D
Dacium
Does anyone know a basic phototransister circuit? I can't use an opamp
because I am looking for very low power consumption and quick power on/power
off and very small footprint size (which rules out IR modules etc.). The
circuit is to have its power turned on/off about 20 to 50 times per second,
and I would like the duty cycle of off/on to be around 5:1 or less, giving
only a few milliseconds to test for a carrier. I need a carrier because the
transmission range is about 1-2m in indirect sunlight.
I have a very high speed phototransister (around 3uS full response). The
circuit I can think of is something like this, however I don't understand
how to calculate the resistor values for the amplifing transister:
---------Vcc (5V)
| \
|c /R2
|< \
|e /
| |c inductor L1 Cap C1
| b +-----()()()()()----||----+------ Vout to uP comparator/ADC
+--------|< |
| |e |
/ | \
\R1 | / R3
/ | \
\ | /
/ | \
| | /
+---------+-------------------------+GND
The idea is the second transister amplifies the signal which is then put
through band pass filter. Obviouly the size of L1 C1 R3 is just calculated
for the correct frequency but what about R1 and R2, and do I also need a
resister to the base of the second transister and a resister from emitter to
ground?
The spec sheet for phototransistor says 0.03mA to 6mA Icc when the IR light
hits it. I believe for the phototransister to be in active mode then that
Vcc > R1*Icc. This means that R1 < 833Ohm at 5Vcc for the phototranster to
be in active mode and not saturate. Does this sound correct? I have tested
this on a CRO and it appears to give voltage across R1 changing with light
intensity without reaching any saturation voltage, but the magnitiude is
very small over distance greater than about 3cm from transmitter (IR diode
at powered at 25mA).
So to increase signal I need second transister also in active mode. How do I
do this? Thanks for anyhelp!
Also for the filter are the values critical over any range? Like I have some
47uH inducters handy so can I base the C1 and R3 values from this, or is
there some sort of range I should be sticking to? The carrier frequency is
likly to be 20-30KHz.
because I am looking for very low power consumption and quick power on/power
off and very small footprint size (which rules out IR modules etc.). The
circuit is to have its power turned on/off about 20 to 50 times per second,
and I would like the duty cycle of off/on to be around 5:1 or less, giving
only a few milliseconds to test for a carrier. I need a carrier because the
transmission range is about 1-2m in indirect sunlight.
I have a very high speed phototransister (around 3uS full response). The
circuit I can think of is something like this, however I don't understand
how to calculate the resistor values for the amplifing transister:
---------Vcc (5V)
| \
|c /R2
|< \
|e /
| |c inductor L1 Cap C1
| b +-----()()()()()----||----+------ Vout to uP comparator/ADC
+--------|< |
| |e |
/ | \
\R1 | / R3
/ | \
\ | /
/ | \
| | /
+---------+-------------------------+GND
The idea is the second transister amplifies the signal which is then put
through band pass filter. Obviouly the size of L1 C1 R3 is just calculated
for the correct frequency but what about R1 and R2, and do I also need a
resister to the base of the second transister and a resister from emitter to
ground?
The spec sheet for phototransistor says 0.03mA to 6mA Icc when the IR light
hits it. I believe for the phototransister to be in active mode then that
Vcc > R1*Icc. This means that R1 < 833Ohm at 5Vcc for the phototranster to
be in active mode and not saturate. Does this sound correct? I have tested
this on a CRO and it appears to give voltage across R1 changing with light
intensity without reaching any saturation voltage, but the magnitiude is
very small over distance greater than about 3cm from transmitter (IR diode
at powered at 25mA).
So to increase signal I need second transister also in active mode. How do I
do this? Thanks for anyhelp!
Also for the filter are the values critical over any range? Like I have some
47uH inducters handy so can I base the C1 and R3 values from this, or is
there some sort of range I should be sticking to? The carrier frequency is
likly to be 20-30KHz.
