The circuit consists of 3 major components:
- Reflective sensor
- Pro Trinket microcontroller
- 7-Segment Display w/backpack
You will also need a power supply, enclosure, connectors, wire and soldering supplies.
An optical reflective sensor is easily adapted to work with most machines. It only requires a light/dark target pattern on the rotating part to produce a signal. The target pattern can usually be added safely and easily with tape and/or paint.
You will also need a 470 ohm current limiting resistor for the emitter. This resistor is included in the package when you purchase the sensor.
Since the Pro Trinket has internal 20K pullup resistors, we won't be needing the 10K pullup resistor packed with the sensor. You can save this in your parts bin for future projects.
It is also possible to use a Hall Effect sensor and a magnet. But this can be dangerous for machines operating at high speeds. A magnet can turn into a projectile if it detaches from the shaft at high speed
Most any small microcontroller has the horsepower necessary to implement a tachometer. We’re using a Pro Trinket here because it is inexpensive, small and easy to work with. We’ll use the i2c interface to drive a 7-segment led display, and the external interrupt on pin 3 as a trigger for timing the revolutions.
Other than routers and rotary tools like a Dremel™, most shop machinery runs at 3,000 RPM or less. So a 4-digit display is sufficient. The 4-digit 7-segment display backpack makes these simple to wire and control via the i2c bus.