Twenty-five years ago this month, Clive Sinclair unleashed his third microcomputer onto an unsuspecting UK public. What had previously been a niche interest hobby, was about to become a mainstream obsession, as home computer mania gripped the UK. The ZX81 came with 1K of RAM, BASIC in ROM, and a black and white character display. It was initially sold through branches of WHSmith and by mail order. It was available to buy ready assembled for £69.95 or as a self-assembly kit for £49.95. Within two years Sinclair Research sold over a million units.

For many, myself included, the ZX81 was a revelation. It plugged into a regular TV set and came with everything you needed in a neat Styrofoam box. A whole series of new businesses sprouted up around this machine. Magazines were founded to promote home computing the 'Sinclair' way, mono audio cassette players become popular once more (software came on tape), and devotees stuck sheets of transparent green plastic over their portable black and white television screens in a rather pathetic attempt to reduce eyestrain. I know it all sounds quite awful, but really, it wasn't! In our imaginations, at least, we were a million pioneers charting unexplored territory - from the comfort of our bedrooms!

For those lucky enough to be able to afford a 16K RAM pack, which plugged loosely into the rear of the unit. Nothing was less 'satisfying' than typing in a two hundred line BASIC program, then accidentally knocking the computer, unseating the RAM pack, resetting the power and losing all your work. All manner of moulded plastics were offered as solutions to the problem, but in the end one of the most widely adopted solutions to the problem could also be purchased at WHSmith - Blu-Tack, liberally applied between the ZX81 & RAM pack. It worked just fine for me and many others.

If you actually managed to type in a whole BASIC program, without resetting the ZX81, you still weren't home and dry, although you could at least now attempt to save your efforts to tape. Often, the listing would contain a 'bug' or a 'typo' and you'd either have to figure this out yourself, or wait a month for corrections in the next issue of the magazine you were working from.

Also, if you think sending texts on your mobile phone is bad for your thumbs, you should have tried typing on a ZX81, the 'keyboard' was a pressure sensitive membrane of plastic, which gave very little in the way of feedback. If you imagine trying to type on your mouse mat, it's a similar sensation. Ingenious companies came out with third party keyboard solutions for us to attach - many were just plastic templates that sat on top of the keyboard; others re-housed the ZX81 motherboard in a new enclosure; all of them a poor substitute for a proper keyboard.

The really brave would fish old mainframe teletypes out of skips and graft the keyboard to the ZX81 one wire at a time - many innocent ZX81s were lost in this pursuit.

Software for the ZX81 never quite lived up to your expectations; you would go into WHSmiths and see a cassette with a colour inlay card promising you all the thrills of the arcade. You'd hand over your money, rush home, load up the title, and then marvel at a flickering centipede or space invader rendered in monochrome Os and Xs. To be fair, some games made good use of the block graphics available - Monster Maze was, I think, one of the better ones, a kind of primitive version of Doom.

Psion were one of the original innovators in Sinclair software. They produced Chess and Flight Simulator for the ZX81, as well as several titles for the Spectrum, before going on to build their own handheld computers.

Most recently, alongside teams from Nokia and Ericsson, they developed the Symbian software that runs in some 39 million mobile phones.

When the ZX81 was replaced by the Sinclair Spectrum, which boasted colour graphics and better games, most people packed up their ZX81s and stored them away in their lofts. Sinclair continued to sell the ZX81 alongside the Spectrum. By May 1983, it could be bought at the discounted price of £39.95

Today, if you want to buy a ZX81, the place to look is either on eBay or a car boot sale. On eBay, mint examples in original boxes can fetch £100 and these are the ones to collect as they may appreciate in value, working examples fetch around £25, and broken ones can be had for a fiver. Also, if you can find a mint example of the much rarer ZX80 with all its packaging, then expect to see the bidding reach £200.

Perhaps the best bet though, for a truly collectable ZX81, is to find one unassembled in kit form. Currently an American company, called Zebra NYC, who imported several thousand into North America just as demand was slowing in 1983, and subsequently left them in storage for twenty years, have started reselling them through their website and eBay. They only have around 200 left in stock, which they are selling for £79 each including shipping to the UK. You will also need a 120v to 240v adapter (about £10) if you intend to build and use it. I have dealt with them personally and they provide a professional service. Also, the US kits seem to be available in the UK from ZX81kit.com.

If you don’t fancy spending hard cash on a ZX81, you could always check out some of the emulators available on the Internet. Also a browse around eBay throws up people selling CDs with ZX81 and Spectrum emulators on, along with hundreds of old games titles, for just a few pounds.

When you look back at the ZX81, it really makes you appreciate just how far computers have come in the last twenty-five years. It's scary to think that a complete chess game could once be made to fit in just 1024 bytes of memory and still be playable. And how do I know this, you may ask? As a spotty teenager I typed in every byte of a machine code chess game by hand from a magazine listing. And how did my first computer repay my patience? Checkmate.

Happy 25th birthday ZX81.

Sealed kits in original boxes from the USA:
www.zebrasystems.com/zebrasystems/zx81/

Planet Sinclair - ZX81 beginning of the boom:
www.nvg.ntnu.no/sinclair/computers/zx81/zx81_sst.htm

Monster Maze - the first 3D game for a home computer:
en.wikipedia.org/wiki/3D_Monster_Maze

Full technical specification of the ZX81:
en.wikipedia.org/wiki/Zx81