Intent
Mounts inside the soundhole as a pass-through harness — it plugs in between the battery and the preamp's existing clip, no cutting or splicing the guitar's wiring. Draws zero current until a button is pressed, then reads out battery health as a red/yellow/green LED indication.
How it works
- A momentary pushbutton gates the positive rail only, so the circuit's loop is open — no standby drain — except during an actual check.
- An LM339 quad comparator, referenced against a TL431 precision voltage source, lights red/yellow/green LEDs at fixed thresholds: below 6.8V red, 6.8–7.5V yellow, above 7.5V green.
- Originally designed around an LM3914 LED-driver chip. Checked directly against DigiKey stock: the entire LM3914/15/16 family turned out to be obsolete with zero stock in any package, so the design moved to the comparator approach — which also happened to fix a dead-zone bug the original chip's fixed 10-step ladder had.
Hardware
| Part | Role |
|---|---|
| LM339 | Quad comparator — 3 of 4 channels used, one per threshold LED |
| TL431 | Precision shunt reference, independent of battery droop |
| 3× LED (red/yellow/green) | The readout |
| J1 / J2 (9V snaps) | Pass-through harness — battery in, preamp out |
| Momentary pushbutton | Gates V+ only — the zero-standby-drain trick |
23 parts total on the schematic, all confirmed in stock at DigiKey (checked 2026-08-27).
Status & next steps
- Validate threshold math (comparator/reference ladder) against target voltages
- First-draft schematic — all 23 parts placed with footprints
- Wire the nets in KiCad, run ERC
- Order parts, breadboard prototype
- Solder to perfboard, mount inside the soundhole