Turning on a DC motor in a control circuit

Thread Starter

bingdog

Joined Oct 29, 2018
11
I am working on a control circuit to run a 12 volt DC motor. The circuit uses a 555 timer to turn the motor on and off based on input from a sensor. The output from the 555 goes to a motor driver (IXF007T half bridge driver) which runs the motor until the 555 output goes low. I would like to be able to manually run the motor with a switch that can turn the motor on and off independent of the 555. I have the +12 volts from the switch connecting between the 555 output pin 3 and the driver input pin 2. I placed a diode between the 555 and the switch connection point to prevent any current from running back to the 555 when I switch on the motor.

When I run the motor manually with this circuit everything works fine for a period of time (2 or 3 minutes) until the motor wont shut off when I release the switch. It appears that the 555 output goes high and that keeps the motor running until I reset the 555. I can only conclude that some current is flowing through the diode to the 555 pin 3 and this is causing the 555 to go high when the motor is being run manually.

I wonder if there is something I am doing wrong here. When the manual switch is not included in the circuit everything works fine.
 

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AlbertHall

Joined Jun 4, 2014
10,249
Or maybe the electrical noise from the motor is getting to the '555 and triggering it.
Do you have good decoupling on the '555 supply, close to the '555.
 

Tonyr1084

Joined Sep 24, 2015
4,925
SPDT switch AFTER the driver. You can switch from automatic control to manual control and back without bothering the automatic control circuit.

Automatic.png
 
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Alec_t

Joined Sep 17, 2013
11,538
the motor wont shut off when I release the switch. It appears that the 555 output goes high and that keeps the motor running
The motor, being inductive, will tend to generate a big voltage spike when its current is interrupted. This spike could get back to the 555 and re-trigger it. If you haven't already got a reverse-biased diode connected across the motor to suppress the spike, add one. The diode current rating should match the motor current rating.
 

Thread Starter

bingdog

Joined Oct 29, 2018
11
This is the complete schematic. The driver is providing braking for the motor so I would like to run the motor through the driver. When I run the motor using the sensor to start and stop the motor there are no issues with the timer so I wouldn’t think inductive current from manually turning the motor on and off should be the problem.
To try and figure out the problem I disconnect the wire running to pin 3 on the timer and tried running the motor with the switch. Everything worked fine then. It is only when the timer part of the control circuit is hooked up that the problem develops. My thought is that there is a small reverse current through the diode that is causing the trouble. Maybe there are diodes that are better for this application. I also considered using an or-gate instead of the diode to separate the inputs to the driver. I am open to any other ideas too!
 

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Alec_t

Joined Sep 17, 2013
11,538
I doubt that diode leakage is the issue, but by all means try a low leakage diode.
I've spotted a possible cause of your problem. The presence of the 1N4004 and D5 mean that the IN input (pin 2) of the IFX007T is presently floating except when pulled high, so is prone to picking up interference and could be held high by leakage currents. Try adding a 10k (or less) pull-down resistor to pin 2.
 
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Thread Starter

bingdog

Joined Oct 29, 2018
11
I did try a 3.3k pull down resistor earlier and it didn't solve the problem. I guess I should keep it in there at any rate. When the motor continues running after switching off the manual switch, the 555 pin 3 is high. Hitting the reset on the 555 stops the motor. I am not sure the problem I am seeing is associated with the motor driver. I ordered the low leakage diode and will give it a try. I will let you know how it turns out. Thanks for your suggestions.
 
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