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 informationThe 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 designThe 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: 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! Siililand note: 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.
an accurate Tachometer ready for some serious testing :-). Disclaimer!I apologise if the above circuits are not shown correctly. I have checked my design both with the made circuit and the data sheet of the LM2917. If you have any problems then please mail me, but you may find the answers are in the LM2917 data sheet. I was planning on showing the output verses input graph but my circuit doesn't want to work today - the chip seems very easy to blow up and I have had a long day :-(. I'll put up the results when I have obtained them.
by Paul Hill Northavon Mini Drivers Club |
