Posted on Feb 5, 2014

This simple pc scope probe is nothing more than a fet follower. It has a high input impedance and a low output impedance tomatch a mic or line input socket on a pc or laptop. This means it is then practical to examine waveforms under test, for example logic square waves of 5v peak-to-peak, withoutloading the circuit source point. The resulting term

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inated output will thus be within a safe mVrange as can be accepted by the sensitive sound card input. Installed oscilloscope/spectrum analyser software willdisplay 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. Both 47mfd capacitors pass down to low frequencies. The first 47mfd cap blocks dc from the fet source pin and the second 47mfdcap blocks the phantom +3v dc outputted from the pc mic socket. If a simple open-plan construction will suffice, the nine componentswill 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 wiringconnections. 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.

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