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
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)
2 off disposable pens
2 off crock clips
you may find http://www.sciencetronics.com/greenphotons/?p=459&cpage=1#comment-312 an interesting read too
Step 1: Soundcard Test
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.
Step 2: The Circuit Explianed
Input stage and attenuater 2M ohm input so High impedence, cosists of a 12 way rotary switch and a resistor chain. Its purpose is to attenuate the input voltage to a manageable level, 0 to 2mv signal.
A Diode conects the +ve input to 5v this ensures that 5v is max input voltage if switch wrongly positioned.
non inverting unity amplifier for a 0 to 2mv signal
a times 2000 non inverting amp. The purpose of this stage is to amplify the signal after attenuating it. to produce a 0.5 to 2v signal an ideal measurment for our sound card
Step 3: Biuld the Circuit
Note how the resister chain is solderd to the switch this is neater than a load of wire it excludes 2 values, the 1M and the 2R. these are in a diferent place.
The 1M is biult into the probe we shall make later,(if you want to use yr own probe add the 1M resistor at the back of the BNC connecter) the 2R resister (I used 2 x 1R in series) are on the strip board for neetness.
The power connector, input and output I have used BNC connectors for the input so as to be able to use scope leads, the output connects to a chassis mounted stereo 3.5mm socket and power conector uses an old USB cable and a DC plug and socket.
The board is small and fits in a small space near the switches wired to the input output and power connectors.
Remember to connect the screen of the two BNC conectors to the 2.5v rail.
The box I have made from an old peice of laminate flooring and machined (routed) it is far bigger than the area needed to contain the circuit and components but is devided into two compartments one of which houses all the leads as my scope is AC only I have managed to squeeze in a dvm into the box
Step 4: The Box
Two scope probes,
3.5mm jack plug to 3.5mm jack plug stereo lead and
USB to 5mm power connecter
I am using a small project box (120 x 65 x 36mm) to house the switches and electronics. I could have got away with a box (120 x 65 x 30). for an even lower profile
the switch stem was shortend
the BNC connecters are inset below the box cover with a larger hole for BNC access, so as to create a lower profile partly to reduce chance of damage by snagging, plus keeps overall dimensions to a minimum
Step 5: Making the Probes
This is relatively simple but has a number of steps and very little cost however your first few steps are the most time consuming.
Dissmantle an old pen, and remove the refil with ball nib.
carefully fit the header so it protudes aprox 3 to 5 mm (the plastic pin spacer may move if it does note position)
solder the 1M resistor to a male header pin end that doesnt protrude see photo
mix small amount of epoxy resin using cocktail stick apply apoxy to resistor and pin taking care not to glue the testing tip and inset into pen nib see photo
Put to one side and leave to cure
while waiting for the epoxy to cure
Taking the 3m video cable and cut it half gives 2 x 1 1/2 m cables and no messing fitting BNC connectors.
slide up a peice of heatshrink tube
run the cassing up the cable
attach a crock clip to a piece of wire at least 2 1/2 times the length of the pen
trim back 15 to 20 mm of the outer insulation and the shield from the cut end of the coax cable
trim back the inner insulation about 5 mm and tin the central core
when the epoxy has gone off (check by feeling resin on mixing cocktail stick and mixing place)
solder the 1M resistor to the central core
reassemble the shaft and mark the cable
remove the shaft and trim 5mm of outer insulation
Solder a length of wire to screen
slide down heatshrink over exposed solder and shield and top of pen shaft
I tested these leads using my osciliscope calibration point. The output from which is a square wave however was a little saw tooth looking but for use as mobile test leads they will be fine. If you want to be more finikey I am sure you can squeeze a smd adjustable capacitor between the screen and core withing the pen tube and drill a small hole to make adjustments.
1 final tip I have lots of scope leads and many look identicle (especially factory made) I use either couloured insulation tape or coloured heat shrink a differnt colour for each lead. two peices one at the BNC end of the scope lead and identicle at the probe end this also allows you to see which probe is which channel