Dead Mans Switch

[ferret_guy]

Does anyone know where to get a Dead Mans Switch like those found on lawn mowers, we want to incorporate it into a project so if someone panics or becomes suprised it will stop (we are trying to build a person carrying robot)

[Maldridge422]

Is your robot going to carry a person in a seat? If so, a seat switch could be useful to determine occupancy.

Alternatively, use any sort of handle you want (such as is found on many lawnmowers) and simply use a touch sensor with it.

[EricH]

If it were up to me, I'd route the main power through a large E-stop switch, and the drive power through the dead-man. Wiring the dead-man switch so that you can only drive when contact is made (and releasing the switch breaks the contact) is about as "fool-proof" as you can get, though it might be a bit more difficult to do.

[DonRotolo]

A deadman switch is not a special kind of switch, it is a normal electrical switch that interrupts power when it is not held in position. In the case of a lawnmower, it interrupts the ignition, which stops the engine.

[ferret_guy]

I was going to just grab one off an old lawnmower, or somthing, but I was wondering if you had any suggestions to purchase something like this there doesn't seem to be anything like this on MCmaster

[Siri]

Originally Posted by ferret_guy

I was going to just grab one off an old lawnmower, or somthing, but I was wondering if you had any suggestions to purchase something like this there doesn't seem to be anything like this on MCmaster

Aren't you basically just looking for a momentary switch that that's default is off? No one pushing = off, someone pushing = on, let go = off, etc. Or do you need it not to restart if you push again after letting go?

[pntbll1313]

Do you want it to put the robot in disabled mode or actually cut the power? If you just want it in disabled mode you could do that with some simple software and a limit switch mounted to the joystick. When the limit switch is closed it can be enabled, when it's open (the hands let go of the joystick) then it cannot be enabled.

[ferret_guy]

I was looking for a switch that needs to be held against a bar, when its released, the software will immediately stop all the motors, I am using a limit switch, but was hoping for something with more red plastic

[Mr V]

The limit switches commonly used on FIRST robots are a viable option. If you want it bigger and redder make what ever operates it big and red. Or you could connect it to one of these.

[MrBasse]

We make use of some parts we got through First Choice on our electric car. It uses a bicycle brake lever and cable that is attached to one of the large rotary limit switches we got last year. The cable lets us remote mount the switch anywhere we want and keeps the electronics protected from contact or in the case of our last race, water. Bike brake handles come in red too...

[D.Allred]

Originally Posted by ferret_guy

I was looking for a switch that needs to be held against a bar, when its released, the software will immediately stop all the motors, I am using a limit switch, but was hoping for something with more red plastic

Mark,
I'm not familiar with standard off-the-shelf deadman switches configured as a grip bar. Typical industrial applications would use mushroom cap momentary limit switches. A typical one is McMaster Carr number 7403K41. It would meet your "big red plastic switch" specification. They come in normally open or normally closed configurations. This model can be used in a control circuit and switch about 2.5 amps.

David

[ferret_guy]

my main concern is that it gets away from someone so I wanted to implement something that required no thinking to engage, I thought that a big red button may not get pressed by the time something bad happens.

[Chris Hibner]

Originally Posted by ferret_guy

my main concern is that it gets away from someone so I wanted to implement something that required no thinking to engage, I thought that a big red button may not get pressed by the time something bad happens.

Is the person riding the robot, or is the person walking along / behind the robot?

This is an important question for the design of your switch.

If the person is riding the robot, then a lawn mower style deadman switch is probably not appropriate. When people get scared, their natural reaction is to grip tighter, not let go. Furthermore, if the robot is truly getting out of control, then you really don't want them to let go since the robot will most likely throw them causing injury.

If the person is walking along or behind the robot, then a lawn mower style dead-man switch is appropriate.

[D.Allred]

Originally Posted by ferret_guy

my main concern is that it gets away from someone so I wanted to implement something that required no thinking to engage, I thought that a big red button may not get pressed by the time something bad happens.

Mark,
That's not an "Emergency Power Off" button, although it looks like one. It's a spring loaded momentary contact switch. It can be wired so that you have to press and hold the button down just like your grip bar example at the top of the thread.

[FrankJ]

Lawn mower dead man switches typically ground the coil. Often, they are a piece of metal, not a switch in a conventional sense.

You can use the leaf limit switches that were on first choice this year. Or a spring return push button switch. You can also get them at Radio Shack. They will not carry motor current though. You can use them with a relay is rated for the current (Best) or drops the signal to the motor controllers.