Basic Stamp/Electronics 101 Question?

kenwtn

···I had read somewhere but can't remember where that a 1K resistor should be put on any of the 16 I/O lines when using them. The reason they gave for this was that would protect the PIN should a un-expected reverse surge take place. Is this fact fiction or did I mis-unstand what was being said? What pre-cautions should be taken if any to protect the basic stamp?

kenwtn

I know I mis-spelled understand and maybe others words as well, but hope you get the general idea of what I am asking.

Russ Ferguson

Common practice is to put a 220 ohm resistor between the pin and the circuit that it is connected to. A 1K ohm could be used in some cases, but the standard current limiting resistor that is used is 220 ohm.

The goal is to keep the current flow through a pin to less than 25 ma. Ohms law tells us that when 5 volts is across 220 ohms of resistance then ~23ma of current will flow. Since the 220 ohm resistor is in series with the input/output circuit that you have built, there will be more resistance than 220 ohms and even less current through the pin.

kenwtn

That would make sense and I will try that. My goal as is the goal of everyone I would think is to keep from frying their Basic Stamp Module.

LoopyByteloose

The 1K just adds 4-5 times the current limiting. In some ways a bit more protection. If you connect a higher voltage by mistake, you would have a bit more leeway. These things are not set in stone, but signifcantly higher values at some point will shut down the circuit.

On the other hand, the 220k is nearly the minimum.

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

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steve_b

Something else to consider, when working with TTL I/O's, is using optoisolation.

This removes the 'direct connection' to the outside world by putting in a little light and a light sensor which will likely blow when things go bad in the outside world.
NOTE, however, that lightning will kill the opto AND anything within its vacinity.
Lightning protection is a bit of a black art....you never know if you're protection scheme works, until it doesn't work!! haha

I've included a simple picture of a 4N35 optoisolator I've used in the past.

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."