Build a fencing ohm tester for cords, weapons and lamés
A handheld tester that measures fencing gear in real ohms and passes or fails it against the FIE limits, for about $33 in parts. Commercial testers that read resistance cost $150 to $300 and more.
No modes, no numbers to read
Plug in a cord, a weapon on a cord, or touch the probe to a lamé: the tester works out which it is. The answer is a traffic light you can see from arm's length and a sound you do not have to look for, so your hands stay on the gear.
| You see or hear | Means |
|---|---|
| green, rising chirp | Passes |
| yellow, steady | Borderline: damp or dirty insulation, or fine for a lamé but too high for a mask wire |
| yellow, blinking | Do the next step: press the foil tip, or let go of the épée tip |
| red, low buzz | Fails; a big word on the screen says why |
| tick | A tip press registered |
| click | A break while you flex a cord |
What it checks
| Gear | FIE limit | How you test it |
|---|---|---|
| Body cord, each wire | 1 ohm | Plug both ends in. Also finds shorts between any two wires, wires crossed in a plug and damp or dirty (leaky) insulation. |
| Intermittent breaks | none allowed | Flex the cord: it counts every break of 0.2 ms or longer. |
| Foil | 2 ohm | Through a cord: resistance at rest, then checks the tip opens when pressed. |
| Épée | 2 ohm | Open at rest, resistance when pressed, and both tip wires insulated from the guard. |
| Sabre | 1 ohm | Guard bond, then a probe on the tip, forte and bell. |
| Lamé, sabre mask | 5 ohm | Slide a 500 g weighted probe over the target; it keeps the worst spot. |
| Mask wire | 1 ohm | Clip to probe. |
Weapon and lamé readings go through a body cord, so the tester remembers the last cord it tested and subtracts it: the number on screen is the weapon or lamé alone.
How it gets accurate readings from cheap parts
It measures the way lab meters do. A small test current flows through the item on one pair of wires while a second pair, carrying no current, reads the voltage right at the jacks, so wire resistance stays out of the result. The current is compared with a precision 10 ohm reference resistor and read by a 16-bit converter, so battery level cancels out. Readings are good to about a thousandth of an ohm at 1 ohm, far finer than the limits need.
What you need
- A Raspberry Pi Pico or Pico 2, four CD74HC4051 switch chips, an ADS1115 converter module and a 0.96 inch OLED screen
- A 10 ohm precision resistor, a few ordinary resistors and capacitors, a diode, a slide switch and a push button
- Eight 4 mm panel banana jacks, a protected TP4056 charging board and an 18650 cell with a holder (the lamp tester's parts carry over)
- Red, yellow and green 5 mm LEDs and a small piezo buzzer for the traffic light and sounds
- For the lamé probe: a banana plug, an M8 bolt, a domed nut and steel washers to make about 500 g
No programming: hold the Pico's BOOTSEL button, plug it in, and drag the firmware file for your board onto it.
Build documents
Firmware
| Firmware for Raspberry Pi Pico (drag onto the board) | UF2 · 198 KB |
| Firmware for Raspberry Pi Pico 2 (drag onto the board) | UF2 · 196 KB |
Source code
| Firmware sketch (Arduino-Pico) | INO · 19 KB |
| Measuring logic header (FIE limits) | H · 11 KB |
| Measuring logic (plain C++, no Arduino needed) | CPP · 16 KB |
| Automated tests (run on any PC) | CPP · 22 KB |