Phototransistor sensitivity and frequency meter

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killertoffy
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Posted by killertoffy: Thu. Nov 15, 2007 - 10:56 PM
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I'm trying build a frequency meter to measure the frequency of displays.
So I need to measure frequencies up to 120Hz, no more.

I tried using a phototransistor ( http://www.tranzistoare.ro/datas... )
but sensitivity is too low to detect something...

Is there a design that can increase sensitivity ?
Can a photodiode be more sensitive than a phototransistor ?
I also tried to find a sensor with a peak more in the visual range, but all I found had peaks around infrared.
I also guess bigger sensors will be more sensitive...
But I need to find one...

Thanks again for you help.

I tried the following design :

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Learning and Information, General Electronics
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ka7ehk
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Posted by ka7ehk: Fri. Nov 16, 2007 - 01:07 AM
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Generally, photo transistor has better sensitivity than a basic photo-diode.

You can increase the sensitivity by increasing the collector resistor value. You can also connect the collector to +5V and the emitter to the base of another transistor. Then use THAT transisor as an open-collector input to the micro. This gives you beta more current sensitivity.

Be aware that photo-transistors are pretty slow. They should be fast enough for 120Hz, though.

Jim

 

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Klave
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Posted by Klave: Fri. Nov 16, 2007 - 02:10 AM
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Quote:

Is there a design that can increase sensitivity ?

It's called a lens. Gain depends on area and goes up with the radius squared. The lens is directional so it helps reduce the problems from ambient (fluorescent) lights which can be bad right up to 120Hz.

You also get a reduction in sensitivity from the ambient light but a shield should help here.

Is your circuit working properly at the moment? perhaps the light is too strong and you're saturating the transistor. Also, you may need to add another gain stage if you want to detect it with a micro.

GK

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ka7ehk
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Posted by ka7ehk: Fri. Nov 16, 2007 - 07:51 AM
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GK has a good point. What is the DC voltage on the collector of the transistor with the display off?

Jim

 

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killertoffy
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Posted by killertoffy: Fri. Nov 16, 2007 - 08:05 AM
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Specs says it is a "Very high photo sensitivity"... all is relative ;-)

Yes optical 'gain' is clearly the best solution... but the phototransistor already has a lens... and finding an adequate lens might prove being more challenging that finding a better phototransistor...
I won't have much problems with spill light.
Circuit is working ok, but require more light than a display can provide.
I already tried a second transistor to amplify the signal out, it makes the transition faster but doesn't really make it more sensitive outside the transition range.

I'll give a try with Jim's approach.

Thanks.

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killertoffy
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Posted by killertoffy: Fri. Nov 16, 2007 - 08:15 AM
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Well it works now ! :D :D :D :D :D :D :D

I connected a BC337 as you proposed, an a 22Kohm resistor at it's collector.
It can now pickup dim light even without pointing it at the screen or a flash light...
That's far better than I exepected !

Thanks again for you ideas and expertise.

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