Sunday, January 29, 2017

Memory is the second thing to go

It's said that as we age, your memory is the second thing to go. And I don't remember what the first one was...

Since I have new incentive to learn to use the KiCad electronic design suite, I thought I should start with a small project. Something worth doing, but small enough that I can focus on the tool rather than the project. Also something that wouldn't cost much to re-spin if I screwed it up.

One of the first that came to mind is a test board to drive the Noritake-Itron DS2029H Vacuum Fluorescent Display (VFD). The connections to this display are a row of flat fingers along the top edge, kinda like an integrated circuit, as you can see in this photo:

I'd just plug this thing into my solderless breadboard except for one thing: the fingers have a 2.00mm pitch, not the 2.54mm pitch of my breadboard. Bummer.

I'm weighing two options for PCBs. I could make a board that mates with these fingers and connects them to pins on 2.54mm (0.1 inch) centers. Then I could wire test circuits on a solderless breadboard. The other is a complete VFD test board supporting a PIC, a USB serial interface, and the 5V-to-36V boost regulator needed to drive the VFD anode. Right now I'm leaning toward the latter.

How does the title of this posting figure into this? Keep reading...

Before I can make a PCB, I need to select the components I'm going to use. Noritake-Itron suggests the following circuit for the 36V supply:

As I searched DigiKey for the proper parts I started to notice that the parts I needed were showing up in my browser as if I'd looked at them before. Maybe I'd looked at some of them them a couple nights ago when I started thinking about this. But before I got too far I took a look at the list of my previous orders and noticed one labeled "VFD Supply" from April last year. Opening that one I discovered that I'd already bought all the necessary parts. Strange.

Sitting down at my workbench I dug through my box of parts and came up with the packets from this order. Then I noticed that one of the three MC34063AP ICs I'd bought was missing. I have several solderless breadboards, and I discovered that one of them hosted a partially-assembled circuit:

Yup, there's my missing MC34063AP. And the missing 150 uH inductor. And the 1% resistors that make up the voltage divider setting the output voltage. Not only had I forgotten that I'd bought the parts, I forgot that I'd actually assembled the circuit. And apparently tinkered with it, as there is an extra 10 nF capacitor in parallel with "CT". But it's missing the +5V connection to pin 6, which either never got installed or was stolen later for use elsewhere. Well, it has been 9 months.

Not surprisingly, the completed circuit works just fine, producing 36.25V (VFD spec "typical" is 36.0, "max" is 40V). With a 390 pF capacitor for CT the oscillator runs at about 83 KHz. Using a 33 uF capacitor for CO shows only a very slight ripple. Loading the circuit with a 2.2K resistor (about 16.5 mA; spec "typical" is 14.0, "max" is 25.0 mA) doesn't seem to affect it.

Once I find some sort of socket for the VFD pins I'll be ready to design my first circuit with KiCad!

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