If have several 1 watt LEDs that I want to drive off of a TTL gate's ouput. What can I do to increase a 74LS TTL logic chip's output fan-out (current) to be able to produce a true 5 V output that is capable of driving as much as 500 mA (or more if possible) worth of devices (at 5v)?

As a side question to this one, is it better to use a TTL chip's high output with the cathode of the LED connected to ground or vice versa (use a low output with the LED anode connected to +5 V)? Also, on which leg of the correct orientation should the current limiting resistor go, if that matters? Finally, to complicate things more, should the LEDs be connected in series or parallel for higher output?

  • \$\begingroup\$ The simplest way is to use a NPN transistor in common-emitter mode. \$\endgroup\$
    – m.Alin
    Nov 1, 2012 at 15:35
  • \$\begingroup\$ I presume you mean this configuration NPN transistor in common-emitter mode, but more detail regarding part numbers and values would be great. \$\endgroup\$ Nov 1, 2012 at 15:36
  • \$\begingroup\$ It depends on your LED's specifications; could you link to its datasheet? \$\endgroup\$
    – m.Alin
    Nov 1, 2012 at 15:40
  • \$\begingroup\$ Sadly I don't have one, but if I want a high TTL output to be converted to 5v capable of powering 0.5 Amp, what kind of circuit would I need. I can go from there. Let's assume that the power supply powering the TTL circuitry and any other components has enough current to handle this scenario. \$\endgroup\$ Nov 1, 2012 at 15:41
  • \$\begingroup\$ Your circuit should look like this: circuitlab.com/circuit/uz3rr4/npn-common-emitter-led-5v. I won't go into how to calculate the values of the components, because there are plenty of answers on this site that explain better than I could how to control a LED through a NPN transistor. Have a thorough search. \$\endgroup\$
    – m.Alin
    Nov 1, 2012 at 16:02

1 Answer 1


Check out the uln2001:


You can put them in parallel and they have built in protection. Should also be as cheap as dirt.

  • 1
    \$\begingroup\$ Very interesting, I am looking into this... \$\endgroup\$ Nov 1, 2012 at 16:47
  • 2
    \$\begingroup\$ What's cool about the ULN2003 is that it's dirt cheap, supports direct TTL outputs, contains seven darlington pairs per chip and the required external part count to support the chip is ZERO. \$\endgroup\$ Nov 1, 2012 at 17:04

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