I have a circuit where 220VAC mains is stepped down by transformer to 48V and through an 5k1 resistor I'm activating optocoupler that drives a LED through a resistor on the other side. I am measuring frequency at this side and when I use one-diode optocoupler I see 50Hz which I expect.
now, when I replace the optocoupler with the one that has another diode in the reverse to the first one in it:
I would expect to get double frequency, becouse as my thinking goes, the transistor gets activated twice as much.
I get this instead:
so as you can see, it's still 50Hz. a even tried to hook it on the arduino and count for trigering the interruption, I get 50 in both cases. and that's what I don't get. in this case I' using H11AA1 opto-coupler module. EDIT: as advised I put pullup resistor over LED terminals to compensate it's forward voltage drop and signal gets readable and 100Hz-ish!
can you explain what going on here, where I make an error in my estimations or measurements?
thank you! EDIT: adding full diode bridge before optocoupler is really getting me to 100Hz altho...would be nicer to have it simpler. the signal that I measure is still kind of strange to me; I wouldn't say it's doubled 50Hz: