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Forum Index : Electronics : Pixaxe - where from?
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| RossW Guru  Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Hey Glenn, I've been pondering how to build my new solar tracking head. It has enough functions to make doing it in discrete bits just over the "practical" limit. By the time I put timers, dutycycle control, power-up test/calibrate functions etc, and comparators, window/deadband, return-home logic etc, its just... well, not worth it. Soo... I remembered your little pump controller - and it's actually kind of "close" to the mark as it is. Where do you get the Picaxe 08M from? None of my local bods seem to know or stock it. How much do they cost? and perhaps more importantly, how much current do they draw? I'm going to have to drop down from a 50V (could be up to 60V) supply. Its not worth building a switching supply, so it'll be a series regulator, but I don't want to dissipate too much power. If it only takes a few mA, I can probably get away with a zener dropping element, or a basic zener pre-regulator to a 78L05... I'm planning either a H-bridge with N mosfets on the low side and P mosfets with bipolar switching for the high side, or perhaps a much simpler single N-ch mosfet as the (single) switching element and a relay for direction control. Obviously the H-bridge has benefits, but I'm not sure the advantages outweigh the additional complexity. Thoughts? |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I buy my PicAxe chips from Oatley Electronics http://www.oatleyelectronics.com/componentsa.html, and I think Altronics also stock them, or is it Jaycar. Either way, Dick Smith dont 
The little 08M is about $4, and has 4 i/o pins, so you could use two for the light sensors and 2 for the H bridge. A 18X is about $14, and has a lot more pins to play with, so you could have a lot more functions built into your tracker. The software is free, easy to use and comes from www.picaxe.co.uk, as well as manuals and data sheets. The manuals also go into using sensors, like LDR's and driving mosfets, relays, etc. Current draw for my PicLog is about 8mA, but most of that is just to drive the power LED  , so I would guess 1 or 2 mA. A series zenner and 7805 should work on 30+ volts. As you can tell I'm a big fan of these things. Once you use a PicAxe for the first time you will wonder how you ever did without them.
Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Jaycar don't, I just checked, and I knew DSE didn't. I guess I can't do anything this weekend then, I'll just have to order some. Well, I was thinking of something a little trickier actually. I'm a big one for doing in software - once it works, it never wears out or gets affected by heat or cold, or damaged by lightning. I was "playing around" with, and have got comfortable with using a couple of high-intensity yellow LEDs. They produce about 1.6V in full sunlight. Two of them, connected back-to-back, one facing east-of-centre, and one west-of-centre. I'll get either 0V (if the illumination is identical both sides), +V if the sun is one side, -V is its the other side. An artificial "zero" by a simple voltage-divider across the rail (lets say, 1V bias, should give me from 0V to +2.6V (clamp it on the -ve side). Feed that to the A-D input and I can do the whole east/west detection, along with deadband in software using really, no more than two resistors and two LEDs, direct into the chip. (I hope it's a fairly high impeadance input?). That leaves me one more I/O bit if I ever need it. Beaut, I'll go check it out. Oh, I've been using bigger things for ages - one of my earliest projects (~1979) was a Z-80 based controller for our local ham repeater. Did all the morse ident, timers, tails, radio linking, trunking, lockout, status query/transmission, all in 2K of EPROM and a small amount of RAM. And it fitted on a board about 1/16th the size of the original controller, took under 1/10 the power and ran for years without ever being touched again. Of late, I've been using the ATmel chips - 90S8535 etc, but that's just waaaay overkill for this application. Thanks Glenn, I'll go have a look-see. |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Glenn, I'm confoozled. In your (revised) Piclog, you say: All the guff I can find on the picaxe says the analogue input (except the low-res stuff, which is only 16 steps, so clearly you're not using that) - is 0-5V. However! .004V per bit = 1250 levels, not a 2^n, so I'm guessing its really a 10-bit conversion (not 8 bit as the docs seem to say), which would mean 4.882 millivolts, or close enough to 5mV. That said, with a 0.1 ohm sense resistor, you'd need 50A to get full scale, which is a monster 250 watts dropped across the sense resistor! (P=I^2 * R) What am I missing here? What started this, was I am about to build two little toys: one is a (small) battery exerciser. (I have probably a hundred 7AH/12V SLA cells in unknown state kicking about here). I'd like to monitor their voltage and current, charge them at a known rate (watching voltage, current etc), then discharging them at a known rate and check their capacity (mA-Hrs), recharge and then either get rid of the dud ones, or keep them if they're ok. (This will require bi-directional monitoring - I built up an opamp circuit in a bridge, biased to Vcc/2 at 0A, and going above/below that proportional to current and direction. My single op-amp circuit works great, except that it can't swing rail-to-rail. TL071 is about 800mV short of either side, which is somewhat of a limitation! I'm pondering shelling out for a better opamp that will go rail-to-rail, putting up with the loss of resolution, or if I need to synthesize a negative rail (yuck!)) The other project is one to monitor energy from whatever RE sources I have - HAWT, VAWT (when I get it made) and PV arrays - to provide useful information in terms of what energy I've got over the day, and where from. Both projects will have an LCD character display, and the logger will have an ethernet connection (using a Lantronix XPort). I'm thinking they'll need to use something more grunty than an 08M though :) a monitor for my place - |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeah Ross your right, the ADC in the Picaxe is 10 bit capable. And you right about the shunt resistor wasting so much energy as heat. I dont like shunt resistors, but I tried to get a linear accurate hall sensor to work without much luck. And I even tried using a op amp, like you said, to increase the sensitivity, but I'm a coder, and not that good at analogue circuit design. The ACS750 discussed here http://www.thebackshed.com/windmill/forum1/forum_posts.asp?T ID=289&PN=1 would work perfectly, its a calibrated linear hall sensor, meaning no shunt resistor. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 1344 |
Eh Ross, I be finally posting the packeage to you on friday and I can throw in a ACS750 or one similar as I can't remember which ones I got as samples. I'm sure you'll love some of the pic chips as most if not all of them have plenty of analogue inputs. Cheers Bryan  |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
You lil' bewdy mate! Can't wait :) I had to do a nasty bodge job today. I'm almost scared to admit it - but damn, my eyes have gone to bits over the last 15 years. I ordered replacement chips for some (believed) damaged drivers - but they supplied the right chip in the wrong package. Being as patient as I am (Ha!), I couldn't wait for them to ship the right ones... this bugger is only 4mm long (my index finger shown for scale) 
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