Probe for Oscilloscope

curtis

Hello,
I found an Pocket Pc based oscilloscope by www.virtins.com that is very economical. They are about to have a probe that plugs into the earphone jack for sale. There is a trial version that I downloaded that I will try this weekend. I would like to ask you electronic experts if there is a way to make a homemade probe so I can test the demo with. I will order it but I'd like to try some experiments this weekend. Any suggestions would be great on making a probe. Thanks, Curtis

kjennejohn

Hello, Curtis.
Nice toy you found. It is very underpowered for digital work, as it will only handle voltages 2 - 3 Volts, and frequencies from 24 kHz to 96kHz. As they say in the literature, it all depends on the characteristics of your sound card. However, it should be adequate for low-voltage audio work. It will also help teach you how to use PC-based digital test equipment. IMHO, all the basic elements are there. BTW, I doubt that this uses the earphone jack. It should be using the microphone jack.

As for the probe, if you limit your work to less than 2V you could use the cord off of a defective or cheapo headset. Cut the wiring off at the headset. Expose both (or three) wires inside and use a multimeter to do a continuity test to determine which goes to the tip and which goes to the barrel, usually referred to as the ring. If it's stereo, you'll have two signals. Use the conductor that goes to the farthest tip and, if stereo, ignore the next one in. The ring will be the barrel part that goes right up to the plastic. Solder the tip conductor to a 1kOhm resistor for the probe tip. Cover the resistor with shrink tubing or electrical tape, leaving the free lead exposed. Solder the ring conductor (which may be a braided wire sorrounding the wires within) to an alligator clip on about 6 inches of stranded wire (for flexibility). Be sure there is no contact between these two conductors. This will short the ground to the signal and produce a flat line reading. Take a resistance reading between the ground clip and the resistor's end. This should read open. Now do a check from the resistor's end to the 'tip' part of the plug. This should read the resistor's value. Now check from the ground clip to the plug's ring (or barrel, whatever) and it should read as a short.
With this set up you can now probe LOW VOLTAGE circuits, less than 2V. The resistor gives you some overcurrent protection. Plug this into the microphone jack on your computer. Crank up the software. Select the instrument function you want. Be careful not to hold the probe tip while taking measurements as this will put an added signal in you don't want, usually 60 Hz emanating from the house AC wiring. Then again, you can test the probe by placing a finger it to see if the 60 Hz is there.

And that is basically the SAFEST minimal probe. All this is Radio Shack stuff. You could also just solder zip cord to a microphone plug and solder the resistor and alligator clip to this conglomeration. How badly did you want that test circuit, and PC, to survive this experiment?

You could buy the probe they sell. It looks like overkill, but is self contained, not terribly expensive and has a few useful features. This sells seperate from the software. I actually thought for a moment the fancy Pocket PC came with the purchase. Must have been a senior moment.

Hope that helped.
kenjj

curtis

Thanks, I never realized that it only measured up to 2 volts. I may just purchase the velman handheld scope for under $200. , Curtis