ZoneMikel's Blog

Before I dissapear in the confines of ultiboard and multisim again I thought I had better write something. The Power circuit and the Ping Circuit were great sucesses. I’m glad I decided to use this modular design, I see now that the all in one mainboard was really a bad idea. Once everything is working I can send off to have a “all in one” board created for about $2.50 each if i buy 20 of them. So the goal now is to get everything working bare bones then improve as much as i can and then order some professionally made boards. I want these boards to be a good starting point for any robot that we decide to use in the future. So a robot that moves on tracks might have the same circuitry as a robot that moves over water, or flies etc.

The H-Bridge circuit works . . . but, I finally see why they were using and gates to make sure it NEVER allowed one side of the h bridge to be totally on. Even if you just let the selects float it can burn up some of the mosfets on the little chips. So make sure the selects are always tied to something, I still dont know how but just from floating one of the mosfets burned up inside the chips. I think i’ll be able to repair it though simply soldering a normal mosfet on top of the chip. But i do need to order some more h bridges … ~ None the less it works fine like it is but I may add some protection circuitry to future modular h bridges.

The accelerometer and the multiplexer module are etched but on a back burner. Since they aren’t really essential to the project (yet) I’m putting them to the side while I see if i’ll even be able to use them. I’m realizing with the mcu board that I’m gonna have to remove some stuff to get something I can actually make.

The MCU board was again a nightmare, even at 70 connections i had to do double sided boards and reduce the trace width to about .3mm and the trace clearance to about .15mm. When I did that I got about 64 of 70 connections connected, but there were about 25 vias. After etching the board (which was attractively small) there were a bunch of places that had traces connecting. I could fix the traces connecting but there was also the problem of the vias being really small, when you drill them it just obliterates the pad and you would have to do some fancy stuff just to connect them, plus some of the vias are under the mcu itself which will cause all kinds of problems. From messing with this for the last week I can kinda tell when im jumping in a bottomless pit and pull out early. I realize a lot of these connections are things that we wont even be using, for example the multiplexer circuit wont be used, at least not in this design and the “extra pins” connector is just that extra pins. So I’ve decided to remove as much fluff as i can to get something I can make at home.

I’m not abandoning all the connectors at all, I have full plans of implementing everything but like I said above I want to test as much as I can at home then send the board off and get about 20 of them made professionally. I think the best you can hope for with home etching is about 50 connections and .4mm traces and .2mm clearances, but .3mm clearances are so much better. What I’ve noticed also is that the trace/clearance size does not make that much of a difference sometimes, sometimes its just you have to many wires crossing. I’m going back over the pinout of the chip and the connectors I have maybe by re routing some stuff I can clear up a lot of clutter. In the original design I never took into consideration the placement of the connectors and the pins on the MCU, I’m seeing that that was a bad error. However, if i can limp by on this design (for testing) i should be able to get the professional boards made which can handle everything on one board.

So I’ve creatied a “jinx mcu circuit barebone” circuit to try and get something I can easily make. I’ve reduced it to just the mcu, hbridge, wireless and power circuit so when i finish I should have a glorified RC car with no sensors. But we will see how that goes. I’m still working on the barebone mcu board and hopefully I’ll be able to etch it and test it a little today.

One more thing I should point out, I’ve decided to make all the modules connect at a 90 degree angle from the mainboard, this makes things a lot simpler. I just have to remember to bend the connectors to a 90 degree angle before soldering them into the modules. Instead of the robot being short and fat it will be more tank like with a semi tall body, looks like the board will be about 2 inches tall and maybe 4 inches wide. That still may change though.