Let's Make Robots!

Safety Switch for Large Robot


new to this Fantastic site,but not new to Robotics

Would appreciate any assistance in incorporating a limit or safety device to a modified motorised wheelchair base.

hope to use ultrasonic to detect a min distance,connected to a relay on motor (s) supply

As I use the original joystick controller I do not want to damage It in any way.

Would for example adding a relay in the motor drive circuit ( output from controller )possibly damaged it in any way in the long term

Thank you in advance



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Pat, I too am using wheelchair motors in my bot and I can tell you your plan is flirting with disaster. The white leads supply 24 volts to release the brakes in the motor BEFORE DC is applied to the motor. This is so if granny stops on a hill, she won't coast down the hill into the river. If you break the connection to the brake but there is still power applied to the motor, you will do one of several things, (none of which are good). Either the fuse will pop because you are applying power to a bound up motor, or, you will fry your controller, or you will rip the brake mechanism out of the motor.

Considering all the problems encountered when using the original electronics on a wheelchair, I did away with all of it and removed the brakes from the motors. Now it is much simpler to control, I've eliminated useless weight, I now have a mounting surface for the shaft encoders, and I was able to sell all the parts I didn't need to buy more robot stuff. It really is a win / win situation.

I am detailing my progress on this site, just search for Salvage. Good luck

Thanks for your recommendation

How could I let Granny coast down the hill or dare I say into the river, thats the end of that idea

Basically its probably best not to place any switch or relay before motors ,as I am using the original joystick  controller ( driven by two servo's) to operate.

Will take a look at switching off controller ,but it has a touch pad,and I would prefer not to open unit if possible

Thanks again


I would attempt to break the circuit at or near the motor rather than the driver. Otherwise, if there is an enable line that the driver uses, you could simply pull the enable line low. If you are really worried about runaway, you could always add a "Big Red Button" on the ground circuit.

Thanks for reply

Yes that would also be my preference

Both motors have a 4 wire connection


under normal operation if eg I push joystick forward ,24v is present between both white wires and a lower varing voltage is present

between red and black (1v -10v depending on speed )

I propose to break one white cable and place a relay in circuit ,thereby breaking circuit to stop motor

my concern is would the relay switching on and off cause any damage to control circuit in the long term