Teardown Tuesday: Homebrew Practice Keyer – Part 2
A couple of weeks ago I wrote about En's grandpa's practice keyer. As I was going though it I sketched out a quick schematic.
Yeah... I swapped the symbol for the battery. Deal. At least I labeled it. :-PBasically what I can tell from looking at the circuit (and the chance that I'm wrong is pretty high, so keep that in mind) is this:
- When the key is closed
- The base of the NPN transistor (Not Pointing iN - the top one up there) is pulled high through the 10K resistor which turns it on
- And that pulls the base of the PNP transistor low relative to it's emitter which causes it to conduct as well (the resistor drop should drop enough voltage to forward bias both transistors.
- This drives the collector of the PNP high, charging the cap and pushing the speaker in one direction
- Which pushes the base of the NPN low since it passes the high frequency
- Until it discharges through the speaker pushing the speaker the other way
- Repeating the cycle
So, like I was saying last week I was experimenting around with some circuit design software, specifically I tried out Altium. So I did a bit of rudimentary schematic capture with it and here's what I came up with in the end:
This looks a lot more readable than the sketch too. :-)Then, let's go for the gusto... PCB layout!
Not too bad! All on a single layer, no jumpers, and not too cramped. I'm not going to run a batch of boards though. This is a perfect job for a breadboard, but that's for another time.I have to say that once I figured out how it wanted me to think it wasn't too hard. I'm sure I have some of the footprints wrong... but that's OK for now. It's the thought that counts.