• Country
• Great Britain (UK)

• Manufacturer / Brand
• Sinclair Radionics Ltd.; St. Ives (Cambridge)

• Year
• 1981–1984

• Category
• Signal Processing and Computing

• Radiomuseum.org ID
• 361060

Click on the schematic thumbnail to request the schematic as a free document.

• Number of Transistors
• Semiconductors present.

• Semiconductors
• Z80_IC or. µPD780C µPD2114LC

• Wave bands
• - without

• Power type and voltage
• Powered by external power supply or a main unit. / DC 9 Volt

• Loudspeaker
• - For headphones or amp.

• Material
• Plastics (no bakelite or catalin)

• from Radiomuseum.org
• Model: Home Computer - Lerncomputer ZX81 - Sinclair Radionics Ltd.; St.

• Shape
• Very small Portable or Pocket-Set (Handheld) < 8 inch.

• Dimensions (WHD)
• 167 x 167 x 40 mm / 6.6 x 6.6 x 1.6 inch

• Notes

• The Sinclair ZX81, launched in March 1981, transformed home computing as an affordable, mass-produced device designed by Sinclair Research and manufactured by Timex in Scotland. Priced at £49.95 (kit) or £69.95 (preassembled), it sold over 1.5 million units, becoming the first widely accessible computer in UK high-street stores like W.H. Smith. Its streamlined design used just four chips and 1 KB of RAM (expandable to 16 KB), with a membrane keyboard and TV display. In the U.S., Timex rebranded it as the ZX-81, later releasing upgraded models. The ZX81 catalyzed Britain’s tech industry, fostering a community of enthusiasts and earning its designer, Rick Dickinson, a Design Council award. Its success solidified Sinclair Research’s reputation and contributed to Clive Sinclair’s knighthood.

  Sinclair ZX81 Technical Overview
  Single-board architecture with four primary components: Z80 CPU, ULA2C184E logic chip, 8 KB ROM, and 1 KB RAM.

  Processor

  • CPU: Z80 @ 3.25 MHz (NEC uPD780C variant in some units)
  • Effective speed:
    • SLOW mode (display active): 0.8 MHz (50Hz) / 0.53 MHz (60Hz)
    • FAST mode (display disabled): Full 3.25 MHz

  Memory

  • ROM: 8 KB (BASIC interpreter with floating-point math and 30+ functions)
  • RAM: 1 KB base (expandable to 16 KB via edge connector, third-party expansions up to 64 KB)
    • Note: Timex variants like TS1000 included 2 KB RAM

  Video Output

  • ULA2C184E generates 256×192 pixel output (32×24 characters)
  • RF modulator: UM1233 chip for UHF 625-line TV output
  • Display modes:
    • Text: 32×24 characters (8×8 pixels per char)
    • Block graphics: 64×48 elements (4×4 pixels per block)

  Storage & I/O

  • Cassette interface: 3.5mm jacks for audio storage (300 baud)
  • Expansion port: 44-pin edge connector for RAM/ROM cartridges

  Keyboard

  • Membrane layout: QWERTY with 20 graphic/54 inverse video characters
  • Multi-function keys: 5 roles per key via shift combinations
  • Modern add-ons: ZX-Key interface enables USB/emulator compatibility

  Pricing & Variants

  • US launch: $99.95 (kit) / $149.95 (assembled, Nov 1981)
  • German price: 398 DM assembled
  • Timex models: TS1000 (2 KB RAM), TS1500 (16 KB integrated)

  Operating System

  • Sinclair BASIC: Includes trigonometric functions (8-digit precision) and RND/RAND operators

  Legacy Issues

  • Crash susceptibility: Loose RAM pack connections caused frequent data loss
  • ULA failures: Modern replacements are scarce due to custom chip design

  Note on RAM claims: While some sources mention 2-3 KB configurations, standard UK models shipped with 1 KB. Higher capacities required aftermarket expansions.

• Net weight (2.2 lb = 1 kg)
• 0.342 kg / 0 lb 12.1 oz (0.753 lb)

• Price in first year of sale
• 69.95 £

• Mentioned in
• -- Original-techn. papers.

• Literature/Schematics (1)
• Funkschau (Sonderheft Nr. 08, "ZX81 Kochbuch")

• Author
• Model page created by Alfred Klich. See "Data change" for further contributors.

• Other Models

• Here you find 43 models, 41 with images and 20 with schematics for wireless sets etc. In French: TSF for Télégraphie sans fil.
  All listed radios etc. from Sinclair Radionics Ltd.; St. Ives (Cambridge)