Using Hall effect Sensors to Turn a Vibrate Motor on, But Motor Won't Turn Off.

Thread Starter

thelittleda

Joined Nov 11, 2020
6
Hi! I'm trying to make a simple circuit which makes a vibrate motor come on by introducing a magnet to a Hall effect sensor. I've connected it to an opamp as a comparator with another hall sensor facing the opposite direction.

If I remove the motor, the LED turns off fine and comes back on, but if I put the motor in the circuit, it turns on when the LED goes off, but it won't turn off when I remove the magnet. The voltage across the motor goes from 0v(no magnet) to 3.8v(magnet) but when I remove the magnet, it only drops to 1.3v, meaning the motor keeps spinning. If I put a magnet next to the opposing hall sensor the voltage drops back to 0 and the LED turns back on.

I'm an amateur at this so I'm a bit lost as to how to get this to work, any help with this would be greatly appreciated.
 

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KeithWalker

Joined Jul 10, 2017
1,599
What op-amp are you using?
How much current does the motor take?
What are you using for a power source?
You need to connect a diode (1N400 series) across the motor to suppress the commutation voltage spikes which are probably causing your problem. You may also need an electrolytic capacitor (47uF) across the supply, for the same reason.
Regards,
Keith
 

Thread Starter

thelittleda

Joined Nov 11, 2020
6
What op-amp are you using?
How much current does the motor take?
What are you using for a power source?
You need to connect a diode (1N400 series) across the motor to suppress the commutation voltage spikes which are probably causing your problem. You may also need an electrolytic capacitor (47uF) across the supply, for the same reason.
Regards,
Keith
Hi Keith, thank you for the reply. The op amp is a UA741CN. The power supply is a USB powered board which outputs 5v. I can't see any current value for the motor it but it's rated 3v. When you say a capacitor needs to go across the supply, do you mean straight across from positive to negative before the rest of the components? Or inline with the positive rail?
 

Alec_t

Joined Sep 17, 2013
11,984
Current through the LED is probably keeping the motor running.
Depending on which pins are connected where, the two Hall effect ICs (?) appear to be shorting out the power supply?
A uA741 doesn't work well with a single 5V supply.
Your schematic could benefit from some component labels.
A motor needs a reverse-biased diode across it to protect other components from voltage spikes due to commutation or switch-off.
 

KeithWalker

Joined Jul 10, 2017
1,599
Hi Keith, thank you for the reply. The op amp is a UA741CN. The power supply is a USB powered board which outputs 5v. I can't see any current value for the motor it but it's rated 3v. When you say a capacitor needs to go across the supply, do you mean straight across from positive to negative before the rest of the components? Or inline with the positive rail?
The capacitor should go from +5V to 0V.
The current for small 3V motors usually several hundred milliamps. Make sure you are not overloading the op-amp. If it gets warm with the motor running, shut it off before it does any damage. It would be better to switch the motor with a transistor driven by the op-amp.
 

Thread Starter

thelittleda

Joined Nov 11, 2020
6
You can operate that circuit using a couple of transistors to drive the motor and LED.
View attachment 222112
Hi! Thanks for the circuit, it looks like much more simplified version of mine. I don't know what you're referring to when you say ON/OFF for the hall sensors? Is that showing the LED lighting up because the outer most sensor is on?

When I made my diagram I simplified it to make it easier for me to draw but it actually has 4 hall sensors. 3 to be activated separately, activating the motor, and the 1 to compare to with the opamp. Could your circuit be tweaked to just have the 3 I need and get rid of the reversed one? Is it just a case fo adding the output of the hall sensors to the base of Q2?
 

Thread Starter

thelittleda

Joined Nov 11, 2020
6
Depending on how strong the field, the orientation of the sensors and the distance between the sensors and motor.
Thanks! The circuit works great. Is there any way to do it with one hall sensor? I feel like the second one in this circuit is redundant if I’m only turning it on with the other. Thanks again for your help it’s been super helpful.
 
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