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I am designing a simple circuit to control 80 LEDs using the parallel port.

I am a software engineer and I barely remember the basics of hardware design. I already implemented the software to control the circuit. However now I would appreciate if someone can guide me how to build the hardware.

The following components are used for designing the circuit (a parallel port, 5 4x16 decoders, 80 resistors and 80 LEDs).

Questions:

  • The power required to turn on all 80 leds at the same time will not be enough, right ? so I guess I need a power supply, how can the power supply be connected to the circuit ?

  • From what I read, the resistors are used to prevent burning the leds (since I have an output of +5 voltage), this might sound stupid but is it possible to connect the resistor before the decoders ?

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    \$\begingroup\$ What LED current are you wanting? \$\endgroup\$ – Andy aka Nov 25 '13 at 13:00
  • \$\begingroup\$ ~20mA leds .... \$\endgroup\$ – TeFa Nov 25 '13 at 13:15
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Second question: no. The power supply of the driver can supply too much current, and that has to be limited. You can't do that at another place in the circuit.

First question: the parallel port can hardly supply enough power for 1 LED, let alone 80 of them. So you need an external power supply. I would suggest a regulated 5V wall-wart. (Regulated means that the output is a nice and constant 5V. Unregulated power supplies may see varying voltage, which you don't want.)
I would suggest shift registers like the 74HC595 for driving the LEDs. The advantage of a shift register is that you only need 1 data line + clock and latch. You clock the 80 bits for the LEDs in the daisy-chained shift registers and at the end you pulse the latch.

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