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Investigating which radio-control models contain potentially hackable transmit/receive circuit boards
So far I've examined about a dozen different models of these micro/mini racers and only about half had suitable innards for the purpose I had in mind. I've never paid more than £7 for one and often as little as £2.50, so it hasn't required great outlay to do the research. It was also interesting to compare the different Chinese factory approaches to what is essentially the same functional problem in similar sized cars.

The two most common manufacturers appear to be Rastar and Shen Qi Wei. Shen are very active in the micro racer format, bringing out a new-look range almost every year. Their models nearly all feature the TX-2/RX-2 chipset, which means most of them can be donors for a wireless computer I/O port. The 'globe' range (on right) came out about 2005.
Rastar made the earlier Mini/BMW/Pajero range (below right), which ran on a handful of disposable button batteries. Those Rastar models are rather dated now (as is the chip set they used) - so they are usually available cheaply as 'old stock'. They're useable in principle (particularly the handset transmitter), but the receiver chip is limited due to its 'pulsed' decode format in forward modes.
A Rastar transmitter and a Shen receiver make a good pairing, but then you have to buy two cars to make one I/O port. Shen to Shen is the cheapest, simplest solution, but the Shen transmitter pcb's extra bulk requires a bigger enclosure.
On the right is a typical example of the receiver circuit board, as found in a Rastar branded Mini Cooper. Almost all the components are surface mounted, so a headband magnifier and fine pointed soldering tip are essential tools for carrying out the conversion process.

The received bits need to be clocked into a serial in/parallel out shift register to reconstitute the byte. With a little delay trickery, the received signal can be used to clock itself into the register. The register also acts as a latch, presenting the eight bits cleanly in parallel to whatever item is being controlled.
Currently only the seven-segment display is being controlled, of course, but I have plans to drive a BBC Buggy using one of these units and cut out all the mess of wires that normally get trailed around. Power will need to be supplied by on-board batteries. I've already tried out a crude system using the prototype board and it looks quite feasible - the Buggy chugs around quite happily, if a little slowly.

The software is relatively simple. It can be parcelled up into a single Basic procedure that sits between a program's normal parallel port output routines and the parallel port itself. It just needs to convert each output byte into the relevant button code for the individual serial bits, while ensuring that flow timing is optimal for the fastest reliable transmission rate.

In a similar manner, the wireless interface could be driven via a user port podule when available. And as both a user port and a parallel port are present on BBC micros, a wireless control interface becomes viable for 8-bit users too. More soon.

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