Which comparator type to use as an On/Off switch for a DC motor via FSR (force sensing resistor)

Question

I want to use an FSR (force sensing resistor) to turn on a 300-350mA (steady state) 4 volt DC motor while one press into the FSR. As the image shows my plan is to have a voltage divider between the FSR and some other resistor such that, once you press into the FSR the voltage in the (-) input terminal increases and it turns the output of comparator high which in turn turns the mosfet ON driving the DC motor. I have found those comparators(NCS225x) that will work at low voltages (3-5v) and output enough current (about 40mA) do you guys think this setup will work properly? Also, what mosfets would you recommend using for a low side driver of a 3-4v motor?

Data sheet for the comparators: https://www.onsemi.com/pub/Collateral/NCS2250-D.PDF

The difference between NCS2250 and NCS2252 is that the NCS2250 has a "complementary push pull output" while the NCS2252 has an open drain out put. What does that mean in terms of my application? Thanks!

Answer 1

For driving an NFET as a low side switch you probably want a push-pull output, otherwise your gate drive has to come from the pull-up resistor. A push-pull can handle the gate capacitance in both directions.

If you are confining yourself to through hole packages your choice in NFETs with a good performance at low gate drive voltages is going to be limited. The gate threshold voltage is that needed for miniscule drain current, you need to study the data sheet and find the gate voltage that will readily pass an amp with low loss. You might check what hobby vendors like Adafruit sell for Arduino-ish projects. Without a package constraint you might look at what gets used in micro drones, for example the DMN2041L is a decent small NFET in a SOT-23.

While a comparator probably can work, don't rule out the possibility of using a tiny MCU with ADC, especially as that might be able to behave in a way that adapts to sensor aging, put sanity limits on things... or even poll in a sleep mode which could reduce power consumption of using a divider by powering the divider itself from a GPIO that you only drive high when taking a reading.

And of course consider also if an FSR is really the best sensor to be using for a binary input.