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I/O Board


Now I've shifted my focus to building a I/O board for all the many lights and small devices used on a pinball machine.  In order to reduce the amount of space I would need I opted for Darlington Arrays which are many darlington pairs (remember those?) in a nice DIP package. I am using ULN2003 darlington arrays.  They have 7 outputs and I wished they had 8 outputs then it would line up so nicely with my shift registers.  Unfourtunately, they do not and my arrays and shift registers are staggered. 

Initial Testing of the I/O Board controlling some LEDs via a IDE cable from a computer.

The I/O board uses shift registers to control all the outputs and darlington arrays to sink the current.  It is important to note that they sink the current and do not drive them.  The 'outputs' of the array are hooked up to the LED which is then hooked up to the positive voltage.  The darlington pairs inside the chip, when turned 'on', allow current to flow to ground hence they 'sink' the current and do not drive it.  MOSFETs in particular use this type of setup.  I discovered that some of the problems I was experiencing with my solenoid board was that I had initally set it up the other way around.  The MOSFET before the 'load' of my solenoids instead of after in the circuit.  This is not so much a problem with transistors but I will try to use them in that sort of configuration in the future so as to avoid potential problems.
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