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2N7000 as PIC input

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cn00728

New Member
Hello,

I'm having some issues with an input on my PIC ignition project. I had a 2N4401 setup as a NOT gate to provide an input to the PIC 16F630 from a 3v or 5v signal. Problem is, the input signal is a 2ms pulse, which I believe is faster than the 2N4401 could see in a NOT configuration, as I have been unable to find data on it's performance as a NOT.
So, I swapped that with a 2N7000, no change. Now it's unstable and the 2N7000 will oscillate just by you touching the gate.
I have attached a sketch of how my input is, and was wondering what your recommendations are. I was going to put a 10K pull-down on the gate to prevent the oscillation, but sensing a 2ms pulse will still be an issue.

Thank you,
Alex
 

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Pommie

Well-Known Member
Most Helpful Member
The turn on time for a 2n7000 is around 20nS, your 2mS pulse is 100,000 times longer. Try the pull down. If it still doesn't work then post details of the circuit powering the gate.

Mike.
 
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mvs sarma

Well-Known Member
I feel that adopting circuits around nosy surroundings should not be left unterminated as O P has shown 2N7000. Already experts have suggested towards the same. Even an NPN transistor needs termination.
 

Pommie

Well-Known Member
Most Helpful Member
Whoops, I just noticed your input signal is 3-5V. The gate voltage required to turn on a 2n7000 is around 2V and so it will never turn off. What is the circuit producing the 3-5V signal?

Mike.
 

cn00728

New Member
I should point out that there is a 10k ohm on the gate that was there for current in the previous 2N4401. As it didn't switch with the 2N4401... Too large of a gate resistor changing the capacitance characteristics?

For testing, I have been using another PIC outputting a 2ms pulse train to the gate. If you connect the gate directly to 5v it works, but not with a 2ms pulse. In addition to the PIC, the EFI ecu was used, no dice.

Alex
 

MikeMl

Well-Known Member
Most Helpful Member
Here is input buffer with a variable trip point. The plots are for a values of B from 0 to 20KΩ in steps of 2KΩ. With B=0, the threshold of the NFet determines the turn on voltage.
 

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MikeMl

Well-Known Member
Most Helpful Member
Using a 2N4401. Now B = 10KΩ, 20KΩ, 30KΩ,...
 

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cn00728

New Member
Thanks Mike. That's thorough, what program is that?

I took another look at the 2N4401 I used before. The circuit was as shown below. The only difference between your diagram is that I didn't have a pull-down on the base, which I should add. Remember though, that this didn't work for me either. I prefer to use the 2N4401 over the mosfet as it isn't as sensitive.

Thinking my gains were messed up, too low collector current which results in a large turn on time, I went through it again this morning and used a 100ohm at R3, and 1K at R4. No change.

Alex
 

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