Laser vs phototransistor?

[Don A. Gilmore]

Hi kids:

I'm trying to sense fairly fast (about 5000 Hz) pulses from a laser beam and
keep it cheap for a comsumer-type application.

The commonly-available, cheap lasers all seem to be visible red (650 nm or
so). This is fine except it seems that phototransistors are all infrared.
There are infrared lasers, but they are all very expensive.

Is there a [cheap] source of visible-light phototransistors? Are there
other types of sensors that I could use?

Thanks for all replies.

Don

 

[Fred]

Hi kids:

I'm trying to sense fairly fast (about 5000 Hz) pulses from a laser beam and
keep it cheap for a comsumer-type application.

The commonly-available, cheap lasers all seem to be visible red (650 nm or
so). This is fine except it seems that phototransistors are all infrared.
There are infrared lasers, but they are all very expensive.

Is there a [cheap] source of visible-light phototransistors? Are there
other types of sensors that I could use?

Thanks for all replies.

Don

Don't "IR" phototransistors work with visible light or do they all come with
an integral IR filter?

 

[Don A. Gilmore]

Hi Fred:

Thanks for the reply.

Well, that's a good question. I would think that external light would
interfere with an IR sensor if it wasn't designed to exclusively see IR
light. But that's just a guess.

Don

Hi kids:

I'm trying to sense fairly fast (about 5000 Hz) pulses from a laser beam and
keep it cheap for a comsumer-type application.

The commonly-available, cheap lasers all seem to be visible red (650 nm or
so). This is fine except it seems that phototransistors are all infrared.
There are infrared lasers, but they are all very expensive.

Is there a [cheap] source of visible-light phototransistors? Are there
other types of sensors that I could use?

Thanks for all replies.

Don

Don't "IR" phototransistors work with visible light or do they all come with
an integral IR filter?

 

[Bill Sloman]

Hi kids:

I'm trying to sense fairly fast (about 5000 Hz) pulses from a laser beam and
keep it cheap for a comsumer-type application.

The commonly-available, cheap lasers all seem to be visible red (650 nm or
so). This is fine except it seems that phototransistors are all infrared.
There are infrared lasers, but they are all very expensive.

Is there a [cheap] source of visible-light phototransistors? Are there
other types of sensors that I could use?

Photo-transistors all show a maximum sensitivity - in mA per mW/cm^2 -
at about 850nm, but if you plot the sensitivity in terms of mA per
photon per second, the curve looks more or less flat from 850nm
through to the visible.

A few phototransistors are packaged black epoxy cases deliberately
intended to absorb visible light, but the rest should work fine.

The cheapest parts in the Farnell catalogue (in volume) are the
Infineon
SFH313-2 and SFH314-2

http://www.farnell.com/datasheets/3528.pdf

http://www.farnell.com/datasheets/3542.pdf

The latter has a wider acceptance angle. Both look as though they
should detect 650nm light with reasonable sensitivity, provided that
you stay away from the SFH313/4 FA parts which are packaged in black
epoxy.

Solid-state photo-diode detectors are available at slightly higher
prices. You get a much lower, but more stable, sensitivity in mA per
photon per second.

Avalanche photodiodes and photomultiplier tubes are much more
expensive, and photo-resistors are too slow to be much use at 5kHz.

The collector-base capacitance in a photo-transistor may be
embarassingly high, and you may need to use it in cascode to see your
5kHz if your light intensities are low enough that you need a big load
resistor. Infineon only specify the collector-emitter capacitance
(15pF at zero bias) which may not be the same thing.

 

[Geoffrey G. Rochat]

Is there a [cheap] source of visible-light phototransistors? Are there

other types of sensors that I could use?

Some experimentation would be required, but try using an LED as a
photo-sensor. Forrest Mims used LEDs this way in a few Radio Electronics
and Scientific American projects; one project I remember was a
bi-directional optical communicator that used one LED on each end for
(non-simultaneous) transmission and reception of audio, not all that far
from your frequency of interest. LEDs can be used in photovoltaic mode with
voltage amplifiers, and in photoconductive mode with transimpedance
amplifiers.

 

[John Larkin]

Hi kids:

I'm trying to sense fairly fast (about 5000 Hz) pulses from a laser beam and
keep it cheap for a comsumer-type application.

The commonly-available, cheap lasers all seem to be visible red (650 nm or
so). This is fine except it seems that phototransistors are all infrared.
There are infrared lasers, but they are all very expensive.

Is there a [cheap] source of visible-light phototransistors? Are there
other types of sensors that I could use?

Thanks for all replies.

Don

Unfiltered silicon photosensors are good in the visible. A narrowband
filter near your laser color will improve s/n, as will a
constant-voltage drive (to avoid saturation from ambient light) and of
course a tuned amp. What's the pulse width of the laser?

John

 

[Ian Stirling]

Photo-transistors all show a maximum sensitivity - in mA per mW/cm^2 -
at about 850nm, but if you plot the sensitivity in terms of mA per
photon per second, the curve looks more or less flat from 850nm
through to the visible.

If you plot sensitivity of bare silicon detectors, then you get a pretty
good slope inversely proportional to wavelength.
At 425nm, (violet) it's half of what it is at 850.
Much below that, most of the light tends to get absorbed before it gets
to the sensitive bits.