Basic Tachometer circuit w/ LED readout - Page 1
Forum section: electronics / projects

author: Nick2334
author_rank: Newbie
timestamp: October 04, 2011, 05:48:17 am
content: 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.

author: joelby
author_rank: Frequent Contributor
timestamp: October 04, 2011, 05:55:39 am
content: 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.htmlYou'll need to set up your divider/gating pulse in order to convert Hz to RPM.

author: Psi
author_rank: Super Contributor
timestamp: October 04, 2011, 05:56:56 am
content: heh, got to love the old BCD-to-seven-segment ICAnother 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.

author: Bored@Work
author_rank: Super Contributor
timestamp: October 04, 2011, 08:25:22 am
content: 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.

author: david77
author_rank: Frequent Contributor
timestamp: October 04, 2011, 08:45:09 am
content: Certainly possible. The ICL7107 can be a bit tricky at timesLots 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?

author: Lightages
author_rank: Unknown
timestamp: October 04, 2011, 05:08:33 pm
content: Remember when designing this that the important information is not actually the RPM, but the surface speed of the material. A large diameter piece of wood needs a slower RPM than a small diameter. The easiest way to take care of this is to have a wheel on a spring loaded arm used as the measuring device with the RPM count on it. That way you will have an automatic way of measuring the surface speed.

author: BravoV
author_rank: Super Contributor
timestamp: October 04, 2011, 05:27:35 pm
content: Quote from: Nick2334 on October 04, 2011, 05:48:17 amLM2917N is definitely workable but not accurate since it converts the motor pulse output into a voltage and will not reflect the true rpm when coupled with ICL7107 to convert it to digital form, also I guess this method will give you a rather confusing reading.I've seen similar setup years ago using LM2917N coupled with LM3914 to give a bar graph reading rather than a number for an ac driven motor. The reason was the operator doesn't need a very accurate on the rpm reading, a 10 levels bar graph to represent the rotation speed from low to high is enough for him, not sure if this is suitable for you.Unless you're planning to build your own, there are many china made cheap portable battery operated non contact tacho meter, some probably cost you less than $50 and they're very accurate since this kind of circuit mostly just a counter. Its using marking method, like marking a tiny part of the rotating part with tiny dot of white paint or sticker/label and it reads thru it's optical sensor on that rotating white mark.

author: Nick2334
author_rank: Newbie
timestamp: October 05, 2011, 05:55:03 am
content: Thanks for the replys. Based on the circuit above I really was skipping a half dozen steps trying to go strait from a voltage reference to an output and I've determined that I really don't understand how multiplexing works for the LED display.I've got a 4 digit common cathode LED display I picked up to experiment with. It has 12 pins but I haven't been able to find anything online that explains how to drive it as a 3 1/2 digit display (The rpms range I'm looking for is 0-2k so 3 1/2 digits is what I need). The 7107 has 21 pins going to a 3 1/3 digit display and the linked circuit has 10 pins for a 3 digit display. Right now I'm thinking I'm trying to use the wrong driver for the wrong display and have a knowledge gap with how the information is supposed to go between the two.

author: joelby
author_rank: Frequent Contributor
timestamp: October 05, 2011, 06:08:08 am
content: You'll need to find the data sheet for your LED display or probe it using a diode tester or battery and resistor.There will be one common cathode pin for each digit and one pin for each digit element. A seven segment display will therefore have seven pins, but there may be one for dots in between. If so, you would expect 8 (anode) + 4 (cathode) pins in total.To drive a multiplexed display with, say, "1234", you would output a "1" and sink current on the cathode of the first digit. Then output a "2" and sink current on the cathode of the second digit, and so on. Do this fast enough and the number will appear steadily.If you look at figure 8 in the ICL7107 data sheet, you'll note that it requires a non-multiplexed LED display in this configuration because seven wires are going between each digit and the driver, and there must be a single common cathode. You would need to find a new LED module (or just use four single digit 7-segment displays)