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Light sensors - two part question...

  • Thread starter The Grim Reaper
  • Start date
T

The Grim Reaper

Jan 1, 1970
0
Part 1 - I need to count/log the number of times a 3mm red LED flashes. The
flash (on) period is very short - looks like half a second-ish, and the
period between flashes varies between 5 seconds and 2 minutes. I can't
interact with this circuit, so I've got to read it optically.
I've got a simple 741 voltage comparator setup that works OK with an LDR
(using a green, not red, LED as test input). However, I believe the
response time can be improved. Google as I might, I can't seem to find a
lot of information on phototransistors or other high speed, but cheap
optical sensors.
Any ideas appreciated.

Which brings me to Part 2; I've found what I thought was a phototransistor
in my pile of spare bits - photos here:
http://www.grsoftware.co.uk/PhotoTransistor.jpg but I can't for the life of
me find any specs on this part, and I can't even remember where I got it
from!!
The numbering on the side is R8305462T102. For an idea of scale, it's the
same size as a BC107 (T 192 case?!).
There is no base connector... I tried wiring it up as a simple input (in the
same circuit, replacing the LDR) but get no change in voltage, resistance or
current from any legs!! Think it's bust?!?
Again, any ideas appreciated!!

Thanks
__________________________________
The Grim Reaper
 
The said:
Part 1 - I need to count/log the number of times a 3mm red LED flashes. The

Try searching on keywords:
photodiode amplifier schematic

Here's a simple one: http://amasci.com/freenrg/ufoscope.html

The usual way to do this is to use a photodiode (or phototransistor)
feeding an op-amp that's wired as a current/voltage converter.
Connect your PD or PT between V+ and the op-amp's neg input pin. (If
using a PD, connect it to V+ backwards so it's turned off.) Ground
the op-amp pos input pin, and use a feedback resistor between neg input
and op-amp output. This converts tiny PD currents into large output
voltages. Resistor value determines sensitivity, and would be between
2.2K and 2meg.

Which brings me to Part 2; I've found what I thought was a phototransistor
in my pile of spare bits - photos here:
http://www.grsoftware.co.uk/PhotoTransistor.jpg but I can't for the life of

Try putting 100K in series to limit current, hooking it to 9V supply,
then measure the voltage across the 100K resistor while shining an
incandescent flashlight at it. If the voltage changes a little, or
stays at 0v, then try connecting the power supply backwards. When
polarity is correct, sensitivity will be maximum. If it never budges
from 0v, then it's shorted out.

((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
[email protected] UW Chem Dept, Bagley Hall RM74
[email protected] Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http://staff.washington.edu/wbeaty/
 
C

Chris Jones

Jan 1, 1970
0
The said:
Part 1 - I need to count/log the number of times a 3mm red LED flashes.
The flash (on) period is very short - looks like half a second-ish, and
the
period between flashes varies between 5 seconds and 2 minutes. I can't
interact with this circuit, so I've got to read it optically.
I've got a simple 741 voltage comparator setup that works OK with an LDR
(using a green, not red, LED as test input). However, I believe the
response time can be improved. Google as I might, I can't seem to find a
lot of information on phototransistors or other high speed, but cheap
optical sensors.
Any ideas appreciated.

Which brings me to Part 2; I've found what I thought was a phototransistor
in my pile of spare bits - photos here:
http://www.grsoftware.co.uk/PhotoTransistor.jpg but I can't for the life
of me find any specs on this part, and I can't even remember where I got
it from!!
The numbering on the side is R8305462T102. For an idea of scale, it's the
same size as a BC107 (T 192 case?!).
There is no base connector... I tried wiring it up as a simple input (in
the same circuit, replacing the LDR) but get no change in voltage,
resistance or
current from any legs!! Think it's bust?!?
Again, any ideas appreciated!!

Thanks
__________________________________
The Grim Reaper

I guess this is an electricity meter that has the flashing light - am I
right?

I found this:
http://www.bwired.nl/stroom.asp

Probably a phototransistor or a photodiode should work, but getting it close
to the LED will help a lot.

I would suggest trying the photodiode or phototransistor in series with a
very high value resistor (try 1M as a starting value, or a 1M variable
resistor in series with a 10k safety resistor would allow adjustment),
across a 9V battery. The photodiode would need to be connected in the
direction so that its anode is more negative than its cathode, otherwise it
will always conduct regardless of light level. A phototransistor would be
more sensitive, but the sensitivity would be less stable with temperature
and when substituting different phototransistors of the same type, As the
other poster suggested, you can measure the voltage across the resistor
with a digital multimeter, and this will indicate when current is flowing.
By using a high resistor value, the circuit will be more sensitive. You
then need to use some kind of comparator with a high input impedance, you
could use a 741, but the standby power consumption would be lower if you
use a CMOS inverter chip line CD4069, CD40106, 74C14 etc.

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