General Picaxe Question

[tjetson]

Hi there

I have a few questions about Picaxes in general.

1. Some of the parts have letters (08M, 28A, 18X). Do these letters stand for anything?

2. I've seen an input pin referred to as "high impedance". Is this high impedance to Vcc or ground?

3. Related to question 2, I have some LEDs that will still light if I use a 10k resistor at 5 volts. Could I light an LED with a Picaxe pin set as input?

4. Which of the Picaxes was the first one and in what year was it released?

Thanks

 

[hippy]

1) The letters classify the PICAXE into groups with particular capabilities. Possibly with some connotations, "A" being an advanced version of its former version, "M" having 'music capability', "X" bringing extended program size and capabilities, "X1" and "X2" extended even further - though that's my theorising rather than an official answer.

2) That's high impedance usually to both 0V ( ground ) and +V ( VCC, supply ).

3) Most likely not. High-impedance is usually extremely high.

4) PICAXE-28 was the first, 1999. A more detailed history here ...

http://www.picaxeforum.co.uk/showthread.php?t=10575

 

[westaust55]

X1 and X2 also have the TUNE command that came with the M series
but only the 08M also gets the LED option as part of the Tune command

 

[BeanieBots]

Q1, already answered.

Q2. High impedance = high impedance. It cannot be high to one thing and not to another. An 'ideal' power supply will have zero impedance and most will be very low, so the impedance between Vcc and 0v will always be low. Hence, the impedance between an input when at logic high will be almost identical to the impedance when at logic low. Do not confuse impedance and resistance, they are very different things.

Q3. No. The inputs are VERY high impedance. Their ability to source and/or sink current is WAY too low to illuminate an LED.
Not sure why you are asking the question

Q4. already answered.

 

[tjetson]

Not sure why you are asking the question

It was merely as a "is this possible" type thing, I'm not actually going to do it. I asked because I thought it was cool that my LED works using only 300 microamps, and I thought I read somewhere that an input pin has something like 10k impedance. But, it doesn't matter that much. Thank you all for the quick responses

 

[Dippy]

If you want to know the leakage current of an input then:
a) Measure it.
b) Read the electrical specs in the actual Microchip PIC data sheet.

I don't mean to sound facecious but there are so many variables that it's impossible for any one to answer.
Remember, that low-cost PIC has a number of internal 'peripherals' including the Input module and an output module.
The input module is a 'simple' digital High/Low ('digital') sense with the (>>>10K) input impedance.
The output module in most cases is a simple push-pull (source-sink) driver.

Genuinely: why is it cool?
Personally, if my PIC input allowed enough current to light an LED I'd be worried
The idea of ANY input (whether a PIC input or a multimeter) is that it can DETECT without affecting the external circuitry.

 

[tjetson]

Genuinely: why is it cool?

Well, I was under the impression that an LED needs between 10 and 20 milliamps to work, and therefore I have usually used 330 ohm resistors. My LED runs at ~300 microamps with a 10k resistor. There is a large difference here!

 

[Dippy]

An LED needs a certain Vf and a certain current (tiny) to start conducting.
But at 'what level' it is visible is another thing.
I've got a 190 page document on semi-conductor theory. I'll try and find if you want it. Excellent mathematics.
There's a lot of statistics involved.

Do data sheets show a mimimum?

Some very high brightness ones are visible (under your bedsheets with the lights out) at a few tens of microamps. Some are a real pain when multiplexing if the leakage is high.

But at 300 microAmps it will certainly be very cool.

 

[tjetson]

Do data sheets show a mimimum?

I have no idea. I got these LEDs from my teacher at school for my Tv remote project a while ago. All I know is that they are 3mm, red, seem to drop 1.7 volts and will also work with a 100k resistor (33 microamps)! That document of yours sounds interesting. Is it very complicated? My electronics experience isn't terribly good (~2 years or so). Perhaps it is worth posting your document somewhere (this forum or your site perhaps?)

 

[manuka]

Never mind 330 Ohms, as even the cheapest LEDs are now often ultra-mega-supa-dupa bright, & a 10k-100k dropper may be needed just to dim them to "normal" brightness.

I ran across some red Xmas LED light strings last year (at ~US$5 a string of 100 !) that were almost too evil to look at. Even a 1mA drain was blinding, & at 10mA they were near burning holes in the wall. Although seeing red spots before your eyes is often associated with seasonal cheer,I guess they were clearing them before Health & Safety issues arose!

 

[Dippy]

You're right there Stan.

I never know why people use 'old' standard brightness ones when ever-so-bright ones are cheap now.
They have every advantage. Lower power consumption and less strain on I/O. Availability aside it's a no brainer.
Unless.......

 

[westaust55]

Unless.......

One already has 100 “normal” 5mm LED’s in assorted single colours.

Have not checked with LEDSales in Victoria single I bought a handful of 5mm RGB type LED's form them around a year ago.

Local electronics stores near me such as Altronics, Jaycar and DSE still only stock the older more “normal” LEDs. Futurlec also have some advertised as High Brightness, low current but the specs are fairly “normal”:
http://www.futurlec.com.au/LED/LED5R.jsp
certainly nothing fancy.

Obviously a need to get more of the electronic stores “down under” to stock some genuinely high brightness, lower current consumption LED’s other than the odd 1Watt or 5Watt Cree or similar.