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Driving LED and efficiency

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Newbie level 5
Sep 22, 2019
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I have a question about driving LED in different, inverted way: the LED should light on when the signal/state is LOW and the same LED should be turned off when the signal is HIGH.

I have attached a schematic which this solution with two npn transistors to do it simply without specific IC's.

But I wonder what about efficiency. If we think about using ONLY transistors (bipolar or MOSFET, doesn't matters), is it the best way (I mean this circuit configuration strictly) to drive the LED with a reasonable low current drawn by the circuit? I ask because driving LED circuit in inverted way requires to be powered the whole time.

Regards, Bogdan


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Signal low lights led.
Signal high turns it off.
In other words a 'NOT' gate.

Logic gate construction can be done using families of components:
diode-resistor logic
diode-transistor logic
resistor-transistor logic
transistor-transistor logic

Does your incoming signal have sufficient voltage and current? Then you might manage to create a NOT gate using a simpler circuit than your schematic.


There are several possible solutions, but to give you a suitable answer we need full informations first.
* VCC voltage range
* drive signal HIGH and LOW voltage at about 1mA ... and where does it come from?
* expected LED current and voltage


If quiescent current matters (e.g. for a battery operated circuit), use a single PNP common emitter driver.

2 - 3 mA LED current can be usually sourced by logic circuit (e.g. microcontroller) directly without a driver transistor.

Thanks for the replies. The HIGH voltage od signal od 5V, LOW is 0V. About LED current and voltage... I think there should be values just to make LED lighting and visible. I thought about pnp, but I am not sure how to match IT with my voltage levels of signal.

If the emitter of a PNP transistor connects to +5V and the LED and its series current-limiting resistor connects between the collector and 0V then when the base signal is low (with a series current-limiting resistor) the transistor and LED turn on. When the base is +5V then the transistor and LED are off.

It is inefficient to short the LED with a transistor to turn off the LED.


5V/0V are usual power supply levels... but output voltage always has a lower range.
LEDs...there are so many different colors, different white, from a couple of mA up to several Amperes. From about 1.5V to several 10V.


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