Circuits for bicycle speedometers happen to be pretty common, the difference in this specific design being the digital readout.
The speed sensing is completed by numerous magnets attached to the spokes or rim of the wheel which conduct a set of reed switches.
How the Circuit Works
The principle is highlighted in the drawing in figure 1, where the reed switches are displayed installed on the bicycle front forks.
The main benefit that a digital display has over a moving coil meter is that of robustness in a situation where the younger generation can produce a really tough environment.
Current consumption is placed to a minimum by arranging for the power supply to be switched on only when a readout is needed.
This switch (S2) should preferably be placed on the handlebars (i.e. using an electric bicycle horn button or similar).
The circuit diagram for the digital speedometer is demonstrated in figure 2. The principle behind the circuit is easy: The pulses from the reed switches are fed to a counter (IC1, IC2) for a predetermined length of time.
The counter is than inhibited and the count decoded and shown. Decoding and show drive is conducted by the counter itself. N3 and N4 serve to remove contact bounce from the reed switches, Sla and Sib, whilst the count pulses are fed to IC1 via N7.
The measurement period is determined by the circuit round N5, N6, and can be varied by modifying P1.
The meter can as a result be calibrated using this preset. The charge time of capacitor C1 will ensure that the counters are reset by N1 prior to a new count cycle commences.
Gate N2 stops a count cycle starting prior to the reset is cleared. Taking into consideration the high current consumption of LED illustrates, a continuous readout is not feasible.
A 'push-button'- type display was as a result selected, i.e. each time S2 is depressed the speed of the bicycle at that specific moment is displayed.
This method includes that the components which would have been needed to be certain that the counter is automatically reset after each count can be dispensed with.
In principle any number of magnets can be employed, however in order to prevent extremely long count periods, a minimum of three is suggested.
The bicycle speedometer circuit should be calibrated (i.e. P1 modified for the desired count period) using an active speedometer.
Digital Bike Speedometer Circuit
This circuit delivers push bike speed measurement between zero and 100 km /hr or 100 mph!
The circuit is dependent on the Sintel MOS counter block, which often counts the pulses through the photo transistor Q1 .
Most of these pulses are supplied simply by fixing 18 aluminium 'barriers' towards the wheels. Q1 was an unmarked type in the prototype, in a TO 18 package.
This installs in an old sketch pen case opposite the lamp such that the obstacles disrupt the beam in functioning.
The counter works while PB1 is pushed, but latches after a short time based on RV1 or RV2. 1C1 and connected elements. IC1 forms a square wave oscillator with adjustable mark space ratio.
The time during which pin 3 is pulled low is based on RV1 / RV2 this permits the counter.
The digital bike speedometer circuit precision depends on the reliability of adjustment of presets RV1 and /or RV2.