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Date: Mon, 31 Aug 2015 13:26:12 +0200
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Subject: Re: [geda-user] Need help with MOSFET part selection
From: "Bernhard Kraft (bkraf0j9 AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Using an Op-Amp you could define the exact switching charactersistics
(hysteresies), an all-round part like the TP912 is available for ~80
Cent and features two op-amps. The usually load the input with only a
few nano amps.

greetings,
Bernhard

On Wed, Aug 26, 2015 at 9:15 PM, Larry Doolittle
<ldoolitt AT recycle DOT lbl DOT gov> wrote:
> 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