[OpenTRV-dev] Sanity check please

Fri May 9 16:09:32 BST 2014

3).... its replaced... on schematic the correct symbol is almost like and 
LED... just that the arrows point towards the device... on board it has 
outline like an transistor...

-----Oprindelig meddelelse----- 
From: Damon Hart-Davis
Sent: Friday, May 09, 2014 5:04 PM
To: Closed list for developer discussions
Cc: Damon Hart-Davis
Subject: [OpenTRV-dev] Sanity check please


Preparing a quick REV3 board with RFM69CW and photodiode in place of LDR for 
RoHS reasons:


While I have modified my breadboard and it seems to work some things are 
puzzling me that may indicate that I’m too stupid for this work!

1) I expected to have to use a series resistor of ~4k7 to get 1.1V across it 
with 100lx illumination of the phototransistor, instead 100k seems to be 
giving me that 1.1V in a bright room.  (Cannot find all the bits of my lux 
meter!)  I am assuming that this phototransistor is best treated as a 
current source, not a variable resistor for example.

2) I am taking the short leg to be the cathode as for a (3mm) LED, but the 
datasheet labels the short leg as C which I’d take to be the collector and 
thus +ve for an NPN.  Uh?  In general I assume that using an LED Egale part 
will give a board assembler all the right clues if I can do it correctly.

3) What would be a good circuit symbol tp replace ‘LDR’?  PT, D, ?


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