I recently noticed that when one of my computers entered a sleep state its power LED went off, leaving no way of telling if the computer was asleep or off. When my other computer enters an S3 sleep state, its power LED flashes to let me know. I pulled out the motherboard manual to see if I could find an answer. It turned out that the motherboard supported a dual-colour power LED, which would switch colours when the computer entered a sleep state. If only a single colour LED was present, as in my case, it would go off when the computer went to sleep.
What was required was a bi-colour LED with two leads. Such an LED contains two LEDs of different colours, which are wired cathode to anode and visa versa. If the current flows one way, one LED will light, and if the current is reversed, the other LED will light. I didn’t have such an LED, and a search of my local electronics store’s web site showed that they didn’t either. What I did have in my box-of-tricks was a fancy three-colour RGB LED with four leads; a cathode lead for each colour LED, and a common anode lead. Sometimes a similar arrangement is used for bi-colour LEDs, i.e. they have three leads, one of which is a common anode or possibly a common cathode. What I needed to do was convert my common anode LED into a bi-colour, two wire type suitable for the motherboard. This can be done with couple of normal diodes, as per the circuits illustrated below.
In this circuit, a 3-wire, common anode red/green LED is converted into a 2-wire red/green LED.
Here’s the equivalent circuit for a common cathode 3-wire LED.
With the circuit worked out, I went out and bought two 1N4004 diodes and wired them up to my RGB LED (using only the red and green cathode leads, as they were the colours I wanted).
The end result. The diodes are wired into the cable and covered with heat-shrink tubing (the bulge at upper right). So did it work?
At top, the computer awake with both the new green power LED and blue hard disk activity LED on, and at bottom, the computer asleep with the power LED switched to red!