I need to read the velocity of a universal motor with a microcontroller. The motor is a universal brush motor 230 V AC 50 Hz velocity controlled by a Triac. The motor has a mechanically connected an AC (generator) tachometer that produces a frequency (and voltage) proportional to the motor rotation velocity. I need to read 1 kHz max of sine wave with a variable voltage from 0 to 35V ac peak-to-peak and transform it in a square wave thanks to the BC547 as a switch.
After several experimentations between a comparator circuit with an LM393 and a more "simple" switch circuit, I chose the last because I noticed that in the field of motor & C it is the most used (maybe for better stability under noise?). I simulated the circuit that I found (the original one was with a PNP BJT I don't know why) and modified it a bit, and all seems ok on the PC. I added R7 in order to shift a bit the sine signal and overcome the 0.7 Vb threshold of the BJT, so that when the input signal is <0.7 V RMS I can still read it. The BJT produce a square wave 0-5V that I send to my micro to count the frequency.
I would put this circuit over a PCB with other stuff, so I would like to know some suggestions about it and if I can proceed!
I read about a third harmonic problem with the 230 AC motor that could be induced in the tachometer (motor and tachometer are very close), and a datasheet suggests for a low pass filter. My doubt is that with a low pass filter centred to 100-150 Hz I think I would take off my signal (up to 1 kHz)! A lot of noise I think will be produced by the 230 AC sine partially due to the TRIAC.
My questions: 1. How to deal with this (supposed) 3rd harmonic problem? 2. What could be the role of R8? 3. Without the C2, can I avoid the R6? 4. The capacitor values could be ok?
Thanks a lot
Update: analogKid, whitout the BJT the total shift with V2=1V seems not 2.08 V but 1.3V about. With V2=100mV the output is 0.66V about. I would capture motor rotation starting when V2>100 mV about. Maybe I need to raise a little bit more the R7 to 150-180 kOhm to avoid a too low trigger compared with noise. I'm in trouble to calculate the impedence of the generator (V2). The tacho resistence is 70 Ohm, but when the AC signal is generated the impedence appears and I don't know the XL (inductive reactance). The Z=R+jXL also change when the frequency change!