220 Ohm resistor from each I/O line? y/n?

bulkhead

Hi, I'm setting up my own prototyping breadboard and I was wondering if i should add a 220 ohm resistor from each I/O leg of the stamp to my breadboard (where i will be plugging in leds, sensors, servos, etc). I read somewhere that this is supposed to protect the stamp from accidental miswiring? is this true? Is the value of 220 ok for all legs, or do i need to "calculate" a specific value for each separate application?

Jon Williams

You don't *need* the resistors, but they will usually save you from an accident. Let's say you connect a push button between an input and ground (and you add a pull-up) to the pin. So long as that pin is always in input, no worries. But if the pin is accidentally made and output and high AND you press the button, without the 220 you would have a direct short to ground and could damage the pin -- the 220 limits the current to a safe level and spares the pin damage.

So, to add them is up to you and what you think your experience level is. Resistors are often called "current limiters" -- I also call them "cheap insurance."

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Jon Williams
Applications Engineer, Parallax

bulkhead

Wow, thanks for the quick reply. I think that explanation made sense, so since I'm a beginner, I'll put in the extra soldering time to hook up 16 220 resistors.

LoopyByteloose

Uh... soldering those in place may work for awhile, but you should keep in mind that as you learn more they may get in the way.

It would be better just to use them in a breadboard until you feel that you no longer need them or want to change the value. Some circuits, like the Resistor-Capacitor combinations may work well with them, but others may need to add 10,000 ohms., some may add 1,000 ohms, and some LED circuits put the LED directly to the Stamp and the 220ohm [noparse][[/noparse]or 330ohm or 470ohm] between the LED and +5 volts.

As you learn the math [noparse][[/noparse]and concepts], you will begin to see why.

There is another way to protect the BasicStamp for outputs that may drain too much. You can put a 74HC07 'hex buffer' between the Stamp and the device. If you are wrong, the 74HC07 will suffer the damage, not the STAMP. It allows for six I/O pins.

Jon has written a lot of Nuts n' Volts articles that really explain these things. They are on-line and Free for you to read. Also, you can download a copy of 'What's a Microcomputer?' and use that to guide you through quite a lot of information.

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"When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 10/16/2005 2:37:37 PM GMT