FreqMeasure w/ pulsing ground

M

Magnethead494

Well-known member
Hello,

Using TeensyLC to toggle a darlington array based on frequency input (33hz low end, 48hz high end). Right now I have the example Serial_Output loaded up on the breadboard. While it seems to work fine for a signal that pulses 3.3V on/off, it does not seem to work well for a signal that pulses between ground and floating. I added a 10K pullup, and that still didn't work.

What is the best way to go about this?

Additionally, with nothing connected, I get a steady 60hz reading. I assume from AC bleed through the computer/USB cable. What's the best way to filter that out?

Thank you.
 
addendum:

I'm reading a pulsed ground that fires a fuel injector or ignition coil of a vehicle. As such, technically the wire will be floating high (12v) behind an inductive coil, then pulled ground by the ECU to fire the coil.

I have a 500 ohm resistor in-line to pin16. Do I need to do anything special to 'clamp' it to 5 volts, or is the resistor sufficient?
 
I use a 3v3 zener diode to clamp the voltage to a teensy 4.1 safe value. You would probably use a 5V zener
 
OK, So what about the floating input? I have no idea if it will ever actually have a voltage, or just float high. I know for sure it goes to ground.

12 volts constant
Fuel injector (12 ohms) or ignition coil (3 ohms)
Pulsed ground

I'm tapping the pulsed ground. Given the relatively low resistance of the inductive coils, I am assuming the line will be "high" when ground is removed.

Back to harmonics - If it's picking up the 60hz AC signal through the USB line, am I to expect that I'll see some amount of resonance from the vehicle's alternator? I'm using a 3V3 regulator with 10uF and 1000uF on the input and 47uF on the output to pull down from the 12V vehicle voltage.
 
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