From: Ben Harris <bjh21@bjh21.me.uk>
Date: Wed, 27 Jan 2021 09:45:46 +0000 (+0000)
Subject: Brief notes on the requirements for Q1
X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~bjharris/git?a=commitdiff_plain;h=d4d9fd71dd675c77752d06b52b5472422976d735;p=clunkpi.git

Brief notes on the requirements for Q1
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       achieve 1.24 V across R1.  D1 suppresses any voltage spike
       caused by the inductance of the clock coil.  The whole thing
       turns off when the GPIO line goes low.</p>
+    <p>The specifications of Q1 are critical.
+      It needs a high enough <var>h<sub>FE</sub></var> and a low enough
+      <var>V<sub>BE</sub></var> that it can sink 250 mA when driven
+      by the relatively weak output of a Raspberry Pi GPIO.</p>
     <h2>Use</h2>
     <p>Plug the board into a Raspberry Pi with the square solder pad
       connected to pin 1 of the GPIO connector.  Connect the clock