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Output pins

readersreaders Posts: 10
edited 2009-04-08 03:12 in BASIC Stamp
Can one drop a 5 volt·dc onto a pin that was made to be a output pin without hurting a BS1.
The reason I ask such a question,·the project that Im building I need to use·six BS1 output and·six·switches
to make the my project work as in auto and manual mode.

Comments

  • Carl HayesCarl Hayes Posts: 841
    edited 2009-02-06 15:21
    No.

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    · -- Carl, nn5i@arrl.net
  • Mike GreenMike Green Posts: 23,101
    edited 2009-02-06 15:24
    If the output pin is set HIGH, then it's already at +5V and wouldn't be damaged. If the output pin is set LOW, then it's at 0V and you have a direct short circuit between the 5V and ground through the output circuitry. Likely the output pin circuitry would melt. The HomeWork board has 220 Ohm resistors in each of the I/O pin leads for just this possibility. These limit the current to about 20mA in case of a short circuit which is tolerable for the Stamp as long as the total current for the Stamp is under around 100mA.

    Can you attach a schematic drawing of what you're trying to do? It may be that a similar resistor is all you need, maybe 330 Ohms. What's the output controlling?
  • readersreaders Posts: 10
    edited 2009-02-06 18:01
    Mike

    Here is a small part of the schematic that I made for you.
    Please note that it only shows one output and one button all other are the same.

    Readers
    bmp
    bmp
    One switch output.bmp
    385K
  • Mike GreenMike Green Posts: 23,101
    edited 2009-02-06 19:43
    You will destroy the Stamp I/O pin if the pushbutton is pressed any time that the pin is a low output. Since you can't guarantee that the button will be pressed only when the Stamp I/O pin is an input or high output, you will at some time blow one or more I/O pins.

    I don't understand what you're trying to do with the 100K resistor. I would suggest putting a 330 Ohm resistor between the Stamp I/O pin(s) and the switch. Normally the Stamp will drive the 4013 and the 7404. When you push the pushbutton, the switch will override the Stamp. If the Stamp is expecting this to happen, it can set its I/O pin to input and read the switch setting. The 330 Ohm resistor is only about 3% of the value of the 10K pulldown, so it shouldn't have a significant effect on the logic high voltage for the 4013 and 7404 since it forms a voltage divider with the 10K resistor.
  • readersreaders Posts: 10
    edited 2009-02-06 20:26
    Mike
    Can I not install a diode in series with the lines from the output pin to stop this from happening?
    readers
  • Dave-WDave-W Posts: 94
    edited 2009-04-08 03:12
    Readers,
    Listen to Mike. Answer his question and do not try to over engineer a circuit. Mikes simple answer to the major problem of your circuit will work correctly as you want it to. I also do not know what the 100K is for?
    Dave...

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    Dave W.
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