From: W. Trevor King Date: Fri, 19 Nov 2010 17:54:42 +0000 (-0500) Subject: Add MultiMode photodiode access post. X-Git-Url: http://git.tremily.us/?a=commitdiff_plain;h=a4211e58cdb3e5dc25aef1b75fccaa512234fbbe;p=blog.git Add MultiMode photodiode access post. --- diff --git a/posts/MultiMode.mdwn b/posts/MultiMode.mdwn index 4b596f1..f9b8747 100644 --- 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 index 0000000..4977a7d --- /dev/null +++ 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]]