PC oscilloscope using the MIC (microphone) input.


<h1>The MIC input</h1>

<p>Almost every PC / laptop has a MIC input - and this can be used as a basic 2 channel oscilloscope ! Of course it's band-width limited - the often 'claimed' 20Hz-20kHz will be more like 50Hz to 10kHz, however for tracking faults in audio amps it should work just fine !</p>

<pre>'All' you need to do is feed the signal to the MIC socket and record the 'sound' using any Open Source application such as Audacity</pre>

<h2>Input voltages</h2>

<p>To diagnose audio-amp faults, you will need to probe voltages up to 50v or even more.</p>

<pre>The typical MIC input will accept voltages from an actual microphone (approx 10mV) up to a low volumn Line input (about 100mV)

The max. you should count on is thus 25mV or so.</pre>

<p>Assuming you are using a standard 'x10' probe (which comes with a 9Mohm in-line resistance), a 1M ohom termination will drop 50v to 5v. To get this down to 25mV, you need an over all 2000:1 reduction (50v to 25mV)</p>

PC-MIC-osc-input.gif

<pre>The 47k pot. together with the 9M ohm probe sets the 2000:1 division ratio.

The additional 470k has mninimal effect (other than to protect the circuit in the event that a x1 probe is ued in error) 

The back-to-back Sckottly dioeds will provide some additional protection (they should keep the levels below about 300mV) 
 
The 1uF cap. prevents DC levels reaching the MIC in circuits (chances are MICin has an input cap anyway)
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