Why can't I see bouncing of switch on oscilloscope screen? - Page 1
Forum section: electronics / beginners

author: denizoguz
author_rank: Newbie
timestamp: April 05, 2019, 08:10:52 pm
content: I'm trying to view bouncing of a simple switch on the oscilloscope. I have prepared a simple breadboard circuit (power->switch->resistor->ground). The problem is, it is displayed as perfect square/rectangle on the scope. I have attached a photo of scope screen and circuit. Please help me to identify why I can't catch bouncing of the switch on the scope. I don't think it this is a non-bouncing switch. As you can see it is rising 0 to 9v within 150uSec and staying there. I have tried a few different switches. Resistor in the picture is 220ohm 0.5w resistor.

author: denizoguz
author_rank: Newbie
timestamp: April 05, 2019, 08:12:29 pm
content: Sorry, I forgot to include other attachments. Here they are.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2019, 08:19:34 pm
content: Are you sure you are triggering on the switch closing - they sometimes open cleanly but always bounce on close. Try triggering on the falling edge.P.S. Welcome to the forum.

author: Nerull
author_rank: Frequent Contributor
timestamp: April 05, 2019, 08:20:24 pm
content: Are you capturing on a slow timebase and then zooming in? That nice slow linear transition between 0 and 9V indicates a lack of sample points between them. It's drawing a smoothed approximation.

author: denizoguz
author_rank: Newbie
timestamp: April 05, 2019, 08:25:59 pm
content: Quote from: denizoguz on April 05, 2019, 08:10:52 pmYes. I have just learned zooming in and capturing is different than capturing first and zooming after. I'm having difficulty to capture when I zoom on time scale. I can set my scopes trigger to capture on rising edge and it is doing it but when I do it is stopping recording immediately. That is I can't see what happens after rising edge. It just stops.

author: amyk
author_rank: Super Contributor
timestamp: April 06, 2019, 01:36:58 am
content: Quote from: denizoguz on April 05, 2019, 08:25:59 pmChange the trigger position.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 06, 2019, 01:54:13 am
content: Is that a 220 Mega Ohm resistor (red red violet)!?!If so, then such a big resistor can be considered an open circuit, because the oscilloscope's probe impedance is usually much lower (usually 1 Mega Ohm for x1 probes, or 10 Mega Ohms for x10 probes). 1 or 10 Mohm together with the few pF parasitic capacitance will make a great hardware RC debouncing circuit, so all you'll see on the oscilloscope will be a ramp.If you want to see the dirt transitions of a switch, you need a much lower resistor. Try for example with a 2.2 Kohm (red red red), or lower. You'll see a ragged edge over about 5-50 ms, depending on the switch quality.

author: radiolistener
author_rank: Super Contributor
timestamp: April 06, 2019, 05:10:41 am
content: Quote from: blueskull on April 06, 2019, 01:57:02 amprobably his oscilloscope has too small memory but he record sample with very slow timebase and then zoomed it while in the hold mode.So, there is just two sample points - low and high and line between them.Bouncing happens somewhere in the middle of that line, it just didn't captured due to small memory and slow timebase.Due to a small memory, there is insufficient horizontal resolution in order to see the bouncing.There is need to capture it with more fast time base.This is the same as trying to use zoom in the picture viewer in order to see microbe on the low resolution photo, where actual microbe size is much smaller than one pixel. With zoom you will see just a big pixel

author: vk6zgo
author_rank: Super Contributor
timestamp: April 06, 2019, 01:00:12 pm
content: Surely there must be a way to do this with a modern DSO.(I could understand the early ones having problems).I've observed (obviously not recorded) switch bouncing with a very basic analog 'scope by winding up the intensity control & running the time/div at a very low speed.Wouldn't "roll" do this on a DSO----it seems to have no other use!

author: bd139
author_rank: Super Contributor
timestamp: April 06, 2019, 01:35:29 pm
content: Best way to do this on a modern DSO is use single trigger mode. Set it to trigger on the first leading edge just above the normal trigger voltage. Make sure sample rate is as high and sweep is relatively slow. 1ms/div -> 200uS/div approx if you dont have lots of sample memory. 2.5kpts is enough!. Hit SINGLE, whack the switch. Then use the timebase control to zoom into the event.To catch bounces you should probably use a pull up to 5V. I had the stuff on the bench already so here's what you should see:Note the trigger position and the fact I've triggered on leading edge on a pull up. That will eliminate any events where the switch does not bounce (it does happen as well!)Experimental setup. Resistor is 1k - was just there on the bench. Switch is a SPDT momentary on/on - first one out of the box.Scope output:

author: StillTrying
author_rank: Super Contributor
timestamp: April 06, 2019, 01:41:54 pm
content: Quote from: vk6zgo on April 06, 2019, 01:00:12 pmYes it's dead easy.Resistor about 1K to 10K.X speed about 5ms/div.With the trigger on AUTO set the trigger LEVEL mid-way between the high and low trace levels of the signal, then switch the trigger to NORMAL. Every switch change will show a capture, if it's bouncing.Quote from: vk6zgo on April 06, 2019, 01:00:12 pmI think it's very useful for slow changing signals, you get to choose when to press STOP by eye.