Add MultiMode photodiode access post.

diff --git a/posts/MultiMode.mdwn b/posts/MultiMode.mdwn
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--- a/posts/MultiMode.mdwn
+++ b/posts/MultiMode.mdwn
@@ -8,7 +8,7 @@ II stumbling along.  If you're in a similar situation, check out my
 notes on:
 
 * [[Stepper motor control|stepper]]
-* [[Photo-diode signal access]]
+* [[Photo-diode signal access|photodiode]]
 * [[MultiMode internals|internals]]
 * [Closest thing][connection] to a description of the
   NanoScope-MultiMode connection that I can find online. (see Ch. 10)

diff --git a/posts/MultiMode/photodiode.mdwn b/posts/MultiMode/photodiode.mdwn
new file mode 100644 (file)
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+++ b/posts/MultiMode/photodiode.mdwn
@@ -0,0 +1,27 @@
+[[!meta  title="Photodiode signal access"]]
+
+Relying only on the measured deflection voltage to measure the AFM
+tip-surface interaction is risky, as the loss of the signal may not be
+obvious. In order to validate incoming photodiode signals, you should
+also monitor the total light hitting the photodiode. This is
+especially true in fluid cell measurements, as bubbles of air or
+buffer solution can deflect the laser enough to miss the photodiode
+completely.
+
+As far as I can tell, the total photodiode voltage is *not* carried on
+one of the DB-25 lines connecting the [[MultiMode]] to the NanoScope
+controller (see the MultiMode [[internals]] page). It is displayed on
+the MultiMode LCD, so I monitored the lines connecting the main board
+to the LCD board to isolate the total photodiode line (LCD connector
+2). I build a small connector to tap out that line (despite
+appearances, none of the huge solder blobs actually short anything out
+:p).
+
+TODO: image of connector.
+
+We sent the tapped line out through Auxilliary D (DB25 line 4), so we
+could tap it again at out main break-out box. Comparing this signal to
+the photodiode ground (DB25 line 12), the signal is actually inverted
+(-7V signal when there's lots of light, 0V when there's none).
+
+[[!tag tags/hardware]]