BS2, a pin as input and output

Comments

• Archiver Posts: 46,084

  2003-12-11 05:55 edited 2003-12-11 05:55

  Why don't you try using a multiplexer/demultiplexer, 1 to 8 type. It
  works like selector switches and work in both direction. You just need
  3 pins for addressing. If you use the 1 to 16 type you need 4 pins for
  addressing. In this case you need to redesign your circuit a bit to
  free 3 or 4 pins but in the end you get more pins.
  
  - Johari
  
  --- In basicstamps@yahoogroups.com, "Scott" <scott@m...> wrote:
  > I'm out of pins in my (now infamous) Alarm Clock project, and I'd
  like to
  > wedge in one more input pin. So now it's time to resort to a trick. Will
  > this work? My concern is damaging the BS2.
  >
  > I have an output pin that goes through a 1k resistor, to an input on a
  > ULN2003A (Darlington pair base). The Darlington's output is a
  speaker, and
  > the output pin is used for occasional FREQOUT purposes. (The 1k resistor
  > reflects my obsessive fear of hurting pins - the darlington already
  has a
  > built in 2.2k resistor).
  >
  > Periodically, I'd like to redeclare this BS2 pin as an input, and
  test it.
  > I'm assuming that base-emitter current from the darlington (through 1k
  > resistor) would make the pin appear low, without damaging the BS2. Now
  > assume +5v is applied, through a separate 1K resistor, to that same
  BS2 pin.
  > This would presumably drive the pin high (and also turn the
  darlington on,
  > but that will just make the speaker click and would be harmless.)
  >
  > It's certainly possible that the +5v could be applied to the pin
  while it is
  > in an output state, during a FREQOUT or otherwise. This means an
  output pin
  > is connected through 1k to +5v. The output might be low or high
  (during a
  > FREQOUT). Any harm in that?
  >
  > +

  ---

  1K

  ---

  switch

  ---

  +5v
  > |
  > |

  ---

  BS2 pin (sometimes in, sometimes out)
  > |
  > +

  ---

  1K

  ---

  darlington input
  >
  > Comments welcome.
• Archiver Posts: 46,084

  2003-12-11 14:24 edited 2003-12-11 14:24

  It can and has been done.
  However, in your case the input would be marginal, the output resistor should be
  at least 5 to 10 times the value of the input
  resistor.
  If the input and output resistors are of the same value the input would see
  2.5v.(which should be enough for the stamp, but is a bit
  low for a logic high in general)
  If you made the input resistor 330 ohms and/or increased the value of your
  output resistor you should be ok.
  
  - KF4HAZ -
  

  ---

  From: "Scott" <scott@m
  
  > I'm out of pins in my (now infamous) Alarm Clock project, and I'd like to
  > wedge in one more input pin. So now it's time to resort to a trick. Will
  > this work? My concern is damaging the BS2.
  >
  > I have an output pin that goes through a 1k resistor, to an input on a
  > ULN2003A (Darlington pair base). The Darlington's output is a speaker, and
  > the output pin is used for occasional FREQOUT purposes. (The 1k resistor
  > reflects my obsessive fear of hurting pins - the darlington already has a
  > built in 2.2k resistor).
  >
  > Periodically, I'd like to redeclare this BS2 pin as an input, and test it.
  > I'm assuming that base-emitter current from the darlington (through 1k
  > resistor) would make the pin appear low, without damaging the BS2. Now
  > assume +5v is applied, through a separate 1K resistor, to that same BS2 pin.
  > This would presumably drive the pin high (and also turn the darlington on,
  > but that will just make the speaker click and would be harmless.)
  >
  > It's certainly possible that the +5v could be applied to the pin while it is
  > in an output state, during a FREQOUT or otherwise. This means an output pin
  > is connected through 1k to +5v. The output might be low or high (during a
  > FREQOUT). Any harm in that?
  >
  > +

  ---

  1K

  ---

  switch

  ---

  +5v
  > |
  > |

  ---

  BS2 pin (sometimes in, sometimes out)
  > |
  > +

  ---

  1K

  ---

  darlington input
  >
  > Comments welcome.