# Digital Tachometer Circuit for Bikes

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.

## Related Tutorials

• zul

hi..
u have project diagram for car tachometer that can support 3 and 4 cyclinder?

• Shivam

can you please tell me the specifications of the reed relay used in this circuit

• Mubashir Mehdi

Hi,
I want to assemble this circuit so before that i want to know:
” in which units does it shows speed(meter/sec or kilometer/hour)?”

• Jim Keith

It can be set up to indicate ft/s, M/s, MPH or kM/hr via calibration of the time base. All of these units easily fall within the range of the two digit display for bicycle speeds. If cal pot P1 has insufficient range, fiddle with the multivibrator component values to bring it into range.

The circuit may be deficient in that it reads the correct value only once after power is applied. This needs to be corrected by adding a counter reset to the time base. Check out Adrian’s comments. I agree with his observations.

• dilli budah

please send me pcb layout as possible..

• INNOCENT

please explain to me about how monostable multivibrator gets its inputs and how it operates.

• kartik

i think that it should reads only to 99 but a bike can go faster than 99 thats why it should be a bicycle trachometer

• raj ...naem to suna hi hoga.....

adrian: tumhe to circuit ki spelling hi nahi pta hai…….read ur comment…