RNG

Random Number Generator PCB

A digital die: press ROLL, ten LEDs blur, release, and exactly one stays lit. Two 1970s logic chips, no microcontroller. KiCad 10, two layers, 50 × 40 mm.

Status: Rev B fabricated, hand assembly in progress.

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How it works

An NE555 in astable mode free-runs as a clock, driving a CD4017 decade counter. The 4017 asserts exactly one of its ten outputs at a time and advances on each clock edge — ten outputs, ten LEDs, no decoding logic needed.

The ROLL button gates the 4017's clock enable against a 10 kΩ pull-up. Held down, the counter runs at the 555's frequency, far faster than the eye resolves, so all ten LEDs appear lit. Release it and the clock enable goes high, the counter freezes wherever it was, and one LED stays on. RESET returns the counter to output 0.

Randomness comes from the gap between human reaction time and the oscillator frequency. It is not cryptographic — the counter is perfectly deterministic — but for picking 1–10 it's indistinguishable from it.

Design

Notes:

• 1 kΩ series resistors on all ten LEDs — ~3 mA each off a 5 V rail with a red LED's ~2 V forward drop. Dim enough to be comfortable indoors and to keep draw low in the sweeping state.
• 100 nF decoupling at both ICs. The 555 injects current spikes onto the rail when its output stage switches, and the 4017 shares that rail. Local decoupling is what stops that showing up as false clocking.
• Separate power indicator LED, so the board shows it's alive even when reset with no output asserted.
• Silkscreen carries 1–10 above the LEDs and labels both buttons.

Bring-up checklist

1. Measure the 555's actual oscillation frequency against the calculated value.
2. Scope the 4017 clock input — confirm it advances cleanly on every edge with no double-clocking.
3. Confirm the clock enable genuinely freezes the counter rather than letting it drift.
4. Confirm all ten outputs assert in sequence and only one at a time.

Tools

KiCad 10.0. Gerbers for Rev B under RNG Rev B/.