Wanted: Simple Sensitive Light Detector Circuit

[Radio Man]

I need a sensitive light detector that i can place in front of the eyepiece
of
a binocular. This detector should sound a "beep" whenever it detects light.
I would also like it to be powered by 6 vdc or less.

 

[Ken Smith]

I need a sensitive light detector that i can place in front of the eyepiece
of
a binocular. This detector should sound a "beep" whenever it detects light.
I would also like it to be powered by 6 vdc or less.

Unless you super cool the binoculars, you could just wire the battery
straight to the buzzer. There will always be some infrared light.

What color light?
How bright?
Is it a focused image or a wide area you care about?
Do you only care about a change in light level?

 

[Radio Man]

My interest is detecting "Starlight"

 

[Jim Miller]

a very bright star such as vega well focused on a silicon photodetector only
produces a few picoamps of signal.

not an easy task...

jtm


My interest is detecting "Starlight"

 

[Ken Smith]

My interest is detecting "Starlight"

Is it starlight as in you are pointing it at a star?
Are you looking for a variable star?
Is it one specific star at a time or a general average?

The light detector you want sounds quite simple.

Chances are you will need some type of PIN diode detector, a
transimpedance amplifier and a comparitor.

A PIN diode can be had as a very small photovoltaic cell for quite a low
price. For more money, you can get a better one.

A transimpedance amplifier sounds scary but it isn't really. More on this
a bit later.

The comparitor can be a slight misuse of an op-amp.


Go look up the LM324 or TL074 op-amp. Many quad op-amps all share the
same pin-out. If you use a socket, you can change your mind about the
op-amp by just plugging a different one in.

Lets imagine we are going cheap. You can improve it later.
Lets also say we use the TL074.

First we need some voltages like this:

V(+)
!
\
/ 100K
\
/
!
+------!+\ U1A
! ! ----- V1
\ --!-/ !
/ ! !
\ -------
/ 5K
!
+------!+\ U1B
! ! ----- V2
\ --!-/ !
/ ! !
\ -------
/ 100K
!
V(-)


Now the transimpedance section:

0.1uF
-----!!-----
! 100K !
+----/\/\/---+
! !
----+--!-\ U1C !
! ! -------+-- Bright
--- V1-!+/
^ D1
!
V2

Note: The photodiode D1 has a very small back bias on it. If D1 is one
that is intended for photoconductive use, you can ground the end of it
instead.

If light hits D1 "Bright" will go to a voltage greater than V1. The more
light the higher the voltage.

You may want to raise the 100K to 1Meg to get a higher gain.

Now the comparitor:

10K
Bright ---/\/\/---!+\
! ---- BuzEn
V(+) -----!-/
! !
\ !
/ !
100K \ !
/ !
! !
/ !
10K \<---
10T / This is a 10 turn pot.
\
!
V1

When "bright" goes above where you've set the pot, BuzEn goes positive.
Now we just need to drive the buzzer. A power MOSFET is likely to be all
you need here.

 

[Ken Smith]

a very bright star such as vega well focused on a silicon photodetector only
produces a few picoamps of signal.

not an easy task...

I assume he is just after "there are stars up there".

If he is trying to detect just one star, matters are even worse than the
few picoamps you suggest. The collection area he has is a very small part
of a square meter.

If he is gathering from a significant fraction of the sky, he may be able
to do it with an op-amp like the TL074 so long as he is willing to adjust
out the bias error and keep the temperature fairly stable.

 

[Jim Miller]

my picoamp number was one calculated recently for vega and a 75mm collection
area, 4pa actually for a detector matched to the visual response curve
(approximately.)

vega is a magnitude 0 star and there aren't many of those around. much more
visible are second magnitude stars like polaris. each magnitude is a factor
of ~2.5 dimmer than the previous.

i think all he is going to detect with a wider field of view is light
polution rather than stars.

jtm

 

[Winfield Hill]

Ken Smith wrote...

Is it starlight as in you are pointing it at a star?
Are you looking for a variable star?
Is it one specific star at a time or a general average?

The light detector you want sounds quite simple.

Chances are you will need some type of PIN diode detector, a
transimpedance amplifier and a comparitor.

[ snip rest of interesting post with schematics ]

Radio Man, chances are you'll need a detector that's much more
sensitive than a PIN diode, such as either a photo-multiplier
tube (PMT) or a low-dark-count avalanche photodiode (APD).

APDs are very expensive, with their specialized power supplies.
http://optoelectronics.perkinelmer.com/Service/WhitePapers.html

Photo-multiplier tubes are also potentially expensive, but you
can get them for low prices on eBay. You'll also need a high-
voltage power supply for the PMT, or you can make it yourself.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3844055848
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3843377888
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3844421285

In either case you may want to explore photon counting, rather
than current measurement, as a way to capture the signal.

 

[Rich Grise]

my picoamp number was one calculated recently for vega and a 75mm
collection area, 4pa actually for a detector matched to the visual
response curve (approximately.)

vega is a magnitude 0 star and there aren't many of those around. much
more visible are second magnitude stars like polaris. each magnitude is a
factor of ~2.5 dimmer than the previous.

i think all he is going to detect with a wider field of view is light
polution rather than stars.

I agree. I live about 50 miles from downtown LA, CA, and even when
visibility is unlimited, I can see about a dozen stars, and it usually
turns out that 3 or 4 of them are airplanes. You have to drive about another
50 miles to get a good night sky. Or maybe on top of Mt. Wilson or
something.

Cheers!
Rich

 

[Steven Swift]

One of the most sensitive (but still cheap) circuits is to put a
phototransistor as one-half of a differential long-tailed pair. With
an op-amp as bias stabiliziation, you can build an extremely
sensitive circuit.

 

[Winfield Hill]

Steven Swift wrote...

One of the most sensitive (but still cheap) circuits is to put a
phototransistor as one-half of a differential long-tailed pair.
With an op-amp as bias stabiliziation, you can build an extremely
sensitive circuit.

That idea is so crazy, it just might work! :<)

 

[JeffM]

I live about 50 miles from downtown LA

You have to drive about another 50 miles to get a good night sky
Or maybe on top of Mt. Wilson or something.
Rich Grise

Yeah. To see Halley's Comet
most folks went below the cliffs of the Palos Verde Peninsula or to Big Bear.