NOPE-Switch V1.1 — Build Documentation

Overview & Features

The NOPE-Switch is a self-contained true-bypass switcher built around a CMOS NAND-latch toggle and a small DPDT signal relay. A momentary soft-touch footswitch toggles the latch; the relay then routes the input and output signals either through your effect or straight from input to output. No microcontroller, no firmware, no boot delay — just a 50 ¢ logic IC and a relay.

Compared to a mechanical 3PDT footswitch, you get:

Silent, click-free bypass — no contact pop, no scratchy switch over time
Soft-touch action from any momentary footswitch — easy on the foot, no kick
True bypass — both signal poles are mechanically disconnected when off
Compact — the PCB is small enough to retrofit alongside most existing pedal boards

It is a "drop-in" replacement for the 3PDT in any true-bypass effect: connect input/output jacks to the A_FRM and A_OUT pads, the effect's input/output to the A_TO and A_IN pads, share ground, share +9V, and you're done.

Populated V1.1 prototype. The relay (left), the CD40107 in DIP-8 (right), 78L05 in TO-92 (top centre), and the box-film C1 timing cap (bottom).

Circuit Theory

The CD40107 and why it suits this job

The CD40107 is a dual 2-input NAND buffer with open-drain N-channel outputs. Each gate has only one output pin (no complementary push-pull), and that output transistor can sink roughly 50 mA at V_DD = 5 V. There's no high side — the load supplies its own pull-up to V_DD. That's exactly what's wanted for driving a small relay coil and an LED directly from the gate output.

In this design both gates are wired as inverters (the two inputs tied together) and cross-coupled to form a stable bistable latch:

NAND1 inputs at the relay/output node; output drives the latch-state line through pull-up R3 = 10 k to V_DD.
NAND2 inputs at the latch-state line; output drives the relay coil, the LED through R2, and forms the other half of the latch loop.

Toggle action — R1, C1 and the momentary switch

The latch on its own is bistable — both states are stable. The momentary switch needs to flip it from outside, and the flip must be debounced against contact bounce. Here's how:

C1 = 330 n sits between the latch-state line (through R1 = 68 k) and ground. In any steady state, C1 has charged to the current latch-state voltage. The momentary switch, when pressed, briefly connects the relay-output node to the C1/R1 junction (the SW and SW. pads). Because the latch-state line and the relay-output node are always at opposite polarities (one inverts the other), this momentary connection drags the output node toward the old latch-state voltage — i.e. the opposite of where it currently sits. That kick flips the latch.

After the flip, the new state has to propagate back into C1, which takes one RC time constant through R1:

τ = R1 · C1 = 68 kΩ × 330 nF = 22.4 ms

During those ~22 ms, the latch is "deaf" to further switch events — that's the debounce window. Mechanical bounce on a momentary switch typically settles inside 10–20 ms, so this RC is well-chosen for the application.

R1 and C1 are critical values. The BOM marks them with asterisks for a reason — change either, and you change the debounce window. Too short and you'll get double-triggers from bounce; too long and the switch feels sluggish. Keep R1 = 68 k and C1 = 330 n box film.

Relay and LED drive

The relay coil (Fujitsu FTR-B3CA-4.5Z, 4.5 V nominal, ≈ 160 Ω, ≈ 28 mA) sits between V_DD (+5 V) and the NAND2 output. When NAND2 sinks (latch state HIGH), about 31 mA flows through the coil and the relay closes its two signal poles — well within the CD40107's 50 mA sink rating. D1 (1N5817) is a flyback diode across the coil: cathode at V_DD, anode at the NAND2 output. It clamps the back-EMF when the coil de-energises, protecting the open-drain transistor.

The LED hangs off the same node through R2 (the current-limit resistor — see §03). When NAND2 sinks, LED current flows from V_DD → R2 → LED → NAND2 output → ground. LED on means "effect engaged".

Power supply

+9 V (centre-negative pedal-standard) enters the board and feeds the 78L05 regulator (TO-92, 100 mA). The regulator produces a clean +5 V rail (V_DD) for the CD40107 and the relay coil. C2 (100 µF or 220 µF electrolytic) provides bulk filtering on the input side; C3 (100 n film) handles high-frequency decoupling on the output rail. Total current draw is about 35–40 mA on the +9 V supply when engaged, sub-mA when bypassed (the CMOS logic is essentially zero-current).

Signal-path pinout

The two relay poles route the audio. With the relay energised (effect engaged), the signal goes from input through the effect; with the relay de-energised (bypassed), input goes straight to output.

Pad	Engaged (relay on)	Bypassed (relay off)
A_FRM	→ A_TO (signal goes to effect input)	→ A_OUT (signal goes straight to output)
A_IN	→ A_OUT (effect output goes to jack)	(disconnected from the bypass path)
GND.	Effect's signal ground — must connect to main GND	—

"FRM" = from input jack; "TO" = to effect input; "IN" = from effect output; "OUT" = to output jack. The labelling is read in the direction of the signal flow.

LED Current Limit — Sizing R2

R2 is the current-limit resistor for the indicator LED. It's not a fixed value because LEDs vary enormously in efficiency and forward voltage. The BOM suggests 3k3 as a starting point; the right value depends on the LED you actually use.

Ohm's law on the LED loop, with V_DD = 5 V and the NAND2 output sinking to ≈ 0.5 V when ON:

R2 = (V_DD − V_LED − V_OL) / I_LED = (5 − V_LED − 0.5) / I_LED

Typical numbers (assuming the LED to feel comfortably bright through a panel-mount bezel, not blinding):

LED type	V_f	Modest brightness (≈ 1 mA)	Indicator-bright (≈ 5 mA)
Red, low-eff	1.8 V	2k7	560 Ω
Red/yellow, high-eff	1.8–2.0 V	3k3	680 Ω
Green	2.2 V	2k4	470 Ω
Blue/white	3.0–3.4 V	1k5	330 Ω

How to land on a value. Start with the BOM's 3k3 suggestion. With most modern high-efficiency LEDs that's enough brightness for a stage indicator. If the LED is too dim, drop to 1k–2k; if too bright (or you're using a high-output 5 mm) climb to 4k7 or 10k. Anything above 100 Ω will keep the CD40107 inside its sink rating, so there's no risk in experimenting.

Why so little current? Modern indicator LEDs are 10–100× more efficient than the parts on which the "20 mA = bright" rule was based. 1 mA is genuinely bright with a clear bezel; 5 mA borders on dazzling.

Bill of Materials

Ref	Qty	Value	Colour code	Notes
Resistors
R1	1	68k	Blue · Grey · Black \| Red · Brown	Metal film ¼ W. Critical value — sets the debounce time constant with C1. Do not substitute.
R2	1	CLR (e.g. 3k3)		Current-limit resistor for the indicator LED. See §03 for sizing — depends on LED type and desired brightness.
R3	1	10k	Brown · Black · Black \| Red · Brown	Metal film ¼ W. Pull-up on the latch state node.
Capacitors
C1	1	330n		Box film, do not substitute. Sets the debounce RC together with R1. Schematic library lists ceramic; box film is preferred for stable timing.
C2	1	100µ – 220µ		Polarised electrolytic, ≥16 V. Power-rail bulk filter. Limited height & width — use a low-profile part. Observe polarity.
C3	1	100n		Box film. High-frequency decoupling on the 5 V rail.
Diodes
D1	1	1N5817		Schottky — flyback / freewheel diode across the relay coil. Observe polarity (band = cathode).
LED	1	3 mm or 5 mm		Colour and type of your choice. The PCB has on-board pads, but in practice the indicator LED usually mounts on the enclosure and wires to the LED solder pads on the PCB.
Integrated circuits
IC1	1	CD40107		Dual 2-input NAND buffer with open-drain outputs (≈ 50 mA sink at 5 V). DIP-8. HEF40107 HCF40107 are direct equivalents. Use a socket.
REG	1	78L05		TO-92 5 V regulator (100 mA). L78L05 MC78L05 compatible. Note the pinout — TO-92 78L05 has IN/GND/OUT in a non-obvious order; check the package datasheet before inserting.
Switching, relay, hardware
K1	1	FTR-B3CA-4.5Z		Fujitsu DPDT subminiature relay, 4.5 V coil. FTR-B3GA-4.5Z (same family, pin-compatible) also works. Mechanically latching with hold current — this design uses the non-latching version.
Switch S1	1	Momentary SPST		Normally open. Off-board; wires to the SW + SW. pads. Any tactile or footswitch momentary will work.

"Do not substitute" parts. R1, C1, and to a lesser extent C2 are functionally critical. R1·C1 sets the debounce; changing C1 to ceramic risks unstable timing as the dielectric drifts with temperature and DC bias (use a box film capacitor — the schematic library happens to call it ceramic, but the docx and prototype both use film). C2 size matters because PCB clearance to the enclosure limits the cap height — pick a low-profile 100 µF or 220 µF radial.

78L05 pinout warning. The TO-92 78L05 has an unusual pinout (IN-GND-OUT, but read in a non-obvious order depending on the manufacturer's flat-side orientation). It is not the same as a typical TO-92 transistor (E-B-C). Check the datasheet for your exact part before bending the leads — getting it wrong puts +9 V on the IC's V_DD pin, which the CD40107 will not survive.

Build Guide

PCB top silkscreen, V1.1. Note the secondary +9V1 pad on the left edge — a convenient daisy-chain for the effect circuit it's switching.

Standard population order — lowest first, tallest last.

Resistors

R1 (68k), R2 (your CLR value), R3 (10k). Bend, insert, flip-and-press, solder, clip.

Diode D1 (1N5817)

Schottky flyback diode. The banded end is the cathode — match it against the silkscreen mark. Polarity matters: backwards and D1 will short V_DD to ground through the regulator the moment you apply power.

IC socket and relay

Drop in the DIP-8 socket for IC1 and the relay K1. Match the IC socket notch to the silkscreen, and make sure the relay sits flat (it has a single orientation — the pin spacing only fits one way).

78L05 regulator

TO-92 package. Double-check the pinout against the datasheet for your specific brand — the flat side does not always face the same way as a transistor. Bend the leads to match the silkscreen footprint and seat the body flat on the board.

Film capacitors

C1 (330n box film) and C3 (100n). No polarity. C1 is the timing cap — use a film type, not ceramic, even if the schematic library called the footprint a ceramic.

Electrolytic C2

Long leg = positive. The silkscreen marks the + side; the cap's body marks the − side. Use a low-profile 100 µF or 220 µF to clear the enclosure lid.

Insert IC1

Match the notch on the CD40107 to the notch on the socket. Press evenly.

Skip the on-board LED footprint. The PCB has solder pads for an LED but the indicator LED is almost always mounted on the enclosure top so you can see it. Wire the panel-mount LED to the LED pads on the PCB — anode to the pad next to R2, cathode to the pad next to IC1. R2 stays on the board.

Wiring & Integration

The NOPE-Switch lives between your input/output jacks and the effect PCB. Five signal connections, three power/ground connections, plus the footswitch and LED.

Pad reference

Pad	Connect to	Notes
A_FRM	Input jack TIP	"From" — the signal entering the pedal
A_TO	Effect input	"To" the effect
A_IN	Effect output	"In" from the effect (returning to the switch)
A_OUT	Output jack TIP	"Out" of the pedal
GND.	Effect signal ground & jack sleeves	Audio ground — must link to main GND
+9V	DC jack centre pin (or +9V from effect)	Power input — 9 V centre-negative
+9V1	Effect's +9V input	Daisy-chain — same rail as +9V, separate pad for convenience
GND	DC jack sleeve, effect ground	Main return — link to GND.
SW, SW.	Momentary footswitch	Order doesn't matter; the switch is just a contact closure between the two pads
LED	Panel-mount LED (anode & cathode)	The on-board R2 limits the current — no extra resistor needed at the LED

You must link A_GND (the GND. pad) to main GND. The audio ground (GND. — at the jack sleeves and the effect's input/output reference) and the main power ground are physically separated on the PCB and need a wire jumper between them at one point. Skipping this leaves the audio side ungrounded — you'll get hum, no signal, or both. Make this connection at the effect's ground point, not at the jacks, to avoid ground loops.

Reference wiring diagram

NOPE-Switch wiring diagram for a typical effect

Complete wiring of the NOPE-Switch into a typical effect circuit. Note the black ground wires meeting at the effect's GND — this is the GND. ↔ GND link. The footswitch (top left) bridges the SW and SW. pads; the LED hangs off the LED pads with R2 already on the board.

Choosing the footswitch

Any momentary SPST normally-open contact works. Common options:

Stomp-style soft-touch footswitch (e.g. PBS-24B-2): the classic soft-touch, low-force, click-free option. Best for boards where you want to lose the mechanical 3PDT clunk.
Tactile button mounted on the enclosure: fine for testing or for panel-mount on a control box; not robust under a foot.
Standard momentary footswitch: works mechanically but the louder click defeats one of the main reasons to use this board.

Bringing It Up

Before connecting to the effect's signal, verify the switcher works on its own.

Power and bias check

Connect +9V and GND only. Power up. Measure the regulator output — pin 8 of the IC1 socket should read 5.0 V ± 5 %. If it reads 0 V or 9 V, the 78L05 pinout is probably wrong (see the BOM warning) — power off immediately and re-check.

Insert IC1 and test toggle

With the power off, press IC1 into the socket. Power back up. The relay should be in one state (audible click is unlikely with this subminiature type, but you can hear a faint snap). Press the footswitch — the relay flips state. Press again — flips back. The LED tracks the state.

Continuity check the bypass path

Power off. With a meter on continuity: between A_FRM and A_OUT, the connection should toggle (engaged vs bypass) as you manually swap the relay state via short power cycles or by holding the switch.

Connect signals and test through an effect

Power off. Wire A_FRM/A_TO/A_IN/A_OUT and the GND link as in §06. Plug a guitar into IN and an amp into OUT. Power up, play a note, press the footswitch — your effect should switch in and out cleanly with no clicks or pops in the signal.

Common failure modes. Silent signal in both states usually means the GND link is missing or the jack sleeves aren't grounded. Signal cuts out in the engaged state means the A_TO/A_IN pair is reversed (or one is open). Constant LED with no toggle means the switch isn't making contact — verify continuity across SW/SW. when pressed.

This little tool fits almost anywhere. Replace your noisy 3PDT, or make your pedal true-bypass for the first time.

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