3PDT Breakout 1590A — Build Documentation

Overview & Features

The 3PDT Breakout 1590A is a small, purely passive PCB that sits directly on top of a standard mechanical 3PDT footswitch and brings every connection out to clearly labelled solder pads on its edges. It replaces the awkward direct-to-lug wiring that makes 3PDT installs frustrating, and is sized to fit comfortably inside a 1590A enclosure (and, of course, anything bigger).

The board carries no active components. It implements the classic 3PDT true-bypass topology — input grounded in bypass to mute the effect, audio bypassed straight from input to output jack, and the indicator LED switched on or off in step with the relay state — entirely through the three poles of the mechanical switch.

Short-distance cable runs

All signal pads sit close to the corresponding switch lugs, keeping wire lengths short and tidy inside small enclosures.

Easy power connection

Dedicated +9V and GND pads on the board edge mean you don't have to share a single wire across multiple solder lugs.

Optional 4-pin Molex header

An optional 4-pin Molex connector (X1) brings Board-In, Board-Out, +9V and GND together — plug your effect PCB in with a single ribbon cable.

LED protection resistor on board

R2 sits on the board itself, in series with the indicator LED. No need to add a current-limit resistor elsewhere in the circuit.

1590A-friendly footprint

Sized to clear the LED hole and the I/O jacks of a 1590A enclosure with room to spare for the 3PDT switch hardware.

Standard 3PDT switching

Uses any common 9-lug 3PDT footswitch (Alpha, Mountain, BBE-style). The board sleeves over the lugs — no adapter needed.

Circuit Theory

The board is a passive routing layer for the three poles of a mechanical 3PDT switch. The schematic below shows all eight components — the switch itself (SW1 with poles A, B and C), one resistor (R2), one LED (D1), the optional Molex header (X1) and the four edge pads (IN, OUT, GND, and the implicit signal pads that connect to the Molex).

Full schematic — three poles of the 3PDT (SW1A, SW1B, SW1C), the LED branch with R2, and the Molex header X1 carrying Board-In, Board-Out, +9 V and GND between the breakout and the effect PCB.

Pole functions

Each of the three poles handles a different job. With the switch wiper in position 1 (effect bypassed), each wiper sits on its position-1 contact; in position 2 (effect engaged), it sits on its position-2 contact.

Pole	Wiper (lug 2)	Position 1 — bypass	Position 2 — engaged
SW1A	Board-In (X1-1)	GND (mutes effect input)	IN jack signal
SW1B	OUT jack signal	IN jack signal (bypass path)	Board-Out (effect return)
SW1C	R2 → LED → +9 V	Open (LED dark)	GND (LED lit)

Bypass state — effect off

In bypass, pole B routes the IN jack signal directly to the OUT jack. Pole A connects the effect's input (Board-In) to ground, which mutes any noise the effect circuit might still emit even though no signal is reaching it. Pole C leaves the LED branch open, so the LED is dark.

Engaged state — effect on

When the switch is pressed to its engaged position, pole A sends the IN jack signal to Board-In (effect input). The effect's output, returning through Board-Out, is connected by pole B to the OUT jack. Pole C completes the LED's path to ground, lighting the indicator. Power for the effect circuit and the LED itself is delivered through the +9 V pad on the board (or pin 3 of the Molex if the connector is fitted).

LED branch

The LED indicator sits between the +9 V rail and pole C of the switch:

+9V → D1 (LED) → R2 → SW1C wiper → SW1C-3 → GND (when engaged)

R2 is the LED current-limiting resistor. Because R2 sees the full 9 V rail directly (with no transistor or regulator in between) the same nominal value gives a brighter LED than it would in a typical buffered indicator circuit — see §03 for current values.

Power source for the LED: the +9 V that lights the LED comes from the same supply you wire to the +9V pad on the breakout. If you feed the effect circuit and the breakout from a single DC jack, everything tracks together: pull the power and the LED goes out.

LED Current Selection

R2 is the only value the builder sets to taste. Because the LED sees the full 9 V rail through R2 directly (no buffering), the resulting current is given by the simple Ohm's-law expression:

I_LED = (V_supply − V_LED) / R2

For a typical 9 V supply and a standard 2 V red, yellow or orange LED, the table below gives the resulting current for each E12 value in the recommended range. The board ships with a 1 kΩ resistor as supplied — comfortably bright for almost any common LED — but you are free to substitute any value from 1 kΩ to 10 kΩ to suit your particular LED and the brightness you want on stage.

R2	I_LED	Value	Colour code	Notes
1k	~ 7.0 mA	1.0 kΩ	Brown · Black · Black \| Brown · Brown	Shipped as default. Bright — suits standard diffused 3 mm / 5 mm LEDs.
1k5	~ 4.7 mA	1.5 kΩ	Brown · Green · Black \| Brown · Brown	Slightly less bright; good if 1 kΩ feels too intense.
2k2	~ 3.2 mA	2.2 kΩ	Red · Red · Black \| Brown · Brown	Comfortable on-stage brightness for most modern LEDs.
2k7	~ 2.6 mA	2.7 kΩ	Red · Violet · Black \| Brown · Brown	A common middle-ground value.
3k3	~ 2.1 mA	3.3 kΩ	Orange · Orange · Black \| Brown · Brown	Subtle indicator — pleasant in low light.
4k7	~ 1.5 mA	4.7 kΩ	Yellow · Violet · Black \| Brown · Brown	For high-efficiency / clear-package LEDs.
6k8	~ 1.0 mA	6.8 kΩ	Blue · Grey · Black \| Brown · Brown	Dim setting — when you don't want the LED to dominate.
10k	~ 0.7 mA	10 kΩ	Brown · Black · Black \| Red · Brown	Lowest practical value — only for very efficient blue / white LEDs.

Different supply voltage? If you run the pedal from 12 V or 18 V, recalculate as I_LED = (V − V_LED) / R2. The same R2 value will give a proportionally larger current at higher supply voltages — a 1 kΩ at 12 V gives roughly 10 mA, which is bright but still safely within standard LED ratings.

Blue / white LEDs: these have a higher forward voltage (typically 3.0–3.4 V) so the same resistor delivers slightly less current than the table above suggests. The values still work — the LED will simply be a touch dimmer than the listed value.

Bill of Materials

The breakout is a passive board with only three components on the PCB (R2, the LED, and the optional Molex header) plus the 3PDT switch that the board mounts onto. Every part is through-hole.

Board components

Ref	Qty	Value	Colour code	Notes
Resistor — through-hole, ¼ W metal film
R2	1	1 kΩ	Brown · Black · Black \| Brown · Brown	LED current-limiting resistor. Shipped as 1 kΩ; substitute 1 kΩ–10 kΩ to set LED brightness. tuning
LED indicator
D1	1	3 mm or 5 mm		Standard indicator LED. Long leg (anode) goes through the round-marked hole; short leg (cathode) goes through the square-marked hole. Choose any colour — see §03 for blue / white note.
Connector (optional)
X1	1	Molex 4-pin		Optional. Connector: Molex 22-23-2041 (Mouser 538-22-23-2041). Mating housing: Molex 22-01-2045 (Mouser 538-22-01-2045). Skip if you'd rather solder wires directly to the +9V, GND, B-I and B-O pads.
Switch (mounts through the centre of the board)
SW1	1	3PDT footswitch		Standard 9-lug PCB-mount 3PDT (e.g. Alpha 3PDT, Mountain Switch). Soldered through the 3×3 grid of holes in the centre of the board.

External (wired to pads)

Ref	Qty	Value	Type	Notes
IN jack	1	—	6.35 mm mono input jack	Tip wires to IN pad; sleeve wires to any GND pad on the board.
OUT jack	1	—	6.35 mm mono output jack	Tip wires to 3PDT OUT pad; sleeve wires to any GND pad.
DC input	1	9 V DC, centre-negative	Standard pedal DC jack	+ wires to +9V pad; − wires to any GND pad. Power feeds through to the effect PCB and to the LED.

Assembly

Build the board first, then mount it onto the 3PDT switch. The whole assembly stands as a single unit that you can drop into the enclosure with the switch already aligned to its mounting hole.

Fit R2 on the back of the PCB

Solder the chosen value of R2 (1 kΩ as supplied, or your substitute from §03) on the back side of the board. Fitting it on the back keeps the front clear of obstructions when the board sleeves over the switch lugs. Resistor orientation doesn't matter.

Optional Molex connector

If you're using the optional 4-pin Molex header, fit it on the back side of the PCB at the X1 footprint and solder. If you'd rather just solder wires directly to the B-I, B-O, +9V and GND pads, skip this step.

Mount the LED

Insert the LED through the two holes at the corner of the board. The long pin (anode) must go through the round-marked hole; the short pin (cathode) goes through the square-marked hole. Reversed polarity will not damage the LED but it will simply not light. Solder both leads on the back of the board and trim flush.

Fit the populated PCB onto the 3PDT switch

Slide the 3×3 grid of holes in the centre of the board over the nine lugs of your 3PDT footswitch. The board should sit flush against the body of the switch with all nine lugs poking through. Solder all nine lugs to the PCB on the front side.

Wire to jacks and effect circuit

Connect the input jack tip, output jack tip, DC supply and effect circuit (B-I, B-O, +9V, GND) to the corresponding pads as described in §06 and shown in §07. Use stranded hookup wire long enough to allow the board to be lifted out of the enclosure for service.

Power up and test

Apply 9 V DC. With a guitar plugged in at IN-JACK and an amp at OUT-JACK, press the footswitch to confirm true-bypass behaviour: signal should pass cleanly in bypass (LED off) and again through the effect circuit when engaged (LED on).

Wiring & Pads

All connections to the breakout are made through labelled pads along the edges of the PCB. The Molex header X1 is electrically equivalent to the four pads B-I, B-O, +9V and GND — fit either route, but not both.

Pad function

Pad	Function	Wire to
IN	Audio input from input jack	Input jack TIP
3PDT OUT	Audio output to output jack	Output jack TIP
+9V	Positive supply input (9 V DC)	DC jack centre pin
GND	Ground / 0 V (multiple GND pads — all electrically the same)	DC jack sleeve and audio jack sleeves
B-I	Board-In — drives the effect circuit's input	EFFECT_IN pad on the effect PCB
B-O	Board-Out — receives the effect circuit's output	EFFECT_OUT pad on the effect PCB

Optional Molex header X1

Pin	Signal	Equivalent pad
X1-1	Board-In	B-I
X1-2	Board-Out	B-O
X1-3	+9 V	+9V
X1-4	GND	GND

Fit the Molex header if you'd like to make your effect-PCB wiring detachable — useful for a modular pedalboard design or for swapping effect PCBs. Otherwise, solder the four wires directly to the corresponding pads.

Switching behaviour

State	Audio path	Effect input (B-I)	LED
BYPASS	IN → 3PDT OUT (direct)	Grounded — muted	Off
ENGAGED	IN → B-I → effect → B-O → 3PDT OUT	Driven by IN	On

One supply, one ground: the +9 V and GND used by the breakout are exactly the same nodes used by your effect PCB — they're shared via the +9V/GND pads (or pins 3/4 of the Molex). Wire them together at one place; don't run separate supply chains, or you'll end up with a ground loop.

Wiring Example

Below is the complete wiring diagram for the breakout used as a standard true-bypass switch. The effect PCB is shown above the breakout; the IN and OUT jacks sit on either side; the indicator LED hangs off the corner of the board.

Standard wiring: green = audio (IN to/from B-I, and B-O to OUT), orange = output to OUT jack, red = +9 V, black = GND. The breakout sits over the 3PDT footswitch lugs (visible 3×3 grid in the centre); the LED hangs off the corner; the effect PCB is wired to the four B-I / B-O / +9V / GND pads (or the equivalent Molex pins).

Effect PCB power: the same +9 V rail used to light the LED on the breakout is the supply for your effect circuit. Wire the effect PCB's +9 V and GND directly to the breakout's +9V and GND pads (or to pins 3 and 4 of the Molex header) — you don't need a separate connection from the DC jack to the effect.

1590A layout tip: in a 1590A the LED is usually mounted to the top of the enclosure, well away from the corner of the breakout PCB. Use a length of stranded wire from the LED to the breakout's LED holes rather than fitting the LED directly on the board, and remember to maintain anode/cathode polarity.

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Disclaimer & Licence

PCBs purchased from TH Custom Effects are intended for DIY and non-commercial use only. Redistribution of PCBs and artwork from this document is not permitted. You may use these instructions and PCBs to build and sell your own product based on PCBs ordered from TH Custom Effects.