DorkbotPDX. While there is no way I could be out there for the party, I do like to support group orders and my old bus pirate (many versions behind) is starting to show it's limits.
Yesterday the mailman brought the happy package and while I wasn't planning on soldering yesterday I get that itch and just had to build it.
The purple pcb (classic Laen) always makes me smile.
A ribbon cable, snap on connector, headers also came in the box in a bag with the pcb.
PIC 24FJ256GB106) to handle the usb connection directly and I wasn't certain how it would work... No problems at all (I suspect it will allow higher throughput/lower latency than the ft chips but it will be awhile before I can test that out).
I didn't pay attention to the orientation of the socket because I plan on using other probes and not the ribbon cable...
If you don't have one, you may want to consider picking one up (although I think Ian still recommends the previous version - not certain about that).
Lots of information at Dangerous Prototypes.
Friday, April 6, 2012
Thursday, April 5, 2012
So yesterday Mr Jiggles became a little oscillator - but rather than a balanced sine wave we had a light distortion where the upper half was a bit wider and the lower half was a bit narrower.
I usually like to check the ground potential relative to the output if there is a problem, but this one is fine - maybe it could use a cap across the the supply but nothing to explain the distortion.
Just for kicks let's look at the power coming into the collector at the same time as the oscillator state. Lots to learn from this one... The first thing that jumps out is that our output is a balanced cyclic form but the voltage looks like two different curves. If you haven't guessed, the reason is the led (remember there is a minimum voltage before it conducts and until we hit that voltage on the emitter not a lot gets to flow) - caps cover the transition interval. We could just leave out the led and only have a resistor to limit the current but since we can't always have things linear in real life I'm leaving it in - for character.
Other fun things to do are to tap from the collector or the emitter, to put resistor dividers in line to change the output voltage range, to bias the base to change the operating point of the transistor... lots of things.. endless things...
Anyway, every once in a while I like to just play around a bit and get a feel for how things actually work and see the changes in reality... touch a few wires to see which is sensitive to noise and how that propagates... troubleshoot the unexpected behaviors... see what happens when the loads match and when they don't... it's silly and pointless, but fun.
Wednesday, April 4, 2012
I like to think of crystals as highly ordered structures that deform a bit in the right kind of electric field - remove the field and they go back to their original state and in the process create a tiny electric field of their own. Now depending on how the little crystal is cut, what it's composed of, how it's treated it will have at least one frequency at which it likes to oscillate.
The other thing we need to consider is how we will make the little amount of energy useful - we need to make it bigger. It happens that our little friend (the npn bjt) does a great job of taking a tiny current and making it bigger.
I know I'm going to link the crystal to the base (to control a bigger current collector to emitter) and I know we need a voltage to load the crystal (but not a lot) so let's start with a 220k resistor to pull the base high from the collector.
Ahhhh... silly mistake. If you look back at the sketch I connected the crystal directly to the base (and this isn't going to leave any room for the jiggling to happen unimpaired - like trying to dance in a car - not going to be beautiful).
Enough for today... think about it and we'll see next time.,.
Monday, April 2, 2012
This one has a little offset (doesn't quite rest at zero) but the for now that doesn't matter. The range seems to go to 20 milliamps.
But... if we put the panel meter in the circuit the needle slams full scale instantly (oh no 3.27mA should be less than 20mA - what's wrong?)
Our little meter doesn't say what the actual resistance is (we could measure it but that's means we have to do a bit of math (very simple math) and today I don't want to do math for this.
When we do this it comes to about 117 ohms (and that's easy to make with a 100 ohm and a 15 ohm resistor in series).
Just for the heck of it the math for figuring out the extra resistors is easy: The meter is 73.9 ohms, the resistor we want in parallel is 9.93 ohms and we want a 20 fold change in the scale... so (73.9 + x + 9.93)/9.93 = 20 -> 83.84 + x = 20(9.93) -> x = 198.6 - 83.84 -> x = 114.86
So all this is kind of silly - it's a very simple thing to do but it's fun sometimes to do things in a different way than usual. Now everyone seems to like digital displays - they are great, but there's something about the old style panel meters that I have a fondness for...