8 replies to this topic

[sgraves]

I'm just starting off in electronics and am coming from a basic stamp 2 which allows 20/25mA source/drain on each GPIO pin. It looks like most mini pins are limited to 8mA. Does the 8mA current limit apply to 5v regulated vss/vdd as well? I have an LED that works best around 15mA and was wondering if it would be safe to use on the regulated source.

[NooM]

yes you can use that from pin21. i use: random pin -> resistor (2kohm) -> transistor (bc 547/557) -> stuff. so nothing will burn and the led or whatever gets all the current it needs

[sgraves]

yes you can use that from pin21.

i use: random pin -> resistor (2kohm) -> transistor (bc 547/557) -> stuff.
so nothing will burn and the led or whatever gets all the current it needs

Thanks. I was assuming 200mA for vdd/vss but wasn't entirely sure. Still making my way through the basics.

[NooM]

i honestly dont know what the maximum for that pin is. i wouldnt use more than 200ma. since switching regulators are cheap as hell (and quite efficient) i use them for the power rails on my breadboard

[Paul Newton]

Hello sgraves,

Welcome to the forums!

There is a wiki page by CW2 giving current limits for the family.

He has a figure of 800mA for the DC output. This is a lot, and you should note this is a maximum, and that the regulator on the board will get hot (especially it you have a higher suply voltage).

Again refering to CW2's page, there are infact two pins on the mini that can drive up to 16mA. You could use these without a transistor, but will still require a resistor.
Asuming the LED has aforward voltage of say 2V:
R = V / I = (3.3 - 2) / 0.015 = 86 ohms
Since resistors come in "preferred values", go for the next highest value - 91ohms.
I was going to post a link to a table of resistor values, but Android has decided to drop my cut and paste ability mid post.
Do a google search for "resistor prefered values"

Hope this all helps - Paul

[chizz]

Just for passing noobies,

"i use: random pin -> resistor (2kohm) -> transistor (bc 547/557) -> stuff."

works great,  but will logically invert the signal.

ie

  myStuff.Write(1)   will give 0V

and

myStuff.Write(0)     will give 3.3/5V

 

bear it in mind!

[NooM]

i do it the same way, good thing of not blowing something up

 

theoretically you can connect the transistors different so its no more inverted, but i also go the standard way. (inverting)

[sgraves]

Thanks for the info. I'm avoiding the vdd/vss/io pins altogether now and simply using a direct connection to the battery. I wound up using all the i/o ports anyways.

[Sigma7]

By the way...

 

The Basic Stamp 2 (BS2) can source/sink 20/25mA per PIO pin.. but only 40/50mA TOTAL (for 8 I/O pins) (per BS2).

The AT91SAM7X512 can only source 8mA per PIO pin.. but 200mA TOTAL.

 

Tom