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How  To Make Your Own Digital Tacho

 This circuit gives a voltage that corresponds  the the current RPM. The advantage over a analogue meter is a reading that is  more accurate since the "needle" is much quicker to react. In fact since the  output is a voltage you could connect it to a moving needle if you require.

Background information


The RPM can be converted to a frequency quite  easily. There are two sparks per rotation and we are working in seconds so simply  devide the RPM by 30! (thus 8000rpm is actually 266 Hertz).

The frequency input to the circuit can be taken  from the -Ve terminal of the coil. You would expect the voltage to swing by  about 12 volts since the contact breakers simply switch the battery to the coil.  Unfortunately the coil is an inductor and the condensor is a capacitor thus  we have a highly resonant circuit which can hit over 40 volts. Protection is  shown in the circuit diagram.

Circuit design


The conversion from frequency to voltage is  done by the LM2917 chip which is a fairly simple 14 pin device. Only a few components  are required to make it perform.

The formula is shown below:

Vout can be found by using the formula: Vout  = Vcc * R1 * C1 * Fin

Where Vcc is the supply, Fin is the frequency  input.

It is a good idea to make R1 a variable resistor.  On making the circuit you may find that the output of the chip isn't quite right,  twiddling the value of R1 helps to get the right output.

If you choose values of Vcc=8, R1=40K, C1=47nF  and Fin=266 then the output is 4 Volts.

Therefore if the output is linear then we have  1V for every 2000rpm.

The ripple voltage has to be considered but  for this design the ripple is 33mV at 2000rpm (or a swing of 66hertz around  ideal voltage).

The circuit is shown below:


Now that we have the output corresponding to  the input frequency we need some way of showing it. Below is a circuit that  I have found to be very simple and reliable... This is a somewhat simplified  version only showing 10 leds but my final version will have around 40!


You can buy one of those LED bars ready from  an electronic shop for next to nothing. Make it back-lit, and you have a real-cool  looking bar graph right next to all your other wiggits on the dashboard.