Use transistors for controlling two LEDs

Question

I'm sure this is a pretty basic question, but I wanted to run it by you guys before creating my PCB.

I want to have two LEDs on the board to indicate the global state of the board. When the board has power, but the MCU is not in an active mode I want the Red LED to light up, and when the MCU puts a pin high I want the red led to be replaced by a green led instead.

I want to solve the problem using simple electronics, and not with RGB led, etc.

My idea is to use two transistors. One which is normally closed (the green one), and one which is normally open (the red one), and then toggle them on/off using a signal from the MCU.

Does the following circuit make sense?

Edit: Lots of good suggestions here. The MCU is a Teensy 3.2, which has 3.3V output, and 25mA Max current on the digital GPIO pins. I have both 3.3V and 5V powersources available on the board.

Answer 1

You can use a transistor as a switch here.

^{simulate this circuit – Schematic created using CircuitLab}

So when the GPIO is low or zero then the transistor would be in cutoff region and the green LED glows and when the GPIO is 3.3 V the red LED glows since the transistor goes in saturation state the Green LED would be switched off.

The resistors values are so chosen that the transistor work as a switch.

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EDIT : Writing KVL's will let you know the values of resistors.

Let 0.7v be across the base emitter junction when 3.3V is given at GPIO pin, 1.8V across red LED, Let the voltage at collector emitter junction be 0V when saturated so at this voltage the green LED will not glow.

Also let Ib be the current and is generally 20mA for a normal 5mm LED

1. KVL at B-E of transistor : $$3.3V-0.7-1.8V-I_bR_1=0 $$ We require about 20mA in the red LED so R1=40 ohm

2. When the transistor is in cut-off state i.e., no voltage at GPIO pin & the voltage at collector is around 5V so KVL at C-E junction of transistor : $$5V-2V-I_3R_3-I_CR_2=0$$

2V= forward voltage of green LED and let 20mA of current is required for the green LED so R2+R3=150ohm, I3=Ic=20mA assuming no current flows through the transistor since Vc=0V.

So choose R2=100ohms and R3=50ohms

Note : we choose R2 to be 100 because we do not want large current to be flowing through the circuits at collector when transistor is cutoff.

Answer 2

Something like this should work:

^{simulate this circuit – Schematic created using CircuitLab}

If the signal from the MCU is Low, the red LED will be on, if High, the green LED will be on.

An NPN transistor (such as 2N3904) will conduct when the base is more positive than the emitter. A PNP transistor (2N3906) will conduct when the base is more negative than the emitter. Resistors are required in the base lead to limit the base current.

Answer 3

When the MCU goes HIGH (5V) the NPN transistor is turned ON, the PNP is turned OFF and the red LED comes ON.

When the MCU goes LOW (0V) the PNP transistor is turned ON, the NPN is turned OFF and the green LED comes ON.

With NO input from the MCU (mid point floats to 2.5V) both transistors (and LEDs) turn ON.

EDIT ADDITIONAL: For a 3V3 MCU reduce the top 4k7 resistor (E-B of the PNP) to 2k2.

Answer 4

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Transistorless option. The 0.9 V reading is obtained with GPIO turned off. 1.1 V reading is with GPIO turned on. Figure 2. Negative rail switching version.

I haven't tested this but the theory is ...

• With GPIO off D1, RED, lights. The voltage drop across D3 and D1 will be about 0.6 + 1.8 = 2.4 V.
• When GPIO pulls high D2, GREEN, lights. The voltage drop across the green led will be about 2.0 V. Since this is less than required by D1 / D3 the current will fall in the RED led and it will go quite dim. I've assumed some voltage droop when the GPIO is supplying current.

This circuit can be inverted if GPIO pull low is preferred. No transistors.

Answer 5

Wow the anwsers to this are complicated. But your circuit is already pretty close. What you're describing is actually a simple differential / ltp circuit. Just disconnect one of the bases and connect it to a voltage divider making 2.5V. When the transistor connected to the MCU is low, it will not provide an current and all current will be diverted to the other transistor. When the MCU is high, that transistor will consume all current and the other transistor will turn off (because the common emitter resistor of the differential / ltp will present a voltage higher than Vbe of the second transistor).

However, you may also need to move the LEDs to the collectors because the voltage drop may cause issues with the differential action.

Answer 6

This is an inverter circuit. I have used this circuit to answer other questions and it works perfectly. The Resistor in the middle should be about 1KΩ.

Answer 7

I tried Peter Bennett's circuit, and like Antonio, I too had the red LED on at all times. At first, I tried Antonio's modification to the circuit, but then I realized that it was wrong (even though it worked). Below is my modification of the circuit. I substituted a BC547 NPN and a BC557 PNP because it's what I had. I changed R2 to 8.5k, and added the 2 pull-up resistors(R5 and R6). When the MCU pulls the bases of both transistors to ground, the green LED turns off and the red LED turns on. I know this was an old post, but I figured I'd add to it since it somewhat helped me.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 8

The following circuit should work nicely provided that the MCU pin has sufficient current drive. Both Microchip PIC and Atmel AVR chips have pins with sufficient current drive.

^{simulate this circuit – Schematic created using CircuitLab}

I am assuming that you are using Superbright LEDs.

A typical Green Superbright LED drops about 3 Vdc and is blinding when run from 5Vdc with a 10k resistor.

A typical Red Superbright LED drops about 1.7 Vdc and is reasonably bright when run for 5 Vdc with a 2k2 resistor.

Operation is extremely simple: if the MCU pin is either high-impedance (set as Input) or is Low, the Red LED is lit and the Green LED is NOT lit.

When the MCU pin goes Hi, the Green LED is lit and the Hi level raises the cathode of the Red LED high enough that it is NOT lit.

This circuit works well with the PIC 16f microcontrollers that I use.