IR-Tachometer

This is my learning experience building a very low cost tachometer that I wanted to build to measure the RPM of the spindles of the various CNC Routers that I have been using.

Hardware

• Arduino Mini-Pro ( under $4.00 at AliExpress ) These are now obsolete, but any arduino will do. I just happen to have a few of these
• OLED Display ($3.50 at AliEpress (Uses Adafuit SSD1306 library)
• Infrared Sensor ($2.00 at AliExpress ( 3pin IR Infrared barrier Module obstacle avoidance sensor)
• Lithium Battery Charger Module ($1.00 at Aliexpress )
• battery 3.7v 450mAh or so. ($3.65 at AliExpress )

Design

There are several ways to do this. I am testing all 3.

1. Count pulses for a period of time using a loop. This is way too sketchy and was only used as a reference ( See this article )
2. Use interupts and measure the time between pulses. My first attempt at this was less than satsfying. I added a running average function to smooth out the jitters and it now works pretty well..
3. Use interrupt to time a window and measure how many pulses occur during this window of time, such as 500msec or 1 sec. See Blinky with Timer1 OVF
4. Use a Frequency Count library such as from PJRC for the Teensy

Final Code

I implemented option 2, which handles lower RPM's pretty well.
Essentially, the program sets up an interrupt to trip every time there is a pulse on the sensor, and measures the time since the last pules.
The RPM is calculated using the width of the pulse. A 1 Second pulse as an example would be 60 RPM. 10mSec would be 6000 RPM.

I also implemented a "No Pulses detected" and "Too Close" based on the sensor being either always on or always off.

The correct operation is when the LED on the sensor is flickering.

Help

• Github-flavored Markdown info page.
• Mermaid diagrams