Author Topic: Photoresistors vs Phototransistors vs Photodiodes!  (Read 15103 times)

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Offline blackheartTopic starter

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Photoresistors vs Phototransistors vs Photodiodes!
« on: March 13, 2009, 10:15:38 AM »
Hello! I am a little confused about the 3 types of light detection parts.

What is the difference between them? Can they be used the same way?
I'm out of ideas!

Offline SmAsH

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #1 on: March 13, 2009, 12:57:10 PM »
i dont know what photodiodes and phototransistors do but i can tell you that photoresistors higher or lower their resistance according to the amount on light falling on them.
try reading this for help: http://www.societyofrobots.com/schematics_photoresistor.shtml
Howdy

Offline pomprocker

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #2 on: March 13, 2009, 12:58:02 PM »
First learn how resistors, transistors, and diodes work ;)

Offline blackheartTopic starter

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #3 on: March 13, 2009, 05:24:59 PM »
I deserved that! I should probably learn.
I'm out of ideas!

Offline Soeren

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #4 on: March 14, 2009, 07:08:36 PM »
Hi,

What is the difference between them?
Resistors are just a (light controlled) variable resistance.
Transistors are like regular transistors, but instead of by base current, it's controlled by photo current (letting more or less current through the collector-emitter according to incident light.
Diodes can be used in either photo voltaic mode, acting like a tiny solar cell, or in photo conductive mode, where it allows more or less current through it according to the amount of light hitting it.


Can they be used the same way?
No, then there would be no reason to have all three of them.
That said, the transistor can be made to mimic the diode by shunting the base to emitter (although it will not be quite as fast as a diode).

Speed of response and sensitivity:
Resistors: Slow and medium sensitivity.
Transistors: Relatively fast (comes in different speeds) and high sensitivity.
Diodes: Fast to ultra fast (pin photo diodes are the fastest responding) and not quite as sensitive.
And then there is the photon multiplier valve if you got a lot of spare money: Faster and more sensitive than the rest.
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Charizard

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #5 on: March 20, 2009, 08:17:31 AM »
It takes me to a question, if i want to calculate resistor needed for photodiode, can I use the same method as photoresistor as described in the tutorial?
 sqrt (resistance of brightest * resistance of darkest)

Offline Soeren

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #6 on: March 20, 2009, 08:28:11 PM »
Hi,

The formula: Squareroot(dark resistance * full light resistance) gives you the "percentile midpoint", which is the value where there is the same percentile distance to either of the extreme values.

This works aces for anything with a linearly change, since it gives the highest tolerance to errors.

A phototransistor (or -diode) does not give a linear change however, so the easiest way of finding the (close to) optimum resistance here is, to set the amount of light that you want as the approximate shifting point and then make a potential divider with the transistor and a variable resistance (preferably a multiturn trimmer/potentiometer) and dial in the value until the shifting point is reached.
The potentiometer can then be replaced, with a regular resistor of the same value as the potentiometer (carefully detach the "pot" and measure the resistance without disturbing its adjustment). This only counts when using the photodevice in photo-conductive mode.
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Charizard

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #7 on: March 21, 2009, 01:40:53 PM »
Sorry for my weak understanding. So, it is about the same idea as Photoresistor Voltage Divider Circuits shown in the photosensor tutorial right? However, one thing i don't really understand is that the Vin I use is 5volt from the power supply and Vout should be 5volt to the PIC right? Or how much should i expecting my Vout? So, based on the formula (R*Vin)/(R+Rphoto) = Vout, my R will be zero?
« Last Edit: March 21, 2009, 01:47:28 PM by Charizard »

Offline Soeren

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #8 on: March 21, 2009, 02:29:44 PM »
Hi,

So, it is about the same idea as Photoresistor Voltage Divider Circuits shown in the photosensor tutorial right?
Link to the tutorial you're talking about is...?
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Charizard

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Offline Soeren

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #10 on: March 22, 2009, 08:59:33 AM »
Hi,

the Vin I use is 5volt from the power supply and Vout should be 5volt to the PIC right? Or how much should i expecting my Vout? So, based on the formula (R*Vin)/(R+Rphoto) = Vout, my R will be zero?
U_out will be a varying value depending on the amount of incoming light and R definitely shouldn't be 0 Ohm!

Assuming:
U_in = 5V
R=10 kOhm
R_photo in max. darkness = 1MOhm
R_photo in max. light = 100 Ohm
(arbitrary values that might be somewhat different in what you have, but not too far off I assume (have you measured your photoresistor alone with an Ohmmeter in light as well as darkness?).

Uo_Dark = 5V * 10 kOhm / (1MOhm + 10 kOhm) = 49.5 mV
Uo_Light = 5V * 10 kOhm / (100 Ohm + 10 kOhm) = 4.95V

The point that is the best midpoint (i.e. where there is takes an equal percentile change to get to either extreme) voltage vise is then:
sqr(4.95*0.0495) = 495mV

This is the voltage that you should use (in this example) as the detection point between light and darkness, as it will give you the largest tolerance against errors, since it has to vary 10 times to either side to reach an extreme.

If you had used eg. 2.5V (which might seem as a good midpoint for the uninitiated), it would only have to sway 2 times up and 50 times down to reach the extremes.

The effect is that the former solution has a 5 times better error protection.

Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Charizard

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #11 on: March 22, 2009, 10:31:25 AM »
so, can i say that actually the photodiode works with any value of resistor. Just that choosing the best resistor that works with it reduces error when ADC taking the reading?

Offline Soeren

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Re: Photoresistors vs Phototransistors vs Photodiodes!
« Reply #12 on: March 22, 2009, 01:43:08 PM »
It has nothing to do with precision when using an ADC - only when using a single point to shift between light or not.
Of course you cannot use "any value" and you could have found that out yourself, by jotting in hypothetical values like 0 Ohm and 1,000,000,000,000  (1 TOhm) and recalculate.

Perhaps you need to spend additional time on Ohms law and very basic electronics concepts, until you have acquired a certain level of understanding, before taking on the slightly advanced.

I have never understood why people start out with Ácontrollers without really knowing the first thing in basic electronics - a Ácontroller may seem easy on paper, but there's a host of analog issues you have to deal with in the real world.
Too many people just go blank when their Ácontroller doesn't work in the first go. A little analog knowledge would probably reduce that amount.

If you don't wanna Google around and try to find solutions to your problems (a good way to learn, since you get more info than you need at the very moment), at least describe what you wanna accomplish when you are throwing your questions out into a forum - anything short of that shows little respect for those that is supposed to help you and is a waste of other peoples time.
Regards,
S°ren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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