper two-pole true-bypass routing. When energised, the relay routes signal through the effect (A_IN → A_TO → ... → A_FRM → A_OUT); when de-energised, it bypasses (A_IN → A_OUT directly). D1 (1N5817 Schottky) is the standard flyback diode across the coil — it absorbs the inductive kickback when the relay turns off, protecting Q1's collector from the voltage spike.

The microcontroller: IC2 (PIC10F202)

A small PIC10F202 in DIP-8 — pre-programmed with the same firmware as the Ultimate Switch. Pin assignments:

• Pin 2 (VDD): +5 V supply from REG1.
• Pin 7 (VSS): ground.
• Pin 5 (GP0): footswitch input (SW_A-1 pad).
• Pin 4 (GP1): not used in this circuit.
• Pin 3 (GP2/T0CLK): output to Q1's base via R3.
• Pin 8 (GP3/MCLR): Power-On option pin — see below.

Power-On option (pin 8)

By default the effect powers up in the bypassed state. If you want the opposite — effect on when power is applied — solder a wire from pin 8 of the IC socket directly to GND. This grounds the GP3/MCLR pin, which the firmware reads at startup and uses to choose the initial state. Useful for always-on effects (boost, buffer, EQ) where you'd otherwise have to step on the switch every time you power up. Leave pin 8 floating for the default behaviour.

Power supply

9 V DC enters at the +9V pad. REG1 (78L05) drops it to a stable +5 V (called VD internally). C2 (100 µF) is the main 5 V rail filter; C3 (100 n MLCC) is the local bypass at the PIC's VDD pin. This board has no reverse-polarity protection — if the Miniature RTB is the only thing in your enclosure, add a 1N4001 or 1N5817 in series with the +9 V supply externally for protection.

Eight labelled pads, grouped on the four edges of the small PCB.

Pad locations — silkscreen on the actual PCB matches what you see here. Audio pads on the top edge; control pads on the right and left edges.

Audio pads (top edge)

In bypass mode the relay routes A_IN → A_OUT directly (the effect is skipped). In active mode the relay routes A_IN → A_TO (into the effect) and A_FRM → A_OUT (out of the effect, back to the chain).

Control pads (right and left edges)

The LED also has its own pads (visible at the bottom-left of the layout image, marked with the LED silkscreen symbol) — square hole for the long leg (anode), round hole for the short leg (cathode).

Tiny BOM — that's the point. Two resistors, two caps, one diode, one transistor, one relay, one IC, one regulator. Plus the LED and footswitch as off-board components.

Small board, few parts, but two important quirks: Q1 and REG1 sit very close together on the top side and need their leads bent to "bow" toward each other before soldering, and the relay K1 plus C2 mount on the back side of the PCB. Read steps 4 and 7 carefully before starting.

Resistors and diode

Lowest profile components first. Both resistors are 1 k — R2 (CLR) and R3. The colour-band column in the BOM helps verify each. Then fit D1 (1N5817 or 1N4001) — polarity matters; band toward the relay coil + side per the silkscreen.

IC socket — if there's room

Fit a DIP-8 socket for IC2 if your build has clearance — the board is tight but there's usually space for a low-profile socket. Don't insert the chip yet; match the socket's notch to the silkscreen. If space doesn't allow a socket, solder the PIC directly into the board (matching the notch), accepting that you can't easily replace it later.

MLCC capacitor (C3)

100 n MLCC — non-polarised, no orientation. Push it flat against the board.

Transistor and regulator — bow alignment

This is the only fiddly step. Q1 (BC549) and REG1 (78L05) are both TO-92 packages, and the PCB places them facing each other with very little space between. Their lead-spread footprints overlap visually on the silkscreen. Procedure:

1. Test-fit Q1 and REG1 first by inserting them into the board without soldering.
2. Their bodies should sit close to each other but not touching — bend the leads slightly as needed so the two TO-92s "bow" toward each other and clear the limited space.
3. Once they fit cleanly without forcing, solder them in place.

Q1 (BC549) pinout: C–B–E (flat side facing you, leads down) — match flat side to silkscreen flat. If substituting 2N5088, insert it rotated 180° (its pinout is reversed). REG1 (78L05) pinout varies by manufacturer — verify from the datasheet of your specific brand before soldering. Wrong orientation may damage the regulator.

Off-board connections (LED, footswitch, audio pads)

If you're going to mount this in a 1590A you'll route off-board wires for the LED, the footswitch, and the audio in / out / power. You can leave that until §06 / §07 — the board itself is now mostly populated.

Insert the PIC

If you fitted a socket, insert IC2 (PIC10F202, pre-programmed) into it. Match the notch on the IC to the notch on the socket.

Flip the board — relay and electrolytic on the back side

Now turn the PCB over. The remaining two parts both go on the back side: K1 (Fujitsu FTR-B4C relay) and C2 (100 µF electrolytic). Push the relay into its 8-pin footprint — the relay's white plastic body has a small marker indicating pin 1; match it to the silkscreen orientation, then solder all eight pins from the top side. Fit C2 next — polarised, long lead is positive; match the (+) on the silkscreen. Pick a low-profile, narrow can (5 mm Ø) so it sits flush. Solder both from the top side.

Back side of the populated PCB — Fujitsu FTR-B4C relay (white) plus C2 (blue electrolytic, 5 mm Ø). Both mount on this side; the rest of the components are on the top side.

Power-On option (optional)

If you want the effect on at power-up: solder a small wire from pin 8 of the PIC socket (or pin 8 of the chip if it's not socketed) to the nearest GND point on the board. This grounds GP3 / MCLR. Leave it disconnected for the default off-at-power-up behaviour.

The Miniature RTB is dimensioned to fit inside a 1590A — the smallest standard die-cast pedal enclosure. The board sits on top of the momentary footswitch and is held in place by a piece of double-sided tape on the switch body.

Drill template (1590A)

Switch contact clipping

Most momentary footswitches have contact terminals that protrude further than the switch body itself. To get the PCB to sit flat against the back of the switch, clip the switch contacts down to about the same depth as the switch body. Use side cutters and clip carefully — make sure no clipped pin shorts to anything when the PCB is pressed against it.

Mounting procedure

PCB mounted directly on the back of the momentary footswitch using double-sided tape, inside an aluminium 1590A enclosure. The LED's long leads reach forward to its front-panel hole. Visible underneath: the red layer of double-sided tape between the switch body and the PCB.

This looks more complicated than it actually is — at heart it's just an input jack going to A_IN, an output jack receiving from A_OUT, the effect input/output going to A_TO/A_FRM, and the power + footswitch + LED on the control side.

Standard wiring of a Miniature RTB with one effect inside the enclosure. The effect's input/output go to A_TO and A_FRM; the input and output jacks go to A_IN and A_OUT.

Connection summary

Final note

You now have a relay bypass that fits almost any tiny space. Ideal for replacing failing 3PDT mechanical switches with something more reliable, quieter under foot, and with proper LED status indication.