using MOSFET to drive sensor signal to PIC

[jerk]

I need to draw a circuit diagram how to connect pickup coil (2 wire tacho generator) to a 8 bit PIC microcontroler in order to read the signal.
I found some schematics on the internet (attachment 1), that I don't see being correctly done. I don't understand why there is a rectifier diode and 10k resistor on the source pin of mosfet, when source should go to ground. Also don't understand why author uses 220 resistor on the gate.

On the attachment 2 (from wikipedia) there is a simple circuit using MOSFET to switch a LED, so according to this I draw my own schematics (attachment 3), that I would like to discous if it will work to connect AC signal from pickup coil to INT pin of the PIC microcontroler (I intend to use INT interrupt).

On the last attachment I posted the schematics for 2N7000 N channel MOSFET from the datasheet, that I also don't really understand. I don't see why a Rgen resistor is needed and what is the purpouse of Vin.

As far as I understand, MOSFETs work in a way where voltage on the gate open/close the barrier between drain and source. On a N channel MOSFET load is on the drain pin, source goes directly to ground. If I am not mistaken you also don't need to limit current through gate to source as there is sufficient internal resistance.

 

[BobK]

Look at circuit 1 again, the source does go to ground. And the 10K resistor is from ground to the pickup. The diode is to block negative pulses, and the resistor is to limit current to the gate, which is probably not necessary.

Bob

 

[jerk]

Look at circuit 1 again, the source does go to ground. And the 10K resistor is from ground to the pickup.
Bob

Yes the circuit is basicaly same as mine (had to look again), apart from the 220 resistor, that I don't see a point being there.

The diode is to block negative pulses, and the resistor is to limit current to the gate, which is probably not necessary.

what gate current? doesn't fet operate on voltage and consumes practicaly no current? I was told that if you remove the 10k bypass gate resistor mosfet will stay on even if you remove the power. Also none of mosfet circuit diagram has gate current limiting resistor.

I was guesing that author tend to use 220 resistor to lower the signal voltage, but than he/she should use two resistors in series and pull the signal betwen the two resistors.

BTW does anyone know what voltage levels do pickups produce at high engine rpm? Will 20 Vgs mosfet be sufficient or not?

 

[BobK]

MOSFET gates draw current for a short time when they are switched. They act like a capacitor. Some high powered MOSFETs can draw gate current measured in Amps (and require such current in order to switch quickly)

The pulldown resistor is needed to ensure that the MOSFET is off when there is no signal. Otherwise the gate is floating and can hold a charge and stay on for long periods.

Bob