[OpenTRV-dev] Sanity check please

[Bo Herrmannsen]

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

Hi,

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

http://uk.rs-online.com/web/p/phototransistors/7000776/

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, ?

Rgds

Damon
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