Post Project 62: Cycle Computer Diagnostic Tool

Test & Diagnosis

It only took a few minutes to build the transmitter on a prototyping plug board and test it out. The complete circuit of the prototype can be seen in Figure 2. In use, push button S2 can be pressed approximately once per second to simulate the speed of the cycle wheel revolving. The receiving part of the circuit now responded immediately and the red LED flashed in time with the push button action. The piezo buzzer also gave out a click at the same time. Now it was time to move the cycle computer close to the transmitter coil in the test equipment. At a distance of around 20 cm from the coil, the computer began to detect the signal and showed a speed reading corresponding to the rate at which the push button was pressed. That’s a relief, the most expensive part of the cycle computer works correctly and the fault must be in the speed sensor and transmitter. The transmitter in the diagnostic tool has a resonant circuit that can be adjusted by trimmer P2. By connecting a frequency counter at the emitter of T3 it was possible to show that the cycle computer receiver had a relatively wide bandwidth; responding to signals in the range of 105 to 128 kHz.
Thanks to the highlyintegrated receiver chip thecomplete circuit fits on aprototyping plug board.
Figure 2. Thanks to the highly integrated receiver chip the complete circuit fits on a prototyping plug board.

All’s well that ends well

There wasn’t any doubt now; the fault was with the transmitter part of the cycle computer. Further investigation revealed that the magnet had no effect on the reed relay sensor and its contacts had in fact become fused together. It was replaced with a new one and once again the tester was used to check that everything worked correctly. Now when a magnet was passed across the transmitter coil the receiver LED lit up and the buzzer made a click. With the cycle computer refitted to the bike the display showed the correct speed reading. The diagnostic tool had proved its worth; tracking down the fault was a breeze. Finally it is important to note that this circuit is only suitable for the more basic type of bike computer. The more up to date highend digital bike computers operate at a frequency of 433 MHz which is not supported by this design.

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Post Project 62: Cycle Computer Diagnostic Tool
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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