The RPM sensor is made with an infrared LED and an infrared phototransistor (TCRT5000L). The IR LED constantly emits an IR light that is invisible to the eye. the phototransistor acts as a switch that is toggled by IR light. Every time a prop blade is between IR led and IR phototransistor the IR phototransistor switches off. In most RPM tutorials this on/off switching is counted during some time. e.g. 100 times a switch off during 250ms equals to 12000 rpm for a 2 bladed prop. But this means you'll only have an RPM readout every 250ms this would slow down my benchmarking.
To calculate the RPM I use the time difference between the switches, or the time difference between the blades. This gives me a RPM readout every turn of the prop. e.g. 4 miliseconds time difference between the blades equals to 7500 rpm for a 2 bladed prop.
Now I have all sensor data for this project. And I can start benchmarking my motors and props. The current PC application is a bit limited. It just start/stops a benchmark cycle and it captures all data. The data can be saved to a file or copied to clipboard. At the moment there is no fancy graph. But this can be done with any spreadsheet application because the saved benchmark file is a tab delimited csv file that can be imported in almost all spreadsheet applications.