The tool every electronics bench has, from one chip
Touch two probes across anything that conducts and it beeps. A bench continuity tester from one 555 timer and a piezo.
Free build sheet
The parts, every wire with its length and bend points, a bend guide, both sides of the board and checkboxes to track your progress. Print it or keep it open at the bench.
Open the build sheetWhat you need
10 parts on a perfboard 15 × 30 holes, plus hookup wire.
| Qty | Part | Where to buy |
|---|---|---|
| 2 | Ceramic Cap10nF C1, C2 | Buy Ceramic Cap on Amazon |
| 1 | Ceramic Cap100nF C3 | Buy Ceramic Cap on Amazon |
| 1 | Power Input (2-pin) J1 | |
| 2 | Screw Terminal 2P (5.08mm) J2, J3 | |
| 1 | Resistor (axial)10k R1 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)47k R2 | Buy Resistor (axial) on Amazon |
| 1 | Resistor (axial)470 R3 | Buy Resistor (axial) on Amazon |
| 1 | DIP-8 U1 |
Check every pin against the datasheet of the exact part in your hands before wiring.
As an Amazon Associate I earn from qualifying purchases.
Cut list
14 wires, 380 mm (15.0 in) in total, in the order the build sheet cuts them. Wire numbers match the build sheet. Each length includes 5 mm per end to go through the hole.
| Wire | Colour and net | Length | ||
|---|---|---|---|---|
| 6 | BlackTIMING M15 → G15 | M15 → G15 | 25mm | Straight |
| 12 | BlackGND D12 → I17 | D12 → I17 | 35mm | 18mm (90°) |
| 5 | BlueDISCH L14 → M9 | L14 → M9 | 25mm | 18mm (90°) |
| 11 | BlueGND D5 → E11 | D5 → E11 | 31mm | 10mm (90°), 23mm (45°) |
| 4 | GreenDISCH K15 → F15 | K15 → F15 | 23mm | Straight |
| 10 | GreenVCC D14 → F7 | D14 → F7 | 33mm | 18mm (90°), 23mm (90°) |
| 1 | RedPIEZO F26 → E26 | F26 → E26 | 13mm | Straight |
| 13 | RedGND J18 → L18 | J18 → L18 | 15mm | Straight |
| 7 | RedTIMING N14 → F11 | N14 → F11 | 38mm | 13mm (90°) |
| 9 | WhiteVCC I12 → E15 | I12 → E15 | 28mm | 13mm (90°) |
| 3 | WhiteOUT E18 → G11 | E18 → G11 | 38mm | 8mm (90°), 25mm (90°) |
| 2 | YellowCONT J15 → H15 | J15 → H15 | 15mm | Straight |
| 8 | YellowVCC J10 → H11 | J10 → H11 | 18mm | 10mm (90°) |
| 14 | YellowGND K17 → G26 | K17 → G26 | 43mm | 28mm (90°) |
About this build
A continuity beeper from one NE555 timer. Touch the probes across a wire, a fuse or a solder joint, and if current can flow, it beeps.
The trick is where the probes sit. They're wired into the power path, so the 9V battery only reaches the chip when the probe tips are bridged. No connection, no power. Once it has power, the 555 runs as an astable at about 1400 Hz.
The piezo hangs off its own screw terminal through a series resistor and a coupling cap, and the probes get a second screw terminal. The rest is three axial resistors and three ceramic caps.
More builds
See all builds
VideoIs that pin high or low? Two transistors tell you
Touch the probe to a 5V logic signal: red means high, green means low, nothing means it's floating. Two transistors, no chip.
10 parts · 6 wires
Tools and test
VideoThis LED tells you how dead your battery is
A light for a 12V lead acid battery: green when it's healthy, yellow when it's getting low, red when it's nearly flat.
13 parts · 17 wires
PowerTools and test
Premieres Oct 31How bright is it in here, in four LEDs
Four LEDs that light one after another as a room gets brighter. A photoresistor and one quad op amp chip, no code.
18 parts · 21 wires
SensorsTools and test