I want to measure RPM of a metal wheel using an inductive proximity sensor (npn) and a LM2917N which is supose to convert frequency into voltage which I intend to read using an ESP32 since has 12bits ADC.
The wheel has 2 holes which will be "see" by the sensor and a diameter of 50mm. Basically for a complete RPM sensor needs to "see" 2 wholes. Considering the max speed of the wheel which I intend to measure is 5 km/h I made following calculations:
Max rpm of the wheel will be around 530 rpm. For that rpm sensor will get max 1060 pulses per minute which means about 17.67 Hz
Min rpm which I would like to measure is 100 rpm which means about 3.2 Hz.
Now the concerns: I see in LM2917N datasheet that the input voltage can be 0 and 28 V, in my setup will be powered at 12V and supply voltage I assume will be same 12V since proximity sensor is powered also on same power supply as LM2917N.
I am not able to do calculations of the C1, C2 and R1 based on my setup and also have to find a way to put the output voltage in range of 0-3.3V (esp32 limits)
Second help is needed to understand how to match the read voltage to frequency (eg - 1V means 10Hz or rpm...)
Any help will be highly appreciated
Thanks in advance!
1 Answer
after reading datasheet : formula is Vo = R1 × C1 × VCC × f
with Vcc=12V, C1=0.1 mF and R1=100K >> V0=0.12V/Hz
for 18Hz, output will be 2.16V
so you can connect directly output to analog input and convert value as
int getRPM() { float V0 = 3.3 * analogRead(A0)/1024; float F = V0 / 0.12; int rpm = F * 60; return(rpm);
}
