Comparing Photodiode and Phototransistor characteristics

Thread Starter

Sensacell

Joined Jun 19, 2012
2,985
I am working on a product requiring a photo sensor to detect white light, I need the largest possible dynamic range (1000:1 light levels) at the minimum cost.

Response time: 200 us rise/fall
Photo-conductive operation- (needs to sink current in proportion to light levels)
Long term / temp stability not required, device will autocal for every measurement.
No need for good linearity, I am not building a measuring device, just a lights on/off detector, with a large range of potential light intensities.

Looking at the range of options, phototransistors seem to typically have a poor ratio of dark current to light current, but tend to be cheaper.

Photodiodes seem to have a better ratio of dark/light currents and are fast, but the parts are MUCH more expensive.

Are there any tricks to getting the wide dynamic range I seek? As cheap as possible?
 

Thread Starter

Sensacell

Joined Jun 19, 2012
2,985

takao21203

Joined Apr 28, 2012
3,702
I am working on a product requiring a photo sensor to detect white light, I need the largest possible dynamic range (1000:1 light levels) at the minimum cost.

Response time: 200 us rise/fall
Photo-conductive operation- (needs to sink current in proportion to light levels)
Long term / temp stability not required, device will autocal for every measurement.
No need for good linearity, I am not building a measuring device, just a lights on/off detector, with a large range of potential light intensities.

Looking at the range of options, phototransistors seem to typically have a poor ratio of dark current to light current, but tend to be cheaper.

Photodiodes seem to have a better ratio of dark/light currents and are fast, but the parts are MUCH more expensive.

Are there any tricks to getting the wide dynamic range I seek? As cheap as possible?
BPW34 is not that expensive
 

ian field

Joined Oct 27, 2012
6,536
I am working on a product requiring a photo sensor to detect white light, I need the largest possible dynamic range (1000:1 light levels) at the minimum cost.

Response time: 200 us rise/fall
Photo-conductive operation- (needs to sink current in proportion to light levels)
Long term / temp stability not required, device will autocal for every measurement.
No need for good linearity, I am not building a measuring device, just a lights on/off detector, with a large range of potential light intensities.

Looking at the range of options, phototransistors seem to typically have a poor ratio of dark current to light current, but tend to be cheaper.

Photodiodes seem to have a better ratio of dark/light currents and are fast, but the parts are MUCH more expensive.

Are there any tricks to getting the wide dynamic range I seek? As cheap as possible?
Photodiodes (can be) very fast but are not too sensitive - Phototransistors have gain which makes them more sensitive, but they're slower.

Cd-S LDRs are the easiest to interface, and are the closest match to the human eye response curve - but they're *REALLY* slow, and I'm not too sure about RoHS compliance (Cadmium is *VERY* toxic!).
 

Thread Starter

Sensacell

Joined Jun 19, 2012
2,985
Thanks all for your feedback, I will be experimenting with a bunch of sample parts, both photodiodes and transistors.
I have a feeling the photodiode is going to be the winner.
 

ronv

Joined Nov 12, 2008
3,770
You don't state the light levels, but even so you should be able to develop a signal from 10 mv to 10 volts with a photo diode easily enough.
 
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