Main Site Documentation

Scope Shield


#1

There is a lot of questions around interfacing with the analog world on the forum these days - and one of the first answers is “put it on a 'scope”… Well what happens if you don’t have a scope?

Would there be interest in an “Instumentation Shield”? A shield for the Panda II with the following features:

  • Graphic LCD display
  • Lithium battery + charger
  • Measure Voltage, Current, Resistance, maybe also Capacitance, Inductance with auto-scaling like a digital multimeter.
  • Measure analog waveforms and display on the LCD - with auto scaling (and over voltage protection)
  • Frequency counter.
  • Serial sniffer and analyzer (to decode serial streams).
  • I2C sniffer and injector.
  • SPI sniffer and injector.
  • Analog signal generator.

I’m not talking a sophisticated instrument like a Bitscope, but something simple for the beginner in the hobby as the next step after a multi-meter. There are plenty of OSH designs for such devices - maybe it can be harvested easily?


#2

Yes! Such a shield would be great, and even better as a kit. I would also suggest making it larger than shield size so it could have a bigger LCD, or maybe split it into two (stacked) shields with the LCD on top. I would be really keen on seeing the schematic after you’ve packed all that functionality in there.

Great idea!


#3

Hi realiser !
That is a great idea, however the panda would have to be used only as a control device, since netmf is basicaly most of the time not adapted to work with real time signals ? Arduino seem to me more adapted for such shield.

Furthermore, if you want to play a little more with electronics, you may invest (at first !) in a really basic scope. Since this summer I have been testing for an example the “xmega xprotolab” ( http://www.gabotronics.com/development-boards/xmega-xprotolab.htm ) with is more or less doing everything you listed for 50$ (including UART, I2C, SPI analyser). I did also test the DSO quad from seeedstudio ( http://www.seeedstudio.com/depot/dso-quad-4-channel-digital-storage-oscilloscope-p-736.html?cPath=174 ) wich is very portable (with battery) for higher speeds (nut currently no analyser functions, which may come sometimes in the future). Since this last one is opensource, it is a good base to try to modify the software to make you own functions !

Finaly, nothing beats a good old scope in your lab, but you can make a lot of great things to start with with simple tools !

Have Fun :slight_smile: !


#4

I have a scope but other people I try to get into electronics don’t. We’re starting a local electronics club and it would be nice to build your own instruments with knowledge you pick up as you go.

Thanks for the links… I’ll start collecting a bunch of circuits and try and distill the best of.

RLP would definately feature with a little help from the hardware.

And I forgot… Of course another function would be Logic Analyzer.

Thanks for the ideas so far.


#5

Why not use the pc’s screen for visualization, and just use the panda to measure and stream that data to the pc?


#6

Some good ideas here:
http://tomeko.net/miniscope_v2b/index.php?lang=en
and here
http://opencircuits.com/Oscilloscope

And I’ve been eyeing this one for quite a while:
http://www.circuitcellar.com/archives/viewable/Zhang205/index.html

One could stream to the PC, but it’s so convenient to have a little standalone device too.


#7

Here is what I have so far…

Some of the features:

  • Blatant copy of Tomeko.net design - but that means it is proven to work.
  • Mostly through-hole components - so someone could even build this as a first introduction to electronics.
  • Gadgeteer Z connector for logic analyzer input. No particular reason other than I needed I/O.
  • 1 Channel analog with variable gain.
  • Buzzer (good for continuity checker feature)
  • On-board negative voltage generator. Clevel how he used a MAX232 for this.
  • Thinking of further using the MAX232 to front-end the Serial sniffer section.
  • On-board 2 cell LiPo charger.
  • Considering adding another LM317 for variable voltage, current limited general purpose power supply. Good for initial testing of new circuits.

Still can’t decide if I should use buttons or two rotary encoders for UI control. The second feels more natural like a “real” scope.

Let me know what you think please!


#8

I would say that two cells as a power source are little unconvenient. It’s pretty easy for hobbyist to find spare single cell (i.e. cell phone battery, even old one with a fraction of nominal capacity), but pairing two cells would be problematic.
Here are two other suggestions for analog front-end:

  1. http://www.seeedstudio.com/depot/images/product/dsomanual.pdf

    • quite similar to drawn above: gain switching with '4000 series IC, double op-amp: buffer + offset shifting, MAX3232 as a voltage doubler and inverter in one,
    • plus: up to 8 gain ranges with less components,
    • plus: works with single Li-Ion cell,
    • plus: could handle wider range of input voltages,
    • minus: input impedance varies depending on gain range (500 kOhm - 1 MOhm).
  2. http://mkeia.com/urzadzenie.php?Jezyk=2&Urzadzenia_ID=1

    • plus: differential inputs,
    • plus: single supply voltage only,
    • I’m not sure if it can measure voltages negative to GND level.

#9

Thanks Thomaz.

This device is aimed at the beginner - cheap and cheerful - so I’m sure there can be a lot of compromises. It also opens the door for a “Pro” version with all the bells and whistles. The benchmark at the moment is a $35 shield kit.

On the battery issue the reason for 2 cells is to keep the whole Fez Panda supply simple. To keep 5V devices in the design, it needs 7.2V with the linear regulators so this is forcing my hand a bit. I use Zippy 2C packs from Hobby King… Cheap with high capacity. One day I’ll get around to doing a generic boost converter for 1 cell.

I like the DSO’s 8 gain levels. I think I’ll use that design. I also like the NOC2K’s idea how to bypass the op-amp for unity gain. Makes sense when you work with 3.3V signals most of the time anyway.

I wonder why nobody uses digital pots for the gain… any ideas?

A tricky decision is going to be which Op-amp to use. AD8602 vs. TL082. I think this decision should be driven by which one is easiest for the hobbyist to get.

Supporting negative voltages and differential inputs is and interesting question. Do beginners still use this much? In audio I guess it is important but for general purpose “digital” work it seems unneccessary. What do you think?


#10

I think part of the problem with digital potentiometers is their limited bandwith. Looking into DS: 50 kOhm AD8400 is rated for 125 kHz. This still may be fine for some amateur low-speed oscilloscope/signal recorder taking bandwith to sampling frequency ratio 1:10 as a rule of thumb, but they are also not so popular. On the other hand CMOS switches are cheap and easily available.

AD8602 was choosen in NOC2K most likely for it’s rail-to-rail operation - measuring voltages up to GND level without negative voltage supply. TL082 is easier to get for sure. Both (and many others) are pin-compatible.

When thinking of digital/mixed signal circuit I would say that measuring negative voltages is not so usefull. If I would like to look at i.e. line ringing I would still have to use higher speed (I guess ADC in LPC is 400 ksps?) analog or digital oscilloscope. Another not so often used feature in my opinion is AC/DC switch.

With some compromises whole signal conditioning may be reduced to single buffer with rail-to-rail opamp and some overvoltage protection I guess. If noise level from ADC would be low and 9 or 10 bits would be usefull than one gain range 0…10V with 10 mV resolution per bit may be just fine.

Personally, I like the idea of cheap PC-based oscilloscopes. Even with low speed they can usefull as a data recorders beating desktop oscilloscopes in this area. Most popular oscilloscope based on real-time streaming would be USBee (or it’s not-so-legal clones). It’s software looks impressive for sure.

As a side note, some critical thoughts on PC oscilloscopes as a replacement for bench oscilloscopes: http://www.youtube.com/watch?v=Ev121xAt_k4


#11

Thanks (again) Tomasz.

I didn’t realise at first that you are indeed thee Tomasz - designer of Miniscope. Small world :slight_smile:

Can I ask you some questions about the miniscope design?

Why the transistors on the 4066’s inputs? I see in the datasheet that the switching voltage for the 4066 with ±5v supply is 3.5v. Isn’t 3.3V not still too low? Can it be switched directly from the Panda’s IO pins and eliminate the transistors?

The MCP41100 digital pot has 1MHz bandwidth. If it is used for gain control, and the AC/DC switch is ommitted, it could simplify the design quite a bit by eliminating the 4066 and everything around it.

I’m considering adding Resistance, Capacitance and Current measurement too - so it can be used as a DMM. Is it taking it too far?


#12

Since miniscope was mentioned I felt obliged to put my two cents ;).

As for '4066 - maybe there are some series with different specification, but looking to datasheets:
MC14066 (plain old '4000 series, Motorola) @ Vdd = 10V, control inputs:

  • max Low–Level Input Voltage 3.0V
  • min High–Level Input Voltage 7.0V

MM74HC4066 (Fairchild) @ 9V (no specification for 10V, but 9V should be fine)

  • max Low–Level Input Voltage 2.7V
  • min High–Level Input Voltage 6.3V

When shifting these from 0/10V into -5/+5V range they would give -2V and +2V as a Vil and Vih voltages.

Unfortunately transistors are needed because microcontroller pins have clamping diodes so voltage on them could not be lower than -0.7V. This voltage can be shifted with resistor or diode, but without transistor one of the control voltages (or both of them) may be little outside specification. In practice it should still work (worked for me) without transistor, but it’s little risky.
'4051 (aka DSO nano switch) is more convenient than '4066 because of it’s logic level conversion on control pins.