Brief notes on the requirements for Q1

diff --git a/index.xhtml b/index.xhtml
index dd983028c3de368c8a225e2032fe9e83c9b58fa8..957d5ec5924e90db66ad645d348b9861ae0c9f59 100644 (file)
--- a/index.xhtml
+++ b/index.xhtml
@@ -51,6 +51,10 @@
       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