Mini Pinball 07: Designing the PCB -- Episode 314

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The prototype for the mini pinball machine has a 16x2 LCD display, audio amplifier, 16 switches in, 16 lights out, 4 MOSFET controllers, and 4 servo ports. In today’s episode, Ben translates this prototype into an Eagle design. Felix works on finding the libraries and the part numbers while Ben Does the layout.

Ben puts together the PCB for the mini pinball machine in Eagle. He starts with some of the more cumbersome areas first, namely the lights and switches. He’s going to use these molex 2.54 or .1 inch pitch headers.  He puts in the 16 switch molex connectors in and 16 for the lights. He’s got to think about how to orient the circuits in relation to the 16 switch molex connectors.  He creates spacing so that they are able to make an adapter board that can attach for testing.  He won’t run the traces until he has the main components on the board. There should be enough space between these.

He lays out placement for the power, the microcontroller, the screen, and input/output plugs on the lower-left. He shows the schematic of the constant current LED driver.   Ben reuses a layout of an eagle file for the schematic of the constant current LED driver. They used the TLC59282 constant-current sink driver in the persistence of vision episode and Super Space Shuttle.  You source the LED with current and then it sinks into the device. After laying out a lot of the parts for the board to run the digital signals Ben needs to attach the SPI signals between the Teensy/Arduino and the I/O chips. At this point he’s added 2 MOSFET packages, headers for the solenoids, voltage regulator, placement for the Teensy and Arduino, and some resistors.

After running the traces the PCB is done.  Power comes in through the 2.1 millimeter jack and goes to a push button switch.  A large capacitor for the 12 volt line goes to a 5 volt regulator which also has an electrolytic capacitor on it.  This regulator goes to the 5 volt input of the Arduino or the 5 volt input of the Teensy. Both the Teensy and the Arduino output their own built-in regulators.  That 3 volt regulator goes to the switch integrated circuits. There are 16 switches and 16 lights.  The light driver has 16 bits of switch input. There are connectors for 4 servos, a large electrolytic capacitor next to those servos because servos can draw a lot of power and cause spikes in power. The audio amplifier is set to a fixed gain and finally the speakers where they hook up.  Then there is the LCD screen with a silkscreen along the screw holes. The silkscreen is .26 inches in diameter. This represents the head of the screw to make sure that it’s not bumping into anything. After doing a design check, the design is ready to be sent off to get boards made.

Ben uses a schematic of an LCD display that fits his requirements but lacks an Eagle file so he’ll have to do his own.  When working with schematic design software it is often necessary to either hunt down a library or make one. For rare parts it can be quicker to make one.  For the schematic, the symbol is representative of the part, whereas the package detail comes from the datasheet. Ben creates a package that matches the physicality of the hardware.

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