display LED flasher output on an analog oscilloscope

[mdanh2002]

Hi,

I am trying to use a GW INSTEK GOS-620 20MHz analog oscilloscope to display the output of a LED flasher (once per second) using a 555 timer on the screen. I am trying to show a square wave on CRT. Unfortunately I am unable to do so:

+ At low time/div settings (0.1, 0.2, etc. ms) all I can see is a line moving up and down between 5V and 0V.
+ At high time/div settings (0.1, 0.2 etc second) all I can see is a dot moving up and down.

This is logically correct. However, how can I make it display the square wave nicely as in the case with a digital oscilloscope? I have tried both AC/DC coupling, various trigger settings to no avail.

The oscilloscope is working normally and has no problem with other signals with high enough frequency.

 

[Diver300]

You can't.

Analogue oscilloscopes rely on signals that repeat so often enough that the user's eyes see the signals as steady.

The only way to see a steady signal is to increase the frequency to 200 Hz or more.

 

[ljcox]

Alternatively, use an analogue storage scope.

 

[mdanh2002]

Alternatively, use an analogue storage scope.

Thanks

One of my friends has an analog storage oscilloscope like this **broken link removed**

Do you think it will be able to display the square wave nicely? Except for the plotter output, the only other thing it has (and my scope doesn't have) is a HOLD button.

 

[ljcox]

I'm not familiar with that scope, but if it can store a waveform, then it should be able to display your signal.

 

[mdanh2002]

I'm not familiar with that scope, but if it can store a waveform, then it should be able to display your signal.

I tried. Same problem. The HOLD button only helps to retain the screen display at the particular moment the button is pressed. It does not have "memory" like a digital oscilloscope where I can use cursor to scroll back and forth the waveform and examine the various data points. Not sure if the PLOTTER output could have helped - I did not have the equipment to try.

 

[crutschow]

The way we did it way back when, was to use a camera with a "bulb" or time mode that holds the shutter open to capture one complete trace of the oscilloscope. You need to do that in a completely dark room, of course.

 

[ljcox]

The way we did it way back when, was to use a camera with a "bulb" or time mode that holds the shutter open to capture one complete trace of the oscilloscope. You need to do that in a completely dark room, of course.

That is what I was intending to suggest.

I don't know how digital cameras do time exposure.

I assume they measure the received light and close the shutter once sufficient photons have been received.

Put the camera on a tripod or other support so there is no movement during the exposure.

I suggest that you focus first, turn the light off, and then press the button to start the exposure.

If it does not work in the dark, try it in dim light.

 

[#12]

The quality of the camera makes a lot of difference. My phd (press here dummy) camera decides when it has seen enough light and closes the shutter. End of list.

A better quality of the species "modern digital camera" will allow all kinds of adjustments, just like the old 35mm film cameras. Speed dominant, shutter opening dominant, plus and minus F-stops. Gosh, the words I have forgotten are making me look stupid. Anyway, you get the idea. Good camera = good results.

 

[Reloadron]

Years ago Tektronix manufactured a line of scope cameras. Some used roll film and some used Polaroid film packs. They attached directly to the CRT face and for their day worked well.

You turned up the graticule illumination and set the scope for single sweep. Then you opened the shutter and initiated a trigger for the scope. Once complete you close the shutter and in the case of Polaroid film pulled the tab and removed the exposed film. Wait about 60 seconds and peel the film backing off to reveal your lovely black and white image. For their time they were actually pretty cool and very useful devices. Today good luck finding film for one.

I guess if I wanted to capture a scope image with a slow rep rate today I would use a digital camera with a good choice of manual settings. That would include a camera with the mentioned "B" or Bulb setting for the shutter. Set up the focus and F Stop and then in the dark open the shutter. Set the scope for single sweep and remote externally trigger the scope. Once the trace is complete close the shutter and see what you have. You will need a good digital camera with a wide range of manual control. You will need to play around with the scope intensity and graticule illumination.

Ron

 

[mdanh2002]

Thanks all for the information. Surprisingly how much knowledge I have learned just by reading your replies.

I ended up using a portable logic analyzer to show the waveform. My cheap digital camera is the point-and-shoot type and does not support many customizations. Its LCD also shows flickering image when pointed at the scope CRT. I have another very old (and big) film camera since the 1980s with a lot of manual controls but don't have film for it.

One more question, how does the analog storage scope work? I mean, how does it retain the CRT display when HOLD is pressed without any memory to store the input voltage at different times? I have been googling but could not find useful information.

 

[ljcox]

I don't know how it works in detail, but they some how keep the phospher glowing where it has been stimulated by the electron beam. You have to press a reset button to erase the trace.

I have just recalled that I made a hood from cardboard (with a circular hole in the centre) to fit over the scope screen. This sets the camera at the right distance from the screen and makes it easy to hold it steady for the photos.

I set the digital camera to Macro and put the lens into the hole.

I have not attempted to take a photo of a low frequency waveform, but it works well for higher frequency, repetitive waveforms.

I load the photos into my computer and use MS Paint to invert the colours. This converts the black background to white, so I don't waste ink when I print them.