Control RGB LED without microcontroller

Question

I have an RGB LED that I'd like to change from green to red when a circuit is completed. While I know this is very simple with tools like an Arduino, I'd love to not use a microcontroller. Would it be possible using basic components (which can certainly include transistors, diodes or even common ICs like a 555 timer) to achieve this? If so - how would I connect the components?

Clarification: I'm using a common anode RGB LED (a lead for each color plus ground - four in total). I'd like to have it be green when a switch somewhere in the circuit is open (not connected) and red when the switch is closed. The switch is not connected to anything else.

Answer 1

This is the simplest circuit I can think of (2 parts):-

^{simulate this circuit – Schematic created using CircuitLab}

Input = 5V Red Input = 0 Green

The circuit works because the \$V_F\$ of a Red LED is generally much less than the \$V_F\$ of a Green or Blue LED.

Here is a typical RGB LED datasheet:

And here are the typical plots of \$V_F\$ vs. current:

When the transistor is "off", the Green LED will have a forward voltage of about 3.0V at 10mA. When the transistor is "on", the Red LED will have a forward voltage of less than 1.9V. Referring back to the Blue/Green graph, little current will flow through the Green LED at that forward voltage.

This particular arrangement shows a common-cathode arrangement, however it could be flipped and a similar P-channel MOSFET used. In that case, 0V would select the Red LED, and +V in would select the Green LED.

Answer 2

Maybe something like this?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

For this particular case ONLY, there are various ways to do this

Digital gate ICs, flip-flop, ON/OFF transistor configuration, etc. depending on your actual configuration

Assuming common anode RGB, attached a sample circuit using transistors

^{simulate this circuit – Schematic created using CircuitLab}