Hello Everyone. I am hoping someone can help me with a circuit (see attached) I am working on. It generates a pulse of a set length when a button is pressed. I have it working ok, but I am having trouble adjusting the pulse length (see attached) to exactly what I want. Right now the fall time (A) is 400us, the length is 35ms, and the rise time to 75% full voltage is about 11ms. What I can't seem to figure out is how to reduce the fall time and rise time without reducing the length. I really would like to have a fall time of less than 5us and a rise time to 75% of about 4ms with the length staying around 30ms. I feel like I should be able to do this by adding another resistor or capacitor somewhere, but nothing I do seems to work. Any help you could provide would be very helpful. Thank you.

 

Use a 555 in a debounce circuit.

 

I should have added that although I know I can do it better with a 555 or other IC, I need to have the circuit pretty much as is or with different values or just an added resistor or capacitor. Thanks for the suggestion though.

 

You are suffering from the wide variation in gate voltage per current of the mosfet. A bjt would make a snappier transition, but use a lot more current.

An op-amp would be perfect for this job, but you could build a differential pair with 2 transistors to get a sharper switching point.

Bottom line, this is the wrong transistor, and you might need more than one of the "right" transistors.

 

I am going to experiment with some different jfets. Thank you.

 

OK, so I have tried a few different mosfets with no success. Can anyone tell me what specific mosfet specification I should look at? I have tried contacting a few manufacturers, but have not really gotten anywhere.

 

I don't think any MOSFET will get you there on its own, although I suppose there must be some out there with a much narrower gate voltage range.

You could use a BJT or BJT darlington to give a snappier transition of the gate voltage as #12 suggested. But this will draw some current and mess up your timing. Fixable, but a pain. Depending on the current through the MOSFET, you might be able to eliminate it and just let the darlington do the work.

 

Hysteresis. You can add a second transistor and some feedback to make a "schmidt trigger" type circuit, which will snap quickly between one output state and the next, even if the cap voltage is rising slowly.

 

Here's how you could add hysteresis. Both FETs need to have a very low (~1V) turn-on voltage.

 

Nice one.

I think there would be benefits going to NPN bipolar transistors instead of FETs. I've done similar using BC337 NPN which will saturate Vce below 0.1v with very little base current.