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 the two sync signals separated from the RGB (Red, Green, Blue) video signals. Below are the pinouts 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 and convert it for the Z-axis, and the sync signals to trigger the sweep signals to feed into the X and Y axes of the oscilloscope. Below is a schematic diagram of the VGA-to-scope converter.

The sync signals generate pulses, which is used to trigger two sweep "sawtooth" generators. Two 555 timers were used for the sweep generators, which generates ramp signals so the oscilloscope can create a raster (scanning pattern). 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. However, some oscilloscopes do not have built-in inverters, so I added an inverter on the output of the two ramp generators, and you are given the choice to invert or not invert the sweep signals (ramp or sawtooth). The 1458 OP AMP IC is used to buffer and invert (optional) the vertical sweep, and the output is fed into the Y-axis. The TL082 OP AMP IC is used to buffer and invert (optional) the horizontal sweep, and the output is fed into the X-axis.

NOTE: The sweep generators were built for my PC's VGA specifications, which I believe is 800 x 600 at 75Hz refresh rate. Different configurations most likely could mess up the sweep. If you wish to build this circuit on a different VGA configuration, then you may have to change "play with" 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 OP AMP IC and fed into the Z-axis of the oscilloscope. You could omit the TL082 IC on the video and enjoy an inverted (negative) picture on the oscilloscope.

The power supply generates +9V, +5V, and -5V for the circuits. A 555 timer is used to convert the positive voltages to negative voltages.

NOTES OF CAUTION (READ THIS!!!): The oscilloscope must have a X, Y, and Z axis and good gain for this converter to be useful. Second, the PC MUST be on before you turn up the brightness of the oscilloscope (the video signals makes the intensity less bright). The reason for this is because without the PC's sync signals, the sweep generators are not on, so you get a dot in the middle of the CRT, which CAN burn the phosphor face. 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, converter, or before unplugging either.

TRICKS WITH VIDEO: You can flip the picture by switching the inverter's output to invert or non-invert. You can also rotate the picture 90-degrees by swapping the X and Y axes. However, you will have to adjust the gains on the X and Y axes again to make the picture fit correctly.

The converter also has a downfall, the sweep generators are sensitive to voltage swings, and can dramatically change when the +5V goes down a few millivolts, so ensure the voltage regulator creates well-regulated +5V. Also, the 50K pots on the two sweep generators REQUIRE adjustments to sync up the ramp signals to the sync signals. You may need an oscilloscope to help sync them up.

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