By Jörg Trautmann (Germany)

What can you do when your bike computer goes on strike? That’s exactly the problem the author of this design had while on a cycling tour; suddenly no speed indication. A change of battery didn’t help. To find out if the fault was with the transmitter or the receiver unit he built this handy piece of test equipment.

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Once you have explored all the usual suspects and the equipment still refuses to work there is nothing for it but to grasp the nettle and read the manual. In tiny print in the handbook that accompanied the cycle computer it stated that the frequency range of the signal between the transmitter unit near the wheel and the receiver in the computer on the handlebar was in the range of 120 and 122 kHz and the signal strength did not exceed -16 dBμA/m measured at a distance of 3 m. These figures comply with existing regulations for such equipment so it is likely that they also apply to the majority of other makes of basic cycle computers.

Considerations

The information in the manual proved useful to start designing a diagnostic tool for the equipment. Frequencies around 120 kHz are at the lower end of the long wave band. The aim now was to build a transmitter and receiver operating at this frequency. Searching for a suitable solution I recalled an AM receiver chip type ZN414 made by Ferranti in the late 1970s. At the time it was very popular; being one of the first highly integrated receiver ICs for radio use and had a good spec. Its bandwidth extended down to 100 kHz. Bingo! Not so fast… sadly the device is no longer produced but the TA7642 made by Philips is a good substitute. Looking at its data sheet, it seems to have a very similar specification. This chip is also de-listed but there are many still stocked by various suppliers and they are also available from online auction sites.