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Photo-transistor floating baseline removal?

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CaleJo

CaleJo

Mechanical
Aug 29, 2009
10
I am trying to design an analog trigger using a photo-transistor. I started by placing the sensor in a Wheatstone bridge to get a zero volt baseline. This works great if the lighting conditions don't change. However, the lighting does change and the baseline moves.

I attempted to use op amps in a proportional feedback loop to stabilize the baseline to no avail. Essentially, I need the circuit to act as a high pass filter with a stable, zero volt baseline.

My questions: Are there any basic solutions to stabilize this base line using op amps? Is a Wheatstone bridge a good idea in this scenario? Should I consider a different approach?

Thanks
 
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If you want to detect sudden changes and ignore slow changes, then you could LP filter the signal to create a slowly changing reference. Then compare the original unfiltered signal to this reference. If the signal suddenly jumps, then delta will be large. If the signal changes slowly, then reference will keep up and the delta will be small.

Another approach would be to find a schematic diagram that matches your requirement. It's not likely that anyone anywhere could find a new and unique application for phototransistors.
 
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I'm with VE1BLL.

I have never heard of the method you are attempting because any ambient light change or temperature change will mess things up.

You usually want a fairly fast average to compare with the instantaneous to generate a pulse.

What are you trying to do?

Keith Cress
kcress -
 
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VE1BLL: I think I get what you said. Check out my schematic. It is based on your comment. I compare the signals to each other and have them cancel each other out until a fast signal come through. This is where the LP filter comes in.

itsmoked: I am creating an outdoor camera trigger.
 
 http://files.engineering.com/getfile.aspx?folder=285dd862-32a1-4153-95cb-ae36ea9489dc&file=Baseline_Removal.jpg
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You may be able to stabilize the baseline without the op-amps.

Add a base-emitter resistor, to drain the stored charge (from the photons) off the base of the phototransistor.



Mike Halloran
Pembroke Pines, FL, USA
 
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I am using a 2-lead photo-transistor. The base in this case is the photo sensitive surface.
 
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Why do you suppose 3-leaded packages are available for many phototransistors?


Mike Halloran
Pembroke Pines, FL, USA
 
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It's needed to inject more current into the base when the photon's aren't enough.
 
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I tested the circuit I posted earlier and was able to get it to work. Thanks VE1BLL and all others who posted. Your help is appreciated.
 
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