I am working on a project that uses a LED as a light source. I read that I can use another LED as a light sensor/detector but will probably need some amplification.
Here my question: there is something called "current to voltage converter" which involves an op-amp.
Is this electronic component something that takes an input current, like the one generated by the LED sensor, and produces an amplified corresponding voltage signal?
But doesn't a simple resistor convert a current into a voltage? Maybe the resistor does not provide amplification.
I guess most sensors produce a current signal output. Why couldn't we use a current amplifying circuit instead of a current to voltage converter? Is there such a thing?
But doesn't a simple resistor convert a current into a voltage? Maybe the resistor does not provide amplification.
Yes a resistor "converts" current to voltage. I assume "transconductance" or "transimpedance" amplifier works better than a resistor followed by a regular voltage amplifier.
I read that I can use another LED as a light sensor/detector but will probably need some amplification.
I've read that too, but I'd be very surprised if that works as well as a photodiode, phototransistor, or a photoresistor.
Why couldn't we use a current amplifying circuit instead of a current to voltage converter?
It's generally easier to measure/detect voltage. If you have a current source, you'd probably end-up running it through a resistor to "convert" it to voltage anyway.
Fused:
I read that I can use another LED as a light sensor/detector but will probably need some amplification.
Yup! And the reason for using an LED, as opposed to such things as a PhotoTransistor, a LDR, a PhotoCell, etc. IS: An LED will respond to the color it emits [only true for LEDs that emit a direct pure color -- as opposed to LEDs that emit a secondary color, like "white" LEDs that are really Blue LEDs that shine through Yellow phosphor -- also, most Magenta or Pink LEDs are really a combination of a Red LED and a Blue LED, and a Cyan, or Aqua LED is also, likely, not a pure color LED]. BUT, you probably only need a Red, a Green, and a Blue, to make a sensor that can cover the full spectrum [maybe a 406nm UV LED will work to extend the top end, if needed].
Fused:
But doesn't a simple resistor convert a current into a voltage? Maybe the resistor does not provide amplification.
I guess most sensors produce a current signal output. Why couldn't we use a current amplifying circuit instead of a current to voltage converter? Is there such a thing?
Yes, a resistor is a current to voltage converter, and yes, for your case you also need amplification [which a resistor doesn't provide].
Check out this website for more: Color Sensor from a Reversed LED and Op Amp - Robot Room
If I decide not to use a LED as a detector, I would have to choose between a photodiode, a phototransistor, or a photoresistor. Does each one of the three mentioned components have a chief advantage? I am familiar with the photoresistor which is simply a light dependent resistance.
Using an LED would be a good choice if the detector must precisely respond only to light of the same wavelength as the light it emits...
I would like to better understand DVDdoug comment:
"...I assume "transconductance" or "transimpedance" amplifier works better than a resistor followed by a regular voltage amplifier..."
What are "transconductance" or "transimpedance" amplifiers? Can they be purchased as a single component or do they need to be built from other parts?
Photoresistor - very slow, very variable between devices, temperature sensitive.
Photodiode - very fast, some variation between devices, no appreciable temperature sensitivity.
Phototransistor - intermediate - bit slow, more variable, more temp sensitive - but have more gain,
very common in opto isolators simply because of the gain.
Your LED is a photodiode already, because any diode is a photodiode if light can get to it...
Photodiodes are not selective in wavelength so long as the wavelength is short enough, but a
coloured plastic housing of an LED can act as a filter. For instance silicon photodiodes react from infra-red
upto blue and beyond - they do reduce in sensitivity across the spectrum and are most sensitive in red,
but they still respond to all colours.
Specific light sensor chips are available, which include one or more photodiodes (often with
colour filtering), and sense amplifiers and ADC and digital output. Perhaps one of these light
measurement chips is more what you want?