VGA-to-Scope Converter

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I built this project in effort to learn some things about television circuits. I decided to try converting a computer's VGA output for an oscilloscope, which was not as hard as converting composite video. VGA, unlike composite video, already has separate sync signals and RGB (Red, Green, Blue) video signals. Below are the relevant pins on the VGA cable.

PIN	PURPOSE
1	Red video
2	Green video
3	Blue video
4
5	Ground
6	Red ground
7	Green ground
8	Blue ground
9
10	Sync ground
11
12
13	Horizontal Sync
14	Vertical Sync
15

Simply, we take the RGB video signals and combine them into one signal for the Z-axis, and the sync signals are used to trigger the sweep generators. The sweep generators produce sawtooth signals that feed into the X and Y axes of the oscilloscope. Below is a schematic diagram of the VGA-to-scope converter.

The sync signals are pulses that triggers the 555 timers. The 3906 PNP transistor and associated resistors serve as a constant current supply to charge up the capacitor linearly for straight sawtooth lines rather than curved lines. The 555 timers are configured as monostables to discharge the capacitor. The vertical generator creates ramp signals at around 75Hz and is tuned to the vertical sync pulses. The horizontal generator creates ramp signals at around 35kHz and is tuned to the horizontal sync pulses. Feeding the two ramp signals into the X and Y axes of an oscilloscope will produce a raster scanning pattern. However, some oscilloscopes do not have built-in inverters, so buffers and inverters were added to the output of the two ramp generators. The 1458 op-amp is used to buffer and invert the vertical sweep for the Y axis. The TL082 op-amp is much faster and used to buffer and invert the horizontal sweep for the X axis.

NOTE: The sweep generators were built for my PC's VGA specifications, which was 800x600 at 75Hz refresh rate. Different configurations most likely require adjustment of the 3.9K resistor and the 220pF capacitor on the horizontal generator and the 120K and .01uF capacitor on the vertical generator.

The RGB video is converted to monochrome video, which is buffered and inverted by another TL082 for the Z axis. The TL082 could be omitted and an inverted (negative) picture on the oscilloscope is the result.

The power supply generates +9V, +5V, and -5V for the circuits. A 555 timer is used to generate the -5V for the op-amps.

CAUTION: The oscilloscope must have a X, Y, and Z axis, good gain, and X/Y inverters for this converter to be useful. Second, the video MUST be connected before you turn up the brightness of the oscilloscope. Without the video signal connected, the sweep generators are not operating due to the absence of sync signals so a dot appears on the CRT, which can burn the phosphor. Make sure you have a raster before you turn up the brightness. The same goes for shutting down, turn down the brightness of the oscilloscope before you shut down the computer and/or converter.

TRICKS WITH VIDEO: The picture can be flipped by switching between invert and non-invert. The picture can be rotated 90-degrees by swapping the X and Y axes. The oscilloscope X and Y gain will most likely require readjustment to make the picture fit.

Pictures:

VGA-to-Scope converter in enclosed case

The converter's circuits.
Click image to view circuit outline.

Screen Shots:

Windows 95 Introduction

Desktop

Computer Clock - 5:20 on Feb. 13th

Actual desktop for comparsion