XO-1.75 microphone socket

[John Watlington]

The published XO-1.5 spec is very conservative.

But unfortunately, layout concerns prevented me from
providing too much protection.   The parts used (1/16W)
should protect up to 9V continuously, and up to
higher voltages for shorter periods of time.
With excessive voltage, the protective resistor should
blow out isolating the over-voltage diodes and codec from
the input jack and preventing hard-to-repair damage.

Regards,
wad

On Nov 15, 2010, at 3:33 PM, Ed McNierney wrote:

> From the XO-1.5 hardware spec at http://wiki.laptop.org/images/f/f0/CL1B_Hdwe_Design_Spec.pdf
> 
> "2.7.4	External Microphone Input
> 
> A stereo jack is provided on the laptop for connecting an external microphone, switch, or other sensor. This jack is connected to the audio codec’s microphone input, with provides a programmable gain preamplifier, with a gain of 0, 10, 20, or 30 dB. The input impedance is 10 KΩ.
> 
> A DC blocking capacitor may be inserted into the input signal path, under software control. This is used when the input is an audio signal. When used, the frequency response of the input is flat down to 10 Hz.
> 
> A bias voltage (1.5 or 2.5V, with a 3 KΩ source impedance) may also be driven onto this external microphone input, under software control
> 
> This may be used to power external electret condenser microphones, or used to sense whether an external switch is open or closed. Due to the high impedance of the bias source, this input may safely be shorted to ground.
> 
> An external voltage applied to this input is limited to between 0 and 3.5 V. Inputs exceeding this range may cause excessive current to flow through a protective diode.
> 
> When the external microphone jack is in use, the internal microphone is disconnected, and the indicator light indicating microphone use is disabled. The state of this jack may be queried by software."
> 
> 	- Ed
> 
> On Nov 15, 2010, at 3:28 PM, forster at ozonline.com.au wrote:
> 
>>> 
>>> This was done starting with the XO-1.5, and will also be
>>> done on the 1.75.
>> thanks
>> what is the allowable input voltage, i dont think its on the wiki?
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