0 to 500v Sound Card Osciloscope and probes Cost Peanuts (under ý20)

I am sure you have seen many of these, this is my simple version it uses 1 USB port for power and the line in of the sound card. If you dont have a USB port the design will still work if you find a suitable easy access power scource a 9v battery would do.

This scop is limited to AC only for me this is because my sound card line in is AC coupled and DC signals are blocked as i do not wish to modify my laptop and keep it simple i settled for this I can measure DC with a DVM, I have found a freesouce osciloscope program which uses the sound card line in.

The sound ard scope circuit I Used is someone elses but modified so as to measure Voltages from 1mv to 500v possibly lower depending on your A to D converter.

The rotary switch mearly acts as a simple voltage devider, to a 2Mohm input.  The voltage is reduced and then amplifyed by 2000 to between 1/2 and 2 volts (for the sound card input) output. This may seem backwards but it is the simplest way to do it, the pay off is loss in accuracy but this is for testing on the move I have a full calibrated osciloscope I an use if I need accuracy.

If you really want a better accuracy over the range I would sugest using a rotary switch wich first layer acts as a Voltage devider and a second layer that selects an amplfyer resister.   Not so simple I fear.

Components  (I like to salvage parts where i can)

Selection of Resisters (I started with a 610 Piece E12 1/4W Resistor Starter Pack usefull for other projects)
2 off 2R 1/4w resistor  (any small wattage will do or use 2 off 1R to make 2R)
2 off 1N4003 Diode
2 off 1 pole 12 way switch
1 off jfet quad opamp (TL074N)
3 off 90 x 25 x 3 perspex
strip Board
LED (Power indicaator)
USB cable (broken or old data cable as long as 5v dc is available should be 6inch to a foot long)
chassis dc jack
chassis 3.5mm stereo jack
Stereo cable with 3.5mm jack plug
2 chassis bnc conecters
rg58 3m video cable (BNC each end)
Box
2 off disposable pens
2 off crock clips
screws

you may find http://www.sciencetronics.com/greenphotons/?p=459&cpage=1#comment-312 an interesting read too

Step 1 Soundcard Test

First we need to test the sound card both left and right channels.

AC/DC coupling test

using a 100k pot and 5 volt DC supply.

wire the pot acros 5v
conect the wiper to left and right channel inputs and input ground to pot 0v hook up DVM across wiper and 0v to measure output voltage

turn on scope software setup with line in as channels 1 and 2

turn the pot slowly see if voltage remains fixed and equal to the wiper voltage   If it returns to zero when wiper movement stops the input is AC coupled.

AC input range

Now we need a to rewire the pot across a 12v ac signal (i have a signal generator)
increase the signal untill the sin wave just begins to be clipped (flat line forms at top and bottom instaed of smooth curve)
measure voltage at wiper pin. this is our peak input voltage commonly this is between 1 and 5 volts.

take a note of this voltage it is important.

Comments

[profpat]

this is nice,

but you did not mention what pc software you used or suggest.

[lrdforster]

Sorry I didnt feel it neccassery but i use soundcard scope

[Goodhart]

A mention in the ible as well as a link would be helpful to many.

[womai]

The schematic shows a 1:1 driver stage followed by a 2000:1 amplifier stage. This is not a good arrangement for two reasons:

First, high gain makes the circuit prone to instability (oscillations). Second, the op-amps limited gain-bandwidth-product (GBP) means the higher the gain, the lower the bandwidth. The TL074 you use has a specified GBP of 3 MHz typical (2 MHz minimal). In a gain=2000 stage thus means your bandwidth is a measly 3000kHz/2000=1.5 kHz. That's low even for a soundcard scope. At the same time the first (gain=1) stage has a bandwidth of 3 MHz

Better to even out the gain between the two stages. E.g. choose sqrt(2000)= approx. 45 for both. The bandwidth of each stage is then 3MHz/45=67kHz. Much better. The total bandwidth of the circuit is then 67 kHz/sqrt(2) = 47 kHz. Fits nicely with the bandwidth of the soundcard input which is in the 20 kHz range.

[lrdforster]

You make a valid point I must admit i forgot all about Bandwith, as yet ive not tested it over 50 hz. I shall modify my circuit accordinly many thanks for your point

[womai]

You may also want to add a diode between the first stage's input and ground, in case the input ever goes below zero. Right now the input is only protected against overvoltage (by the one diode already in the circuit), not against undervoltage. As for diode, 1N914 is a fast-switching model and very inexpensive. That's what I use for my own scopes.

[lrdforster]

thanks i will, I have a few improvements now, after modifying the two stages to 1 x40 the other x50 making x2000 the sugested root 2000 being 45 didnt sit well I know accuracy is a little iffy but having a close start sems a fraction better.

I now notice the 2 off 1M resisters as a voltage devider doesnt set 2.5 v but 1.5v so i replaced with a preset and still cant set 2.5v so have to look into a better solution. possible lm317 set at 2.5v

[Lectric Wizard]

Could you add a clear schematic. Present one is unreadable Thanks!

[lrdforster]

if u click on the i in the top left corner you can see it enlarged at full resolution

[Lectric Wizard]

Thanks, I have seen other audio card interfaces but they were too complex. This is great because of its simplicity, GOOD JOB !!!

[lrdforster]

Thank you

[Lectric Wizard]

Here is what I mean:

[Lectric Wizard]

If I can make a suggestion, I would put a 1.5A fuse in line with & a 1N5339B zener across the USB power supply so that if something should go wrong & high voltage gets back to the supply the zener will "crowbar" the excessive voltage & blow the fuse. CHEERS!

[lrdforster]

Good idea.

That is the point of the 1n4003 on the amplififyer input it should maximize any voltage to 5.6volts and with the 1M resister on the probe. current is allready minimised however i dont see why you cant take a belt and brase's aproach, if you feel the need. It certainly would protect the sound card/laptop 5v.

[legailutin]

Like it. Just happened to be looking for a quick way to capture more than 5V signals from my circuit. I'll build myself one of these and post feedback.
Thanks for your time.

[lrdforster]

i look forward to hearing yr feedback