GamesMachine

Games Machine

KiCad recreation of "Games Machine — a programmed random number generator" by D. Burn, Practical Electronics, December 1976 (pp. 969–975). PCB by Sideburn Studios.

A TTL-logic random number generator with four selectable modes:

Randomness comes from an avalanche-noise transistor (TR1) feeding a two-stage amplifier (TR2, TR3), squared by a 7413 Schmitt trigger, and gated into a pair of 7490 decade counters while a 2N2646 UJT relaxation oscillator (TR4) free-runs at 3–4 s. Because the operator's press and the oscillator phase are uncorrelated, the stopping point is unpredictable. Game ranges and the zero-detector restart logic come from a diode-resistor matrix (D5–D20) read through an interlocked pushbutton bank (S1, S3–S6).

Status

Rev 1.0 — fabricated 2026-07-09, built and fully bench-tested. All four game modes verified working against the behaviour described in the original article. Three defects were found and worked around on the physical board (see below).

Rev 2.0 — all fixes applied to the KiCad source, DRC clean (0 violations, 0 unconnected), gerbers and drill files regenerated. Ready to order; not yet fabricated.

Fixes applied for rev 2.0

Full detail, including what was deliberately left alone, is in Docs/Rev2_Board_Fixes.md.

Notes for anyone building one

• Identify a UJT's leads by measurement, never from a pinout diagram. Ohms-mode resistive pair = the two bases, the leftover lead = emitter; the can-to-lead short then separates B1 from B2. Two of the three 2N2646s sourced for this build were counterfeit or dead, and one passed a static meter test while being unable to fire under real operating bias.
• TR4 wants rotating roughly 10° clockwise of the silkscreen tab mark when fitting — the TO-18 leads sit on a circle and don't land dead-on the pad angles.

Repository contents

• GamesMachine.kicad_sch / .kicad_pcb / .kicad_pro — the KiCad project
• GamesMachine.kicad_sym — project-local symbol library (custom parts)
• GamesMachine.stl — 3D case/board model
• Gerbers/ — fabrication output, regenerated for rev 2.0
• Docs/PartsList.md — full parts list with sourcing notes
• Docs/BOM.csv — machine-readable BOM exported from the schematic
• Docs/BringUp_TestPlan.md — staged bring-up and test procedure
• Docs/GameMode_Jumper_Reference.md — switch/jumper configuration per game mode
• Docs/Rev2_Board_Fixes.md — defect record and how each was resolved

The first commit is the rev 1.0 project exactly as fabricated, so the whole set of fixes can be read as a diff against the board that was actually built.

Original source

Practical Electronics, Vol. 12 No. 12, December 1976. Article by D. Burn, PhD. The magazine scan is not redistributed here.