December Update

Well some good news this week… I was able to gather some important information about the current draw of the RN-9090 that I wasn’t able to get before. When I first started this project, I did not know a whole lot beyond Ohm’s Law, KCL, and KVL. And even my grasp of those concepts was tenuous. I also, was not very adept at reading and understanding datasheets for integrated circuits. So, when I set out to determine what kind of power supply I needed, I figured 192 LED’s at about 17 mA each is about 3.3 Amps. I also added another amp for microcontrollers and display screen. I thought I needed about 4.5 Amps at 5V and there wasn’t really any easy way to do that from what I saw. All of the linear voltage regulators I was used to working with from the Arduino tutorials had a max output of 2A. So, I found this incredibly expensive 5V 5A power supply on mouser that was $25 and decided to go with that.

I wanted to revisit this while doing the redesign of the PCB for the ARM migration. And so I reread MAX7219 datasheet armed with more knowledge than I had previously and it occurred to me that I hadn’t counted on the fact that the MAX is actually multiplexing the LED’s very quickly and they are not all turned on at once, therefore the current draw should be much less than I had originally planned for. Sure enough, the datasheet even shows how much current is expected on each row if all LEDs are on! Now, I have a multi-meter but, it isn’t the best and I really wanted to make sure I gathered good data so I took the RN-9090 down to the ECE lab at my school and used the multi-meter there. And what do you know? The max current draw for the RN-9090 is less than 500 mA. That means I can significantly reduce the power requirements! I can switch to a more common and cheaper power supply and use a common linear regulator to get the voltages I need! I figure this probably just saved $20 on the cost. Hooray!