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Date: Wed, 26 Aug 2015 12:15:43 -0700
From: Larry Doolittle <ldoolitt AT recycle DOT lbl DOT gov>
To: geda-user AT delorie DOT com, Spacefalcon the Outlaw <falcon AT ivan DOT Harhan DOT ORG>
Subject: Re: [geda-user] Need help with MOSFET part selection
Message-ID: <20150826191543.GA14560@recycle.lbl.gov>
References: <1508261840 DOT AA26986 AT ivan DOT Harhan DOT ORG>
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SF -

On Wed, Aug 26, 2015 at 06:40:50PM +0000, Spacefalcon the Outlaw wrote:
> 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
> 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.

You're on the right track.

> 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.)

OK.  Depending on the LED and the ballast resistor (3, in the
StackExchange drawing), the brightness will vary some with the
state of the battery charge.  Better to choose a Red LED that
has lower on voltage, so you have less dependence on battery voltage.

> 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.

That's perfect.  Go to Digi-Key, set yourself up with
Product Index > Discrete Semiconductor Products > FETs - Single
and under "FET Features" select "Logic Level Gate, 1.5V Drive",
as well as "In stock".  That gets you down to 227 matches.

Adding "Cut Tape" packaging, and the package as
"TO-236-3, SC-59, SOT-23-3", gets you down to 11 choices.
Any of them should work for your purposes, assuming your battery
voltage doesn't exceed +12V.  Cheapest in quantity 10 is
Toshiba SSM3J325F,LF, Digi-Key part number SSM3J325FLFCT-ND,
$0.368 each.

A FET like this obviously does not load down the drive at DC,
but is a fairly beefy capacitor, at least 270 pF for this part.
You'll have to evaluate whether that matters for your application,
and whether adding a gate resistor (2, in the StackExchange drawing)
is helpful or necessary.

You're welcome, and good luck!

  - Larry