I want to measure switch bounce, but all I have is access to an old CRO. Based on inputs from this forum post, I tried the HOLDOFF knobs and other buttons too. Nothing worked. In the case of modern DSO's people have suggested using the trigger and I think that may be of use here, but I don't know if it's possible and how.

Btw even though I'd come across this wonderful guide on debouncing, I'd love to run my own tests with the switch I have.

This is the only one I have access to

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    \$\begingroup\$ This is more how do I use a scope? Apply pullup R, measure And trigger off same channel , disable auto trigger and set base ~ 1ms/div and alternate trigger polarity for open or close bounce. Use eyes in dark ambient for memory. Not every switch or action bounces , while adding a small cap stores Vc and prevents fast pullup. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 5 '18 at 18:43
  • \$\begingroup\$ If you're looking for the manual of that scoop, it looks like Scientific SE scopes are now branded as HTC (link to company website). \$\endgroup\$ – Mast Jul 5 '18 at 21:26
  • \$\begingroup\$ FWIW, there's a PDF of that guide: A Guide to Debouncing August 2004 by Jack G. Ganssle. I came across it last week and thought it most useful. \$\endgroup\$ – Greenonline Jul 5 '18 at 22:16

I've done exactly what you're trying, back in the day. It's possible.

First, learn to use the single sweep function. Let's say your setup looks like


simulate this circuit – Schematic created using CircuitLab

Set your scope to trigger at 2 - 3 volts, negative edge. Set the time base for about 2 - 5 msec/div. Set the brightness fairly high.

Now turn off the lights. Leave only enough light to see the scope and switch, and using a keychain flashlight is a perfectly good approach.

Arm the scope, turn off the flashlight, and hit the switch. You should be able to get a pretty good idea of bounce.

In the old days, you'd use a polaroid camera set up with the shutter held open, then develop the film. That's not likely to be a useful approach these days. However, if you have access to a good digital camera with a B setting, you can use this on a tripod to do the same thing.

  • \$\begingroup\$ I would also suggest to use active probe if possible but yes 10pF isn't much \$\endgroup\$ – Umar Jul 5 '18 at 18:51
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    \$\begingroup\$ Yep, what a difference with modern scopes. I remember pasting tons of polaroids into engineering notebooks full of hand-drawn schematics and calculations. You always ran out of film when you needed it most. \$\endgroup\$ – John D Jul 5 '18 at 19:10

This should be a relatively simple process with a DSO that can store a single shot waveform. With the DSO you simply connect the scope probe into the circuit where the switch connects. Say for example the switch connects to GND and to a pullup resistor to 5V. Connect the probe to the switch/resistor connection. Then set the trigger to activate on the falling edge of the waveform at a threshold of say 3V.

When you use an older conventional CRO you hook it up the same way. But since the CRO has no waveform storage capability you need to repeatedly press the button to that the scope triggers over and over so you can see the scope beam trace across. You want the scope in Normal mode as opposed to Auto mode on the trigger settings.

  • \$\begingroup\$ There have been old analog CROs with a storage tube, see DVBST. I used them sometimes in the seventies of last century. \$\endgroup\$ – Uwe Jul 5 '18 at 19:50
  • \$\begingroup\$ I used the scopes of that type which were simply referred to as "storage scopes" in the labs where I worked. \$\endgroup\$ – Michael Karas Jul 5 '18 at 20:13

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