Buffer Parallel Port to Drive LEDs - Page 1
Forum section: electronics / projects

author: GJS123
author_rank: Newbie
timestamp: November 06, 2016, 01:28:04 am
content: Hi,I want to operate 7 LEDs from the parallel port of a PC. The parallel port is implemented by a Fintek F81866A LPC chip,http://pdf1.alldatasheet.com/datasheet-pdf/view/459084/FINTEK/F81866A.html.The data sheet states that the data pins are "TTL level bi-directional pin with schmitt trigger,12 mA sink capability, 5V tolerance." at 3 volts.What is the best way to drive normal 20mA LEDs from these pins?I think that as I have no easy access to the boards 3V (3.3V?) supply, I will need to source current from the pins, not sink. Since there is no mention of maximum source current in the data sheet, I guess I will have to assume it is extreamly low to be safe. So I think I need a buffer arrangement that will accept 3V logic at the inputs with very high impedance, and is able to pass 20mA at 5V at the output to drive the LEDs via curent limiting resistors. I have easy access to a 5V supply from the motherboard's front panel header. Am I on the right track? Is there an octal buffer IC that fits the bill?Thanks.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: November 06, 2016, 01:47:19 am
content: These are Open Drain output pins, aren't they? So does it matter where the power comes from, as long as you don't exceed the sink current spec?Here's a better link to a data sheet:http://www.nctechnology.co.kr/product-sogae/CPU%20Board/MI970/F81866A_V0.12P.PDF

author: rstofer
author_rank: Super Contributor
timestamp: November 06, 2016, 02:49:23 pm
content: You can certainly use a buffer chip and you can find out how to do this on Google. I would probably just use a transistor with a base resistor around 330 Ohms (to limit pin current) and call it good. Yes, several transistors and resistors will be larger than a chip.

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2016, 04:32:54 am
content: Quote from: GJS123 on November 06, 2016, 01:28:04 amDrive them at 3 to 6 milliamps instead. How bright do they really need to be?

author: GJS123
author_rank: Newbie
timestamp: November 07, 2016, 05:57:51 pm
content: Thanks everyone for your replies.I found that the source current is stated in the data sheet - on page 198! Max current is 12mA so I am going to get some of these:www.vishay.com/docs/82400/tlhp42j2l1.pdfThey're 14mcd at 10mA so I'm going to try them at about 8mA and see how they look.I had always thought that LED driver ICs were for alpha/numeric displaysSo I will look into them, they look like the perect solution where a few indicator LEDs are needed for an embedded PC setup, except they all seem to have SPI input? Are there any with TTL or RS232 inputs?Cheers.

author: Fungus
author_rank: Super Contributor
timestamp: November 07, 2016, 06:56:29 pm
content: Quote from: GJS123 on November 07, 2016, 05:57:51 pm14 mcd is a very, very dim LED. Many green LEDs are two or three orders of magnitude brighter than that.

author: Fungus
author_rank: Super Contributor
timestamp: November 08, 2016, 10:25:22 am
content: Quote from: GJS123 on November 07, 2016, 05:57:51 pm"Clock", "data" and "latch". Not exactly difficult to program.TheTLC5916seems ideal for control from a parallel port.Quote from: GJS123 on November 07, 2016, 05:57:51 pmI don't think so but any old microcontroller can do that. eg. An Arduino's I/O pins have enough current drive for LEDs and an Arduino Pro Mini is only about $2.