TinyTacho, an ATtiny Tachometer
Stefan Wager has designed his own small RPM meter PCB featuring a Microchip ATtiny13A and an OLED display.
After seeing GreatScott’s DIY or buy tachometer – featured here on Hackster – Stefan Wagner decided to create his own version. Whereas Great Scott used an ATmega chip for the job, Wagner found this to be a bit overpowered. His new TinyTacho instead employs the ATtiny13A to run the device. Fitting the code for measurement and calculation, along with I2C output display on its small OLED screen was certainly a challenge, but he was able to pull it off nicely, as shown in the video below.
The final build is stuffed onto a small PCB, including an adjustment potentiometer, an IR emitter and receiver to pick up on reflected IR light, the ATtiny13A, and a few resistors/capacitors. There’s a slot for a CR1220 battery, though this wasn't sufficient to run the device. It's instead powered externally, through soldered-on leads and a battery/external power selector switch. A rechargeable LIR1220 may be a possibility, as it produces more voltage than the CR' version, though this has not been tried yet.
An explanation of the TinyTacho's RPM calculation routine can be found in Wagner's GitHub repo with the code and hardware. More info on how to use an I2C OLED screen within the limits of an ATtiny10 or 13 is available in this separate post. While the accuracy of this build versus a purchased option isn’t explored, once battery power is worked out the compact size of the assembly would be a significant advantage over most, if not all, commercial systems.