Creating this basic tachometer circuit

Thread Starter


Joined Dec 15, 2009

I am trying to construct a tachometer to show the RPM of a rotating shaft. The shaft rotates about 20-120 times a minute. I was thinking of using this sensor ( ) and this basic design ( ).

I understand how to hookup the 555 into an astable circuit with a period T. How do I wire this into a one-shot (74123?) in order to get the desired signals?

Also, I understand how to get the decade counters to count the number of pulses during the period T (from the 555) but how do you hook these to latches in order for the 7 segment LEDs to display the previous count?

Thanks for any help.


Joined Aug 7, 2008
What resolution do you need? If using a 6 sec. counter gate time, each count would be 10 RPM, with 60 sec time base ,ea count =1 RPM, but would take a little over 1 min per measurement. For slow shaft speed it might be better to measurt period of each rotation, then then there would be a measurement every other revolution; the disadvantage is inverse reading, ie slow RPM = high count. It all depends on application as to which is best.


Joined Dec 8, 2009
One solution would be to get a bicycle computer (speedometer) with cadence sensor (pedaling crank RPM) and use that portion of it. Runs off a watch battery, sensor, and a magnet. Sensor clearance to magnet is usually 5mm or closer. I think they start around $50 in stores. Cateye is a good brand. Wireless models are available but sometimes have issues with a lot of fluorescent lights.


Joined Feb 11, 2008
If you feel up to soldering a kit, there is a low cost LCD tacho design on this page, it was deliberately designed for low revving machines like lathes etc. It has averaging function to smooth RPM fluctuations caused by cutting cycles or engine firing cycles;

Cheap open-source LCD tacho

The kit can be supplied with pre-programmed PIC so you don't need to know PIC stuff to build it, just basic electronics knowlege.