Controlling RGB LEDs with Old CRT TV Parts and 555 Timers

There was a time in the recent past when many in the DIY electronics community looked down their noses at microcontrollers and especially at development boards. “Why use an Arduino when you can just use a 555 timer?” they would ask. Practically speaking, the answer is pretty obvious: microcontroller development boards are easier, offer far more flexibility, and are often cheaper to boot. But we still love seeing what people can achieve with discrete components and simple ICs, such as Urs Schmidt controlling an RGB LED light strip with old CRT TV parts and 555 timers.

This project began when Schmidt found a broken CRT (Cathode-Ray Tube) TV in front of his house. It was mostly unsalvageable, but it did have an interesting control panel consisting of several sliders for adjusting volume, brightness, contrast, and more. Schmidt loved the look of those sliders and decided to use them to control RGB LEDs that illuminate his workbench.

Schmidt’s plan was to use three of those sliders, with each one controlling the brightness of a single color channel on the LED strip. It is important to note that this is a conventional RGB LED strip and not an individually addressable RGB LED strip like NeoPixels. One slider would adjust red brightness, the other green, and the final would control blue.

The most common way to control the brightness of LEDs digitally — because it isn’t possible to adjust voltage directly — is to modulate power very quickly to lower the average voltage. This is called PWM (pulse-width modulation) and is why you can often see LEDs flashing in videos. A microcontroller can produce an arbitrary PWM signal based on firmware using an internal clock. But Schmidt didn’t want to use a microcontroller, so he needed another way to generate a variable PWM signal for each LED color channel.

To achieve that, Schmidt used a 555 IC and a 556 IC. The 555 is a timer with an output frequency and length determined by input characteristics (namely resistor and capacitor proportions). The 556 is the same thing, but with two timers on one chip. Together, they have three timers—one for each color. The control sliders are really just linear potentiometers, so adjusting those changes the value of the threshold and trigger resistors (they’re equal in this case). That simultaneously alters the pulse width and frequency, brightening or dimming the corresponding LED color channel.

This is a common way to control RGB LED strips, but we like Schmidt’s use of the old TV sliders for input. Now he has a novel way to set the color and brightness of his workbench LEDs.