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X-Recipient: geda-user AT delorie DOT com
Date: Wed, 26 Aug 2015 18:40:50 GMT
From: falcon AT ivan DOT Harhan DOT ORG (Spacefalcon the Outlaw)
Message-Id: <1508261840.AA26986@ivan.Harhan.ORG>
To: geda-user AT delorie DOT com
Subject: [geda-user] Need help with MOSFET part selection
Reply-To: geda-user AT delorie DOT com

Hello fellow tinkerers,

I seek to implement the circuit shown on this StackExchange page:

http://electronics.stackexchange.com/questions/60865/how-to-drive-a-20ma-led-from-a-4ma-max-gpio-pin

I want to implement the circuit which the OP of that thread had in
mind (the one at the top of the page, the one with a discrete MOSFET),
not any of the suggested alternatives.  The problem I am solving is a
little different from the one the StackExchange OP had: I don't need
to pass lots of current through the LED (it should be bright enough
for me to see even with just a couple of mA flowing through it), but
my issue is that I am absolutely not allowed to put any extra load on
the internal digital signal whose state I would like to show on a LED.

I would like to put a LED on my FreeCalypso GSM development board that
shows whether the chipset is in the "phone on" or "phone off" state.
The chipset has an internal signal (intended to go from one chip in
the chipset to the other, and nowhere else) that signals the state I
am after.  It's a 1.5V logic signal, low in the "phone off" state and
high in the "phone on" state.  I would like to have my LED light up
when this internal ON_nOFF signal is high.  The power for the LED will
come from VBAT (raw battery voltage rail), and not from the ON_nOFF
signal itself or from any of the LDO regulators in the chipset.  (The
latter have complex on/off/sleep modes, and I would like this LED
indicator circuit to be completely independent of them.)

The circuit drawing I've cited above is the best I have found that
seems to do what I seek.  But I have the same question that OP had: I
am clueless when it comes to selecting a specific part for the MOSFET.
What I am really looking for is a Digi-Key part number.  The input
signal whose state I need to sense without loading it is a 1.5V logic
signal, so I assume I need a MOSFET with a switching threshold that's
appropriate for that logic standard.  Can someone perchance help me
with this issue?

TIA,
SF