An increasing number of individuals are utilizing PCs, both conventional and notebooks, for film viewing purposes. The VGA output can be harnessed to create a corresponding ‘Ambilight’ effect for this purpose. By limiting the setup to a single RGB LED, it is possible to draw power for this circuit directly from the VGA connector, in addition to obtaining the RGB signals.
The following pins of the 15-way VGA connector (three rows of five pins) are used for this circuit:
- Pin 1: Red video signal
- Pin 2: Green video signal
- Pin 3: Blue video signal
- Pin 5: GND
- Pin 9: +5 V
Video Signal Processing for RGB LED:
The RGB outputs carry video signals for red, green, and blue channels, each with an amplitude of 1 to 1.35 V. These signals form the visual imagery on the screen, updating at a rapid rate. The circuit described here manipulates these signals to drive an RGB LED, although it’s not a complete ‘Ambilight’ setup. However, it can produce a green light for a rugby match or an orange hue for a sunset displayed on the screen.
Sawtooth Signal Generation:
A sawtooth generator, built around IC1 and T1, produces a steady sawtooth signal fed to opamp IC2a through R6. The frequency hovers around 850 Hz, and the amplitude varies from 1.6 to 3.4 V. IC2a subtracts around 1.6 V due to the voltage divider R4/R5. Subsequently, the peak value is reduced to about 1.35 V using voltage divider R10/R11. The resulting signal drives the comparators in IC3 after being buffered by IC2b.
Signal Averaging and LED Control:
The red video signal’s average level is determined by the R12/C2 network. IC3a constantly compares the generated sawtooth signal with this average red video signal. The comparator circuit acts as a PWM driver for the red LED. Similar arrangements are employed for the green and blue channels. With a notebook, enabling the VGA is necessary, often done through key combinations like Fn-F5. For desktop PCs, tapping into video signals requires an adapter between the video cable and the monitor.
Expanding to Multiple LEDs or LED Strip:
For larger setups involving multiple LEDs or a LED strip, an external power supply is necessary, though the control circuit can still draw power from the PC. The cathodes of the LEDs are connected to IC3’s comparator outputs, while all anodes are linked to the external power supply. Resistors R15–R17 might already be part of the LED strip. If more current is needed, the outputs of IC3 can drive transistors (such as BC547) with appropriate resistors, allowing for higher current handling, expanding the setup’s capability.