The stepper motor controller boards for my CNC router project are based on the L297 controller and L298 H-bridge driver combination. The circuit board design is the same as the schematic shown in the L297 datasheet. To keep the size of the boards small, I decided to make them double sided. I also needed to build a parallel port breakout board so I did that at the same time. The schematic and boards were laid out using EagleCad.
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The positive photo fabrication method was used to create the boards. The supplies are from MG Chemicals and are readily available at Sayal electronics. After the boards were designed, the top and bottom traces were printed onto transparencies in an inkjet printer. Because these boards are double sided, vias are used to connect the top layer traces to the bottom layer traces. The transparencies are cut to size and carefully aligned so that the vias on each side line up perfectly. I staple the transparencies together and then slide a double sided, pre sensitized, board in between.
The transparencies and the board are then sandwiched between two sheets of glass and held together with some paper clamps so that nothing can move out of place when the board is being exposed to ultraviolet light. The side opposite to the one that is being exposed to the UV is covered with light blocking plastic. when the top of the board is finished being exposed, it is covered with the blocking material and the other side is then exposed.
I happen to have a UV eraser unit left over from the ‘good old days’ of microcontroller programming and eprom erasing so I use that as my UV light source but a regular fluorescent bulb will also work. Note that I cover the entire unit up when it is operating so that I’m not exposed to UV.
After the board has been exposed it is developed much like a photograph. The areas that were exposed to the UV light are washed away and the circuit board pattern that is left will protect the copper from the etchant during the next step. This is where the whole process can get messy with chemicals. When Darin White completes his board mill we won’t have to deal with this any more.
What’s that cooking on the hot plate? That delicious black tar is ferric chloride, a metal etchant. The ferric chloride dissolves the unprotected copper from the board and leaves the circuit design intact. The solution gets darker as more copper gets dissolved and it takes longer to etch a board. I heat it up so that it is warm and then turn the hot plate off. This stuff has to be put back into the bottle and disposed of at a chemical waste drop off site when it gets saturated.
Etched boards washed and dried.
The boards are then drilled.
Here are two completed stepper motor controller boards attached to the parallel port breakout board. Note that the cables connecting the breakout board to each controller were fabbed out of an old hard drive ide cable by cutting the header on a band saw and splitting the ribbon cable. That saved me a lot of time soldering.
The Robots are coming.
By Karl Williams