Overview & Specs
The Signal Injector / Tracer Pen is a self-contained debugging tool — a pocket-sized PCB shaped like a pen, with everything an audio-circuit troubleshooter needs in one place: a small audio amplifier feeding an on-board loudspeaker (so you can hear what's at any node) and a signal generator (so you can inject a known test tone). Two slider switches on top of the board configure the modes; a probe tip at one end touches into the circuit under test.
The completed pen. Test pin at the tip (left), two slider switches mid-body, oval loudspeaker right of centre, off-board pads (GND / SIGN / PWR / SPKR) at the right end. The blue lead is the test clip used for signal injection; the metallic barrel jacks are the T-connection power inputs.
What ships in the kit
The PCB arrives with all SMD components pre-soldered. There is nothing to populate on the board itself. You only need to fit and wire up the off-board parts — see §03 for the BOM and §04 for the assembly.
Specifications
| Supply voltage | 9 V DC recommended (operating range up to 12 V max) |
| Polarity | Centre-positive at the PWR pad — reverse polarity will cause unrepairable damage |
| Test signal output (low range) | ≈ 160 mV at 9 V supply (suitable for low-level signal stages) |
| Test signal output (high range) | ≈ 1.8 V at 9 V supply (suitable for line-level / output stages) |
| Loudspeaker | 0.5–1 W, 8 Ω, 20 × 15 mm or 23 mm round |
| Modes (per slider switch) | Burst / Alternate · 200 mV / 2 V signal level |
| Form factor | Pen-shaped PCB with brass-nail or proper test-pin tip |
Modes & Use Cases
The pen has four core use cases. Each one uses a different combination of switch positions, test-pin placement, and signal source. Most debugging sessions use two or three of them in sequence to localise a fault.
1. Signal tracing
Touch the test pin to a node in a circuit that should have audio at it (e.g. the wiper of a volume pot, an op-amp output, a coupling cap). The pen amplifies whatever it picks up and plays it through the on-board loudspeaker. This is the fastest way to find a dead stage: walk the probe from input toward output and listen — the audio will be present up to the broken stage and absent after it.
2. Signal injection + tracing
Connect the test clip to a node early in the chain (e.g. the input jack tip, or just after a coupling cap). The pen feeds a known test signal in. Then move the probe pin downstream to confirm the signal reaches each subsequent stage. Useful when you have a circuit that's missing both an input source and output, or when you want a clean known reference signal instead of guitar pickup noise.
3. Signal generator only
The device under test (DUT) is already plugged into a guitar amp. The pen's test clip injects a signal at the DUT's input. You hear the result through the amp, not the pen's loudspeaker. Useful for checking gain, tone shaping or distortion character at full volume — exactly as the player would hear it.
4. Continuity tester
Touch the test pin and test clip together — or one to each end of a wire / track / solder joint you want to test. The pen sounds when there's continuity. Quick way to check switch contacts, jack soldering, ground integrity and broken traces without unholstering a multimeter.
The two slider switches
The board has two slider switches, labelled on the silkscreen:
| Switch | Positions | What it does |
|---|---|---|
| burst / alternate | BURST · ALTERNATE | Burst sends short bursts of test tone with gaps in between — easy to identify in noisy circuits. Alternate outputs a continuous alternating waveform — useful for steady-state listening tests. |
| 2V / 200mV | 2 V · 200 mV | Selects the test signal level at the SIGN output. 200 mV for low-level signal stages (instrument-level / pre-gain nodes). 2 V for line-level or output-stage testing where the higher level is required to drive the stage cleanly. |
Off-board Parts
The PCB ships pre-populated; the only items to source / fit are the off-board mechanical parts. Everything below is what you wire up in §04.
| Part | Qty | Value / type | Notes |
|---|---|---|---|
| SW1, SW2 | 2 | 1PDT miniature slider | 2.54 mm pinout. If the parts you receive have a second row of pins, cut one row off — see §04 step 1 and the photo there. Make sure no contact remains to the bottom side of the PCB. |
| Loudspeaker | 1 | 0.5 – 1 W, 8 Ω | 20 × 15 mm rectangular or 23 mm round driver. Solder leads to the SPKR / GND pads. |
| Test pin | 1 | 1 mm test pin | A proper sprung test probe is ideal. Brass nail works as a substitute if you don't have a real test pin handy. |
| Test clip | 1 | Microhook / IC clip | Used to inject signal into the circuit under test (see photo below). 6.3 mm phono connector works as an alternative if you prefer to inject via a jack lead. |
| Power connector (male) | 1 | 2.1 mm DC plug | For supply input. Both connectors below are needed if you want a T-connection that powers the pen and the DUT from a single supply. |
| Power connector (female) | 1 | 2.1 mm DC jack | For the T-connection — passes power through to the DUT. |
A typical microhook test clip. The sprung hook clamps onto a wire, IC pin or pad; the wire on top connects to the pen's SIGN pad.
Assembly & Wiring
Total assembly time is small — most of it is fitting the slider switches and dressing the off-board cables for strain relief.
The off-board pads
At the right-hand end of the PCB you'll see four labelled pads in a row, plus two larger holes nearby. The pads are your wiring points; the larger holes are strain-relief holes — pass each cable through one before soldering, so any pull on the wire is absorbed by the hole rather than the solder joint.
Right-hand end of the PCB. Pad labels (top to bottom): GND, SIGN, PWR, SPKR. Two large strain-relief holes for routing wires before soldering.
| Pad | Connect to | Notes |
|---|---|---|
| GND | Power supply ground · loudspeaker (one terminal) · DUT ground (via T-connection) | Common ground. |
| SIGN | Test clip (microhook) | Test signal output. The 200 mV / 2 V slider sets which level appears here. |
| PWR | +9 V from the male DC plug · pass-through to female DC jack for T-connection | Centre-positive. Verify polarity before powering up — see §01 warning. |
| SPKR | Loudspeaker (other terminal) | Speaker drive output from the LM386. Polarity doesn't matter on the speaker. |
Assembly steps
Inspect the slider switches
Some batches of these miniature sliders have a second row of pins on the underside (see photo below). The PCB only accommodates one row. If the switches you received have two rows, gently clip the unused row off with side cutters before fitting. Make sure no part of the cut pins sticks out far enough to short anything on the bottom side of the board.
Fit the slider switches
Push each switch into its position on the board, flat against the silkscreen. Solder the pins from the bottom side. The two switches are mechanically identical — orientation is set by the silkscreen footprint, not by the switch itself.
Fit the test pin
The test pin solders into the hole at the tip end (left in the overview photo). A proper 1 mm sprung test probe gives the best contact, but a brass nail filed to a sharp point also works. Solder securely — this joint takes mechanical stress every time you use the pen.
Wire the off-board cables
Take two pairs of flexible stranded wire (or 4-conductor ribbon, split as needed). For each cable: thread it through one of the strain-relief holes from the bottom side, leave a small loop of slack, then bring the wire up through to the pad and solder. The strain-relief hole takes the pulling force; the solder joint only carries signal. Connections (any colour scheme that you find easy to remember):
- Loudspeaker → SPKR + GND (two leads)
- Test clip → SIGN (one lead, with its other end fitted to the microhook)
- Power-in → PWR + GND. If you're building the T-connection, this is where the male DC plug, the female DC jack and the pen pads all join (see step 5).
Build the T-connection (optional but recommended)
Solder the male DC plug, the female DC jack and a short pair of leads to the pen's PWR / GND so they form a Y. Power comes in at the male plug, the female jack passes through to power the device under test, and the short tail powers the pen. This way both the pen and the DUT run off one supply. Triple-check polarity at every step — the pen has no reverse-polarity protection.
First power-up
Connect a 9 V DC supply (centre-positive). The pen powers up immediately — there's no on-switch by design. Touch the test pin to the SIGN output to verify the loudspeaker plays the test tone. Try both positions of each slider to confirm the switches work. If everything sounds right, the pen is ready for use; if anything is silent or wrong, recheck the wiring against §04 step 4 and the polarity of the power input.
Using the Pen
A typical fault-finding session
Suppose you have a stompbox with no output — input fine, output dead. Power up the pen, connect ground (DUT ground to pen GND), and step through:
- Trace mode first. Switches: burst off / mode-doesn't-matter. Plug a guitar (or any signal source) into the DUT's input. Touch the test pin to the input jack tip — should hear it. Move to the output of the first stage — should hear it. Keep walking forward until the audio disappears. The bad stage is the one you just tried.
- If no signal source is handy, switch to inject + trace. Set the level switch to 200 mV (low level — appropriate for instrument-level stages). Clip the test clip to the DUT's input. Walk the probe forward as before; you'll hear the pen's own test signal at each healthy node.
- For output / line-level stages, set the level switch to 2 V. Low-level signal won't drive a power-stage input enough to be audible after the gain.
- If you're unsure whether a tone is in burst mode or genuinely intermittent, flip the burst/alternate switch to alternate. Continuous tone removes the ambiguity.
- Continuity — touch the test pin to one end of a wire / trace / contact, the test clip to the other. The pen sounds when it's connected. Quick way to check switch contacts and ground integrity.
Tips
Hold ground reference. Tracing only works if the pen and the DUT share a ground reference. Either run a ground wire from DUT ground to pen GND, or use the T-connection so they share supply ground automatically.
Pick the right level. If you inject 2 V into the input of a high-gain pedal you'll hear distortion that isn't really a fault; drop to 200 mV for high-gain stages. If you inject 200 mV into a power amp you'll barely hear anything; raise to 2 V for line-level points.
Don't leave it stuck-on. The LM386 amplifier on the board can get hot with a permanent signal feeding it. For long sessions, give it air, or unplug between probes.
This is a really great debugging tool
Whatever you have in your defect-stack — just pull it out, connect power and pen, and try it immediately. Far quicker than dragging a scope onto every node.
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.
Circuit design and PCB layout © TH Custom Effects 2020–2026. Build documentation V6.3.