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Basic Tachometer circuit w/ LED readout

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I am hoping to get some help/advice on a tachometer circuit for a wood lathe I am building. I have a mechanical engineering background so the mechanical bits have been easy so far but I'm trying to develop some electronics skills and thought this would be a good project to start on. I found the motor speed control to be easier than expected but I am having issues with the tachometer which I thought would have been the easiest part of the whole project.

I am trying to take a rpm measurement and display it on a 4 digit LED display. My initial thought process was to pick up the rpm using a hall sensor and a magnet attached to the rotating shaft, convert the signal frequency to a voltage (using a LM2917N), use a ICL7107 or similar IC to take the voltage and display a proportional value corresponding to the rpm on the display.

I have managed to breadboard the hall sensor and the freq/voltage converter but have gotten nowhere with getting anything close to an output on the LEDs. All similar projects I can find on the internet use micros. Is it possible to make this circuit out of ICs? I don't have anything against micros, only that I have no experience using them and would like to complete this project before I delve into learning about them in detail.

Any help or advice would be greatly appreciated.

Sure, you can keep it old school and build a simple frequency counter using BCD counters and BCD-to-seven-segment decoders.

Here's an example project that uses this approach: http://members.shaw.ca/roma/fc.html

You'll need to set up your divider/gating pulse in order to convert Hz to RPM.

heh, got to love the old BCD-to-seven-segment IC

Another approach might be to convert the Hz to a voltage using a frequency-to-voltage circuit.
Then just display the voltage using a pre-made voltmeter module.
It could be calibrated so although it was displaying voltage it actually related to rpm, eg 1.00V = 100 rpm.
Some voltage modules even let you customize the decimal point location so it would display correctly.

The usual way to do it these days would be to use a microcontroller to count the RPM pulses and to drive a display. No F/V converter, no voltmeter.

Certainly possible. The ICL7107 can be a bit tricky at times ;)

Lots of questions:
What do you have so far? Is the F/V conversion working? Do you get a voltage proportional to the RPM of your lathe?
Do you have a circuit diagram of what you did? What is the exact problem with the ICL7107?


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