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question about turning multiple leds on — Parallax Forums

question about turning multiple leds on

HillsHills Posts: 13
edited 2006-04-03 00:39 in BASIC Stamp
hi

I just want to know quickly how many leds can I turn on and off ( how many outputs can I use ) with a resistor of 360 ohms for each output

quickly please

thank you

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-04-02 23:11
    Directly? Probably no more than two. Using a driver (e.g. ULN2803) you can drive a whole lot more, but that answer will require some more input from you: what is your source voltage, what is your LED forward current....

    You'll get better, quicker answers when you provide more complete information with your questions.

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    Jon Williams
    Applications Engineer, Parallax
  • HillsHills Posts: 13
    edited 2006-04-02 23:19
    ok thanks for the reply

    but if i want to use a digit display ( with number eight ) with the basic stamp 2, is it okay if i put a HIGH value with the ( VDD Voltage 5 volts ) to five outputs all in the same time and each output has a resistor of 360 ohms ?
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-04-02 23:24
    What's the forward voltage of the LED segments?

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    Jon Williams
    Applications Engineer, Parallax
  • HillsHills Posts: 13
    edited 2006-04-02 23:31
    I will explain to you my circuitry, it will be more simple. Look, my basic stamp 2 is powered with a simple 9 volt battery. I use 7 outputs ( P0 TO P6 ) to turn on and off simple leds with a resistor of 360 ohms for each. And the end is all connected to pin 4 ( vss ). Is it okay. Is my basic stamp 2 is safe from burning?
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2006-04-02 23:38
    You still need to know the forward voltage of the LEDs to accurately answer that question -- you can't calculate the current through the 360 ohm resistor without that knowledge. If you assume (which may be dangerous) that the LEDs have a 1.5v forward voltage, the current would be about 9.7 mA ((5 - 1.5) / 360); which means that on the BS2 you can only run four segments at one time -- not enough to do a full digit. Change your resistors to 1K and you'll be safe to run all seven segments.

    First things first: get out your meter and using the diode check function measure the forward voltage of one of your LED segments.

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    Jon Williams
    Applications Engineer, Parallax
  • HillsHills Posts: 13
    edited 2006-04-02 23:43
    Thanks a lot
  • ForrestForrest Posts: 1,341
    edited 2006-04-03 00:39
    FYI, Parallax sells a 7 segment LED www.parallax.com/detail.asp?product_id=350-00027 that can be driven from a BS2 if you use 1K resistors in line with each segment. The circuit is described in Parallax's What's a Micontroller that you can download here www.parallax.com/detail.asp?product_id=28122
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