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Stamp I/O max voltage — Parallax Forums

Stamp I/O max voltage

ltmhallltmhall Posts: 102
edited 2007-03-26 17:34 in BASIC Stamp
I built a sound detector circuit for my robot that has a output of 5.75V whent it detects a
4kHz signal. Is this to much voltage to apply to a input of the basic stamp. Or should I just decrease
the supply voltage or input a resistor on the output ?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-26 15:10
    This is a little too high for a Stamp. You should either reduce it slightly (below 5V) or use a series current limiting resistor, 4.7K to 10K typically. What happens is that there's a protective diode that conducts if the voltage on an I/O pin goes above 5.7V (one diode drop above Vdd - 5V) clamping the excess voltage and needing some current limiting.
  • ltmhallltmhall Posts: 102
    edited 2007-03-26 15:22
    If I use a current limiting resistor this wont' drop 5.7 volts will it?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-26 15:30
    The resistor will drop the difference between the applied voltage and about 5.7V (one diode drop above Vdd). The main reason for using it is to limit the current to well under 20ma.
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-03-26 15:41
    The PIC processor used in the BS2 was designed with "protection diodes" on the I/O pins. These "protection diodes" will be forward biased, and conduct, one diode if the voltage on the pin goes below 0.6 volts, and another if it goes above 5.6 volts.

    Now, these diodes can't conduct a LOT of current. What this means is there needs to be some serial resistance (like 10 Kohms) between the I/O pin and the input voltage.

    Since the 'normal' input resistance of the I/O pin is quite high (meaning the normal input current is quite low), putting a 10 KOhm resistance in series with the pin should not drop the 'normal' signal very much, and will limit the current the protection diode has to deal with.

    As another example, using a 22 Kohm resistor enables the PIC I/O pin to be used to recieve the +- 10 volt RS232 signalling voltage directly. The 'internal protection diodes' will 'clip' the resulting waveform that the BS2 'sees' to 0 and 5 volts.
  • ltmhallltmhall Posts: 102
    edited 2007-03-26 16:24
    The sound circuit I built was designed to use a 5 volt source. However, when I emitted the 4khz signal the output would
    only rise to 3v. So I used a 7.5 volt source and when the circuit detected the signal I got out 5.75v . Do you think
    that using the 7.5 volt source( even though the circuit shows 5 volt source) to get an output of 5.75v out then using a current limiting resistor is a good idea ? The components I used were resistors capacitors, a microphone, and a LM324.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-03-26 17:09
    The I/O pins of the BS2 are designed with a TTL threshold of about 1.5V. Anything over that will be treated as a high level so you should run your sound circuit at 5V instead of 7.5V and not use the series resistor.
  • ltmhallltmhall Posts: 102
    edited 2007-03-26 17:34
    Thanks alot I figured that it would have to be at least 4V to be considered high
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