I would like to get a PC to flash seven LEDs in response to program events. I plan to use the parallel port which is provided on the motherboard I am using by a Fintek F81866A "super i/o" LPC chip http://pdf1.alldatasheet.com/datasheet-pdf/view/459084/FINTEK/F81866A.html. The datasheet describes the parallel port data pins as "TTL level bi-directional pin with schmitt trigger,12 mA sink capability, 5V tolerance.". It seems to also indicate its 3 volt logic level, see table on page 20.

I beleive I cannot sink current through the pins because I have no (easy) access to a 3.3V supply from the motherboard. I do have a 5V supply from the front pannel header so I thought I would source a small current from the pins then use some sort of buffer chip to drive the LEDs from the 5V supply.

Because the data sheet does not seem to specify a maximum source current from the data pins, I think I will have to assume that the current avalible is extreamly low to stay safe, say 500uA maximum. I would like to drive a wide range of LEDs, up to say 25mA forward current from the 5V supply.

So I am trying to find a suitable buffer chip. I have looked at the ULN2003 darlington array but the data sheet says (if I am reading it correctly) that the input current will be up to 1.35mA at 3.85V, so this may be a bit high. Plus, if the input voltage is down around 3V, will the transistors even turn on reliably?

So what is a suitable buffer chip to use? Something with very high input impedance to draw negligable current and that works reliably at 3V logic levels and can switch around 25mA on the output. It also needs to be avalible in a through hole package for hand soldering.

I know I could use discrete transistors but a compact board layout/low component count is important for this application.

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    \$\begingroup\$ Output levels are guaranteed for rather substantial output sink/source currents.. Did you read the entire datasheet? \$\endgroup\$ Nov 5, 2016 at 13:17
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    \$\begingroup\$ This is trending dangerously close to a "shopping" question, which would be off-topic here. Focus on the problem you need to solve, rather than presupposing a solution. \$\endgroup\$
    – Dave Tweed
    Nov 5, 2016 at 14:37
  • \$\begingroup\$ @Spehro Pefhany: No, I had not. I see on page 197 that the electrical characteristics for I/O 12st,5v pins are stated for source/sink instead of just sink on page 20. If you add this as an answer I will accept it. \$\endgroup\$
    – Guy
    Nov 6, 2016 at 10:30

1 Answer 1


Check out the electrical characteristics in the datasheet- normally close to the back, so almost 200 pages in.

You will find that the output levels are guaranteed for relatively high currents, more than enough to drive a ULN2003A.

Although some datasheets brag about drive levels in the large print near the front, it's always advisable to read the fine print, if only to find out how well they actually drive that current (minimum output voltage when sourcing current, maximum when sinking).


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