Triggering scope with foot pedal switch

[slugrustle]

I just jury rigged a foot pedal switch to the external trigger input on my scope with a 1k resistor and 5V supply from scope USB.  It's super handy.  I wanted to hold two nice high speed probes in particular locations with ground springs and needed both hands for that, but I also wanted to trigger the scope.  Enter the foot pedal switch.

Surely other people have done this before.  I'm looking to make a better / more permanent setup.  Do other people have advice on nice ways to do that?  I was thinking maybe a little box with debounce, selectable 3.3V / 5V out, selectable idle high / idle low, a BNC jack for output, that kind of thing.

[dobsonr741]

Food for thought: voice assistant triggering single shot trigger over SCPI.

[schmitt trigger]

Excellent idea, I may build one myself.
The most important feature IMHO, would be a debounce circuit. 

[ejeffrey]

A smart pedal would be super cool.  Something like this might be more complex than necessary but sounds handy:

Hold down the pedal to put the scope in auto or normal mode.  Release to stop/hold.  Tap once to arm single shot mode.  If the scope triggers from the signal, another tap re-arms single shot mode.  If not, subsequent taps act like force trigger button.

[TERRA Operative]

Food for thought: voice assistant triggering single shot trigger over SCPI.

Like this one?

https://www.ebay.com/itm/185712951081

[slugrustle]

The idea to use SCPI is a good one.  Man, if this scope had RS232 instead of USB, I would totally program an MCU to arm a single shot capture based on one pedal input and trigger the capture based on a second pedal input.

As it stands, doing this with USB is a bit beyond what I have time for (given that I would insist upon using a small MCU, not something with an OS).  Interesting idea all the same.

[thm_w]

Food for thought: voice assistant triggering single shot trigger over SCPI.

https://www.eevblog.com/forum/testgear/adding-voice-control-to-rigol-mso5000-oscilloscope/

[LazyJack]

Clever idea!
The old 2400 series Tektronix digital scopes had a similar feature. The scope could be programmed to do all kinds of things at the press of the ident button on the probe. Like saving the waveform, switching to different configurations or starting an acquisition.

[slugrustle]

Ok, I took a brief look at controlling the scope with SCPI over USB.  Was thinking about using a MAX3421E as the USB host and controlling that from an 8-bit MCU.  It would be even cooler to use a WCH CH554 MCU or similar by itself, but I'm not sure how to get the debugger for those MCUs.  Then I looked into the USB protocol a bit and realized that learning it would be a big lift for what I want to do here.

It turns out that setting my scope (SDS2000X+) to normal mode and triggering it externally with a foot pedal switch does just what I want (take a new single shot each switch press), but only if the switch is properly debounced. So I put together a little circuit that debounces the switch upon both open->closed and closed->open transitions, sets a latch on switch debounce closed, and resets the latch on switch debounce opened.  Every time the latch is set, it fires a ~100µs one shot that controls a ~5V level line driver.  I also added an output overvoltage crowbar to hopefully prevent damage to my scope if something fails in the circuit.

Boards are in the mail, and components are ordered.  The circuit simulates well, so I'm hopeful for a good result.

[tautech]

It turns out that setting my scope (SDS2000X+) to normal mode and triggering it externally with a foot pedal switch does just what I want (take a new single shot each switch press), but only if the switch is properly debounced.
I also added an output overvoltage crowbar to hopefully prevent damage to my scope if something fails in the circuit.

Boards are in the mail, and components are ordered.  The circuit simulates well, so I'm hopeful for a good result.

Please see Ext Trigger input specs on P10 of the datasheet.
https://int.siglent.com/u_file/download/22_09_19/SDS2000X%20Plus_Datasheet_EN01C.pdf

From a 1M source 42V is max Ext Trig input.

[slugrustle]

Thanks for that.  Turns out my local copy of the datasheet is one rev older and doesn't have that info.  It's good to know.

I'm pretty sure the 1M/50Ω different for the "Ext Trigger Channel input voltage" line has to do with the EXT channel input impedance, not the source impedance.

In any case, the circuit I'm building runs from a 9VDC source, has slightly under 5V output voltage unloaded, about 50Ω output impedance, and the overvoltage crowbar should trip at around 6V, but only if there is some kind of fault condition.

I'll keep the EXT input impedance at 1MΩ though based on this info from the datasheet; the extra headroom shouldn't hurt.

[tautech]

I'm pretty sure the 1M/50Ω different for the "Ext Trigger Channel input voltage" line has to do with the EXT channel input impedance, not the source impedance.

From datasheet:
Ext Trigger Channel input voltage 1 MΩ ≤ 42 Vpk, 50 Ω ≤ 5 Vrms

To me seems very clear this ^ indicates source impedance.
Therefore a series resistor in the source of your switching output can keep the signal within limits without need of a crowbar.

BTW, threshold levels that you set when selecting Ext Trig:
EXT: ±0.61 V
EXT/5: ±3.05 V

[slugrustle]

Max input voltage for the other channels, the normal scope channels, is listed as
1 MΩ ≤400 Vpk(DC + AC), DC~10 kHz
50 Ω ≤5 Vrms, ±10 V Peak

Do you think 1MΩ and 50Ω for the normal input channels represents the output impedance of an external source?  I'm pretty sure it's the channel input impedance on the scope.  The same 5V limit applies to normal input channels and to the EXT channel at 50Ω, presumably due to a power limitation.

Perhaps you mean something different than I do by source.

The scope can set the input impedance of the normal channels and of the EXT channel to 1MΩ or 50Ω, and the input voltage limits are listed for both input impedance settings.  It would be really weird for the scope to be able to set its inputs to these two specific impedance values, list max voltage limits according to these two impedance values, and then intend the impedance values to refer to the output impedance of some external source...

[jmw]

Food for thought: voice assistant triggering single shot trigger over SCPI.

https://github.com/jmwilson/ollie

Snips is gone, but it's an offline voice assistant, so the image will continue to work indefinitely.

[Psi]

If I was going to do that I'd probably have a switch mounted on the underside of the desk where I can press it with my knee.