5/18/11 - Timer and PCB Design

Since my last post I have determined that the clock source I will use for this project is the HTC555DC timer dip chip. This package is perfect for this application because tying all of the frequency select pins to ground produces a 1 Hz signal on the output line. A testbench circuit using the 555DC timer to produce an led blinking at 1 Hz is shown in the picture below:

Also, I have determined how I'm going to implement the PCB for this project. At first, I had intended to do all of the led and logic circuitry on one board. Because I wanted to etch my own PCB for this project, I thought it would be helpful if I could have a double sided PCB and be able to run traces on either side of the board. However, the type of board I would have needed to make that work doesn't exist and it still would have required a lot of jumper wires to make the circuit work properly. Therefore I came up with a new design that uses two different PCBs. The top PCB will have only the LED circuitry, and the bottom PCB will have the timer and logic circuitry. The number of traces needed to run the LED board is 23 power lines and 1 ground line. Therefore, to connect the two boards I will use a 24 pin ribbon cable. For final packaging, I am thinking of getting two sheets of plexiglass and suspending the boards in between them with risers. This will allow the circuitry to be visible from the front and back.

The process I am following to etch my own PCB requires the design to be laser printed on a piece of magazine paper in order to be able to be transferred to the copper. Therefore, I needed a way to draw a circuit on my computer that was simpler than Cadence or Eagle but more robust than say paint. After looking around, I found a program called ExpressPCB. This software allowed me to create a board of the right size and add pads and traces with relative ease.

I have finished the PCB design, minus any tweaks that may need to be made after finishing the other board, for the LED board. It is shown below.

The red lines are VDD and GND traces and the red circles are pads for led pins. The yellow block at the bottom is the footprint for the connector for the ribbon cable that I mentioned earlier. I have basically inferred the trace widths and pad sizes based on the info given in the PCB etching tutorial. The pads are 0.1" with a 0.035" hole (hole tolerance +/-0.004). The traces are 0.05", and the connector footprint is for a Connector - Ribbon, rt, 24 pin (3M 3627-5002, DigiKey MHR24K).

My next task is to figure out the rest of the timing circuit and do the seven-segment-decoder circuits for the 4 numerals on the clock.