# Which comparator type to use as an on/off switch for a DC motor via a force sensing resistor?

I want to use a force sensing resistor (FSR) to turn on a 300-350mA (steady state) 4 volt DC motor when pressure is applied to 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 the FSR the voltage on 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 these NCS225x comparators that will work at low voltages (3-5V) and output enough current (about 40mA.) Do you think this setup will work properly? What MOSFETs would you recommend using for a low side driver of a 3-4V motor?

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

In such a low power application, almost any comparator (open-collector/drain or otherwise), and almost any MOSFET will work. For an open drain comparator, you require a pull-up resistor at its output:

simulate this circuit – Schematic created using CircuitLab

This means that when the output is low (motor off), the pull-up resistor will be passing some current $$\I = \frac{4V}{R_1}\$$, draining the battery a little more than a push-pull output would.

Even the good old LM393 will function well in this application, so you won't need anything fancy or expensive.

Your choice of MOSFET is also not too critical. The main consideration will be that you require the MOSFET to have $$\V_{GS(TH)}\$$ well under 3V, to ensure that it's fully switched on even if the supply voltage is only 3V. You'll have to consult the datasheets to ascertain whether the device can switch 300mA of current with $$\V_{GS}=3V\$$, while maintaining $$\V_{DS}<0.1V\$$ or so. Since it's either on or off, it won't be dissipating much power, even when passing significant current, so it won't have to be very beefy.

The 20Ω resistor at the MOSFET source is guaranteed to cause problems. It permits the source to rise in potential, above 0V, which is the last thing you want, so remove it altogether, and connect the transistor's source directly to 0V. If its purpose is to limit current in the motor, and you must include it somewhere, place the resistance in series with the motor on the drain side.

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.