I am trying to gather parts for this project to attempt in the near future. Can you point me in the right direction on where to get the failsafe relay or a part number that you used? I tried searching Digi-key for a 60 A 24v SPST-NO relay, but none came up that were switchable by a 5v signal. I noticed on your instructable you stated that you used only a 12v relay, but are supplying the motors with 24v. Why is that?
Also, in my H-bridge (trying my hand at building it myself) I am only able to dedicate 4 output pins total to drive the thing, so do you forsee any problems with using the same PWM signal to drive both corners of the H-Bridge (severe switching losses, cross conduction, etc)?\
Also, did the lawnmower you used have a deadman's switch to stop the blades? How did you deal with that? It would be nice to use some sort of actuator to control it to be able to stop the blades as well, but I don't really know what I'm looking for. Any advice you can give would be greatly appreciated.
I used a standard automotive relay from Radio Shack (like this one: http://www.radioshack.com/product/index.jsp?productId=3020762)
It has a 12v coil and 40-60 amp contacts rating... I have never had a problem with this as the lawnbot only gets above 40 amps for a few seconds at a time which is fine.
I could not find a high amperage relay that was operated at 5v, though now they do have some cheap solid state relays that would be usable (like this one: http://www.banggood.com/SSR-60DA-SSR-Solid-state-Solid-State-Relay-60A-Output-AC24-380V-p-79301.html)
I had to use a small 2n2222a transistor to switch the 12v signal to power the relay, but it worked fine. I used 24v for the motors because 12v did not provide enough speed for the hills in my yard. You can get 12v to power the Arduino and Relay by tapping onto one of the batteries that make up the 24v (the one that provides the ground signal).
Can you provide more info as to why you only have 4 ouput pins? Are you using a small microcontroller or already have the other pins used?
Not sure if you can make that work on the homemade motor-controller board easily... I will check it out.
I made a dead-mans switch for the blades by re-routing the wires that are connected to the handle of the push mower (the one you have to hold closed while mowing to keep the mower running), and basically using a relay to ground out the spark-plug wire on the mower... which kills the motor. I controlled this relay from the R/C transmitter so I could kill the motor remotely. To do this, you will need to solder 2 wires to the spring loaded switch that the original handle operates. The original handle disconnects these contacts when you pull it in so you can crank the motor and when you let go of it, it connects them and grounds out the spark plug.... by bypassing the switch with the 2 wires, you can let a relay do the job of grounding them together and thus kill the motor with your microcontroller.
If you have any questions about this process or need some pics, I can post some of my mower.
I was thinking of doing this as a startup website, but with a twist - have a cheap phone or arduino network board integrated to give everything internet access + camera, and then let someone operate it from a distance for a monthly service fee + lease. What do you think the costs would be for converting a new mower, with all the relays and batteries ? I want to start small, and then work up through different deck sizes.