The July 31, 2014 Arduino Memory Keyer Update

By JonathanGuthrie, 31 July, 2014

I survived ApolloCon just fine, and I'm more energetic than I can remember being, so I have started to work on projects again. I have too many projects, but I still want my memory keyer to be finished, so I'm working on that.

A while back, I finished the board layout for the correct transistor pinouts, and I sent it off to OSHPark to be turned into actual boards. I also ordered parts. Last week I even found out what was wrong with my soldering iron (it wasn't set up right at the factory) and I fixed it. So, I was all set.

I put the parts on the board and powered it up and...nothing. Well, no sound, anyway. The LED's on the Arduino flash, but no noise. So, I get out my trusty oscilloscope and there's a signal on the speaker lines. Sort of. If I put the probe on the collector of the amplifier, I get a nice square wave. So...what? I was talking it over with my coworker, and he suggested that the blocking capacitor was too small. I had replaced the 0.01uF capacitor on the original board with a 100 pF on the new board. And that value is too small. Why did I use it? Well, I'm used to looking at RF schematics where fractional values are microfarads and whole number values are picofarads and this component is labelled on the schematic as "100". However, the legend says "capacitors are uF unless otherwise marked" so I bought a capacitor that's a million times too small.

The question then becomes what to do about it. The capacitors on the board are all surface-mount, and they're all 0805 components. 100 uF surface-mount capacitors exist, but the smallest size available is 1210, which is much larger than 0805. The largest 0805 capacitor I could find is 22 uF. So, I've layed out the board again, with one space for a 1210 capacitor. However, I don't want to respin the board for one freaking capacitor. The surface-mount components are less than $5 and the board is like $9, so it's not worth it, especially since I only need one of these things and I have to order boards in 3's. I'm getting a 100 uF capacitor from somebody in the QRP club, but it's an electrolytic so I'll have to be careful how I use it. I'll wire it in to the speaker wire, but that means I have to plug it in the right way, or it'll be destroyed.

I think I'm going to order a new set of parts including two of the 22 uF capacitors. That way, I can compare the sound output with 100 uF vs 22 uF and see if there's enough difference to worry about. And, if I decide that it's not a big deal, which is what I think will happen, I'll just rework the board and lose the electrolytic.

Next, I'm going to be wiring up the PTT and keying circuits. I don't need the sidetone if I'm keying a radio, and I'm anxious to see if the keyer works as a keyer. Stay tuned, it's about to get functional!