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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?

  • The simplest way is to use a NPN transistor in common-emitter mode. – m.Alin Nov 01 '12 at 15:35
  • I presume you mean this configuration [NPN transistor in common-emitter mode](http://upload.wikimedia.org/wikipedia/commons/a/a0/NPN_common_emitter.svg), but more detail regarding part numbers and values would be great. – Michael Goldshteyn Nov 01 '12 at 15:36
  • It depends on your LED's specifications; could you link to its datasheet? – m.Alin Nov 01 '12 at 15:40
  • 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. – Michael Goldshteyn Nov 01 '12 at 15:41
  • Your circuit should look like this: https://www.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. – m.Alin Nov 01 '12 at 16:02
  • What about the use of a 74LS06/74LS07 to increase the output current? – Michael Goldshteyn Nov 01 '12 at 16:06
  • From its datasheet: _The maximum sink current for the SN54LS06 is 30 mA, and for the **SN74LS06** and SN74LS16 it is **40 mA**._ – m.Alin Nov 01 '12 at 16:11
  • OK, thanks, if you would like to copy your comment into an answer, I can upvote it at the very least... – Michael Goldshteyn Nov 01 '12 at 16:19
  • Related questions: http://electronics.stackexchange.com/questions/6093/how-to-pick-a-switching-transistor-to-drive-leds-or-how-to-read-a-transistor-d | http://electronics.stackexchange.com/questions/23654/simple-transistor-switching-example-should-show-led-off | http://electronics.stackexchange.com/questions/16281/should-i-calculate-a-resistor-value-for-driving-indicator-leds-from-ttl-outputs | http://electronics.stackexchange.com/questions/42581/attaching-a-led-to-pics-rx-pin/ – m.Alin Nov 01 '12 at 16:29
  • At 1W (300..350mA) I would consider regulating the current, but that may be to complicated for you? – starblue Nov 02 '12 at 13:43

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Check out the uln2001:

http://www.st.com/internet/analog/product/65558.jsp

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

Mariano Alvira
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