# Light up an LED with an NPN transistor in active low?

I have a circuit which is turned on at a logical zero. I would like to have a LED indicating this circuit is on, but I only have NPN transistors at this moment, which conduct when there's a logical one on its base.

Is there a way to "emulate" the behavior - i.e. that when there's a logical one, the LED isn't lit up, and when there's a logical zero, the LED is lit up - without having to use a PNP transistor?

• Do you need a transistor at all? If there is no problem with voltage (you have access to "right" logic 1 voltage and it is high enough for LED) and your signal line is able to sink at least few mA, you can lit small LED directly? – Martin Mar 25 '18 at 13:44
• Good point! But as the signal line is an Arduino, and there is a number of these LEDs, I'd rather not go with this approach. – Tomáš M. Mar 25 '18 at 17:19 simulate this circuit – Schematic created using CircuitLab

Figure 1. Non-inverting LED indication.

• Calculate R2 as usual for a series current-limiting resistor.
• When Q1 turns on it will shunt the current away from the LED.

Note that the circuit is not efficient in that it passes more current when the LED is off than when it is on.

The secret here is to use two NPN circuits. The first stage acts as an inverter to convert the active low input to a high signal. The high signal at the collector of that NPN can then feed into the base of the second NPN (via a resistor of course). The second NPN drives the LED in its collector circuit. Something like this: The dumb (or at least wasteful) way: simulate this circuit – Schematic created using CircuitLab

R1 draws 5mA when the LED is off and maybe 3mA when on.