PC Scope Probe
This simple pc scope probe is nothing more than a fet follower. It has a high input impedance and a low output impedance to match a mic or line input socket on a pc or laptop.
WARNING: Please be careful when applying any signal into a sound card. See also the sites General Disclaimer. Circuit Notes
Its useful feature is that the fet input is effectively attenuated by a factor of 30 at the output, when the mono plug is inserted in the mic socket.
This means it is then practical to examine waveforms under test, for example logic square waves of 5v peak-to-peak, without loading the circuit source point.
The resulting terminated output will thus be within a safe mV range as can be accepted by the sensitive sound card input. Installed oscilloscope/spectrum analyser software will display essential elements of the wanted waveform.
Proviso: Sine waves and square waves of 50:50 duty cycle to 20khz are no problem to check. But examination of fast rate clock and transient pulses is not possible due to the sound card's poor risetime response and not by limitations of the probe itself. The probe will faithfully transfer exactly what it receives.
Component notes and final build suggestion
Both 47mfd capacitors pass down to low frequencies. The first 47mfd cap blocks dc from the fet source pin and the second 47mfd cap blocks the phantom +3v dc outputted from the pc mic socket.
If a simple open-plan construction will suffice, the nine components will fit on a piece of veroboard cut to 43mm by 16mm. Close-crop the component legs after soldering. Maplin board pins of code FL24, can be used for all external wiring connections.
The finished board will then fit comfortably within the triangle of three mounting holes on the back of a plastic PP3 battery carrier of Maplin code CK65V.
To secure the board trackside to the plastic, cut to size double-sided sticky tape, apply then press the two surfaces carefully and firmly together. Solder the red and black battery wires correctly to the board.
Red/Black spring loaded probe clip leads of Maplin code N21CB or miniature insulated crocodile clips, are suggested for circuit probing.