How to Make an RPS Counter

This GreenPAK RPS (revolutions per second) counter is designed to count revolutions of rotating objects with blades such as fidget spinner, drone propeller, fan etc. It is able to count up to 199 RPS, which is 11940 RPM (revolutions per minute). Basically, it is a frequency counter modified to meet the requirements:

• Contactless sensor to avoid contact with rotating parts.

• Frequency divider, since the sensor “sees” every blade, the output pulses must be divided by the number of blades.

• Start button, after pressing the button, the device will start counting for one second and then display the counted value for one more second or until the next pressing.

• Low power 7-segment LCD. In order to reduce power consumption, the LED display was replaced with the LCD.

The current consumption (Icc) of the device is determined by two phases:

• Counting phase. Icc depends on IR sensor emitter and is set to 20 mA. However, Icc can be reduced by increasing the resistance of R1 (see Figure 1), but this will also reduce sensor sensitivity. The counting phase lasts for 1 second after power-on or after pressing the button “Start”.

• Displaying phase. Icc depends on GreenPAK’s internal blocks and 7-segment LCD and is about 70 uA. Displaying phase starts right after counting and lasts until power is off or the button “Start” is pressed again.

Below we described steps needed to understand how the solution has been programmed to create the counter. However, if you just want to get the result of programming, download GreenPAK Designer software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

Figure 2 shows the block diagram of the device. It consists of:

• Reflective sensor which includes an infrared emitter and phototransistor. It converts reflections from the blades’ infrared light into electrical pulses.

• ACMP (analog comparator) detects pulses higher than 500 mV.

• Frequency divider is used to divide input pulses by the number of the blades (set manually from 1 to 6).

• Two binary counters along with two 7-segment decoders count and convert divided pulses and display the result on the 7-segment LCD.

• Crystal oscillator together with the frequency divider form a 0.5 Hz reference clock. It is also used to power down the IR sensor and ACMP after counting is done.

• Osc 0. An internal 25 kHz oscillator divided by 16 (~1.5 kHz) provides the AC current to drive the 7-segment LCD. It is powered down while counting.

To build the RPS counter, two GreenPAKs are needed. The SLG46533 was chosen because of the great variety of internal blocks. For full schematic diagram and BOM refer to Figure 1 and Table 1 respectively.

For the project, screenshots refer to Figures 3 and 4.

Conclusion

The GreenPAK RPS Counter is designed to count revolutions of moderately slow objects. However, adding another GreenPAK IC will allow increasing the counting limit to 1999 RPS (119940 RPM). This design is easy to scale and can be used as a reference for many similar applications.