This DIY digital 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.
Schematic of the digital tachometer circuit
The tachometer 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.
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 continous display of speed information. This circuit is not continously active but is activated only after a button is pressed. At least three permanent magnets must be installed on the wheel. It can be calibrated with the help of another precalibrated tachometer.