Electrical – Detecting motor stall with lm311

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I have 9v DC motor that I want to use to lock a door. Control is achieved using an esp8266 board. The esp8266 also switches the direction of rotation via an H bridge. So far, so good..

Now I want to detect the stall at each end of the motor travel. Initially Im trying to do this without the H bridge, just for simplicity.

There is a good amount of info about the subject out there, but Im afraid my electronics knowledge is poor and thus far I havnt been able to get anything to work.
My approach has been to use a .1 ohm sense resistor with an lm311 as in this circuit below.
The supply is an adafruit variable voltage module delivering 9v and Ive tried a number of different pullup resistors. Im afraid I just dont understand enough to work this out.
Regardless of the R pullup value.. 1,5,10 K and even without it, the output seems to wobble arounf 1.8v setteling down to about .9v. When the motor is stalled I detect no change at all in the output.

In an ideal world, I would get I nice 3.3v on the output when the motor stalls so that I can deal with it in software.
Id be grateful for help. Thanks

Best Answer

There are a couple of problems here.

Firstly, you are connecting an input to the positive supply rail (pin 3 -Vin) - this won't work - the LM311 isn't a rail-to-rail device and if you were powering from (say) +15V the data sheet tells you that the usable input range is up to 13.8 volts.

Given that your top supply is +24V you must not assume that inputs can be taken as high or higher than 22.8 volts.

Next, you really need to engineer a solution that allows the voltages on the two inputs to cross each other - this guarantees switching operation on the output. Unfortunately you have -Vin always set to a voltage that +Vin can only aspire to reach.

What you probably want is a high-side current monitor. If I remember correctly Linear Technology make some: -

Maxim have a few options: -

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You insert a small value resistance in series with the motor. The resistance needs to be low value (maybe an ohm or less) so that it doesn't significantly reduce the power capable of being delivered to the motor.

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For some reason I am unable to draw this circuit using the circuit tools? This makes it difficult to visualize for sure and it doesn't make it any easier to describe it so if i've made a mental error will someone please tell me?

Best Answer

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