Digital Bike Tachometer

This digital DIY
tachometer for bikes uses two reed switches to get the speed
information of the bicycle. The reed switches are installed near the rim
of the wheel where permanent magnets pass by. The permanent magnets are
attached to the wheelspokes and activate the reed switches everytime
they pass by it. The speed is digitally displayed.

The tachometer circuit works according to this principle; the pulses
created by the reed contacts are counted within a certain time
interval. The resulting count is then displayed and represents the speed
of the bike. Two 4026 ICs are used to count the pulses, decode the
counter and control two 7-segment LED display. RS flip-flops U3 and U4 function as anti-bounce.

Electronic bicycle DIY tachometer circuit diagram

The pulses arrive at the counter’s input through gate U7. The
measuring period is determined by monostable multivibrator U5/U6 and can
be adjusted through potentiometer P1 so that the tacho can be
calibrated. The circuit U1/U2 resets the counters.

Since batteries are used to power the circuit, it is not practical
to support the continuous display of speed information. This circuit is
not continuously active. The circuit is activated only after a button is
pressed. At least three permanent magnets must be installed on the
wheel. The circuit can be calibrated with the help of another
pre-calibrated tachometer.

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