Aha...oh...huh?

[Private Reid]

Hey

I found these photoresistors of ebay although i hate ebay it was my last resort due to the fact that nooooooo shops or stores in Australia have any! Well i found these on ebay but it comes with phototransistors as well? what are they? can I use them?

http://cgi.ebay.com.au/Lot-of-10x-photoresistors-and-10x-phototransistors_W0QQitemZ350040044264QQihZ022QQcategoryZ36338QQrdZ1QQssPageNameZWD2VQQcmdZViewItem?_trksid=p1638.m122

[airman00]

you can use the phototransistors

http://encyclobeamia.solarbotics.net/articles/phototransistor.html

but be warned

that seller lives in China , so it will take about a week or two(maybe even three) to receive it

[Private Reid]

thanks...I had actually forgot that the seller lives in china...hmm that may be a problem...

Next question...

how would I wire up a phototransitors?
Would i just wire it up like a photoresistor?

[ed1380]

like a transistor.
like a resistor

[Private Reid]

How am I suppost to wire a phototransistor up like a transistor?!?

transistors have three legs! a phototransistor only has two?

and is a phototransistor just another word for IR?

[Trumpkin]

is a phototransistor just another word for IR?

Nope. How could you hate E bay, lol?

[airman00]

How am I suppost to wire a phototransistor up like a transistor?!?

transistors have three legs! a phototransistor only has two?

and is a phototransistor just another word for IR?

maybe search google 

or read the link i sent in my first reply
http://encyclobeamia.solarbotics.net/articles/phototransistor.html

phototransistor is not IR. IR is infrared - search google for that as well , ill give you a hint infrared is a type of light

[ed1380]

How am I suppost to wire a phototransistor up like a transistor?!?

transistors have three legs! a phototransistor only has two?

and is a phototransistor just another word for IR?

ir=infared
a transistor has 3 legs because one is the signal when to let current pass from leg 1 to leg 3. well the phototransistor lets electricity pass when it senses light. the more light the more passes.

on a photoresistor. the more light the less resistance

edit- darn you air. beat me by 50 seconds

[Private Reid]

when I google imaged phototransistors it had a picture of this little black thing (great description!) and it looked exactly like this thing i found in a VCR is it also a phototransistor?

pictures found at this url:
http://no1name.zoomshare.com/

[airman00]

when I google imaged phototransistors it had a picture of this little black thing (great description!) and it looked exactly like this thing i found in a VCR is it also a phototransistor?

pictures found at this url:
http://no1name.zoomshare.com/

yep

surprise surprise  TV remotes use IR to communicate with the TV!

[paulstreats]

to carry on what ed was saying:

a transistor has three legs. A collector , a base, and an emitter.
the load is usually connected to the collector (ie Vin +5v).
another input is also connected to the base(ie Vin +1v). When this input is switched on, current will flow between the collector and the emitter.
Transistors are deemed always on because if you send a small input(0.2v) to the base, you get a small output out of the collector(1v). If you have a medium input to the base (0.5v), you get a medium output(2.5v).
The datasheets explain what the tolerances are.

Phototransistors work in the same way. They only have 2 legs usually. 1 is the collector and 1 is the emitter.(its best to get a datasheet to find out which one is which so you dont risk damaging it with reverse current). the input at the base comes from the led itself so when medium light hits the led it allows a medium output from the emitter(2.5v). When very bright light is shone on the led it allows a large output(5v)

This works because led's also generate a very small amount of electricity when light shines on them. Its enough to run through the base of the transistor and turn it on(it has to be the same colored light as what the led is made for so an ir phototransistor will work only with ir light)

[Private Reid]

OK...

so my little black thingy is also a phototransistors so I could wire it to my robot along with my photoresistors!

SPLENDID

i think ill mess around with the wiring. first ill plug it in like the photoresistor and see what happens

[Private Reid]

This works because led's also generate a very small amount of electricity when light shines on them. Its enough to run through the base of the transistor and turn it on(it has to be the same colored light as what the led is made for so an ir phototransistor will work only with ir light)

Ok so that is why they were in the VCR. cause the remote produces infared and the phototransistor collects it and plays your movie or whatever.

Correct?

So could i use the IR in the remote for obstacle avoidance?

Aswell as my photortransistors and photoresistors

[superchiku]

no photoresistors in australia ... how come this is impossible ... australia is an advanced country and it doesnt have photoresistors?

[ScorpIon]

Maybe photoresistors are obsolete in Australia? :p

[Soeren]

Hi,

no photoresistors in australia ...

Plenty, you just have to search for the right term. Here is one example: http://www.scorpiotechnology.com.au/technologypricelist.shtml#ldr

Phototransistors do have 3 "pins", but sometimes the base is removed, since some people only use them in their simplest configuration. When the base terminal is brought out, the response time and sensitivity can be adjusted.

[Soeren]

Transistors are deemed always on because if you send a small input(0.2v) to the base, you get a small output out of the collector(1v). If you have a medium input to the base (0.5v), you get a medium output(2.5v).

Complete nonsense, I thought you had better knowledge of (bjt-) transistors?

the input at the base comes from the led itself so when medium light hits the led it allows a medium output from the emitter(2.5v). When very bright light is shone on the led it allows a large output(5v)

This works because led's also generate a very small amount of electricity when light shines on them. Its enough to run through the base of the transistor and turn it on(it has to be the same colored light as what the led is made for so an ir phototransistor will work only with ir light)

Wrong. Phototransistors are made with the base junction exposed to light, making it light sensitive. The LED-like encapsulation is just to make them easy to use and they can be made for various angles of incident light pretty cheaply.

All transistors are light sensitive and that's why they are encapsulated in a light proof housing. Any old metal house transistor can be made into a photo transistor by carefully cutting the top off - used to do this decades ago, as photo transistors were quite expensive.

[paulstreats]

Quote from: paulstreats on Today at 01:45:09 AM
Transistors are deemed always on because if you send a small input(0.2v) to the base, you get a small output out of the collector(1v). If you have a medium input to the base (0.5v), you get a medium output(2.5v).

Quote from: Soeren
Complete nonsense, I thought you had better knowledge of (bjt-) transistors?

Quote from wikipedia
NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter.

So provided there is some positive signal on the base (bringing it higher than the grounded emitter), the transistor is always on (the voltages example above is true for 1 particular type of npn transistor but will obviously change with the type of transistor that you have).

Wrong. Phototransistors are made with the base junction exposed to light

True, but the makeup is exactly the same as that of an ir detector led. When you consider that a transistor is made of the same thing and in the same way as diodes, its not surprising to find that the part exposed to light is an ir led and a base junction of a transistor at the same time.

All the same, I usually forget how to use transistors properly. If I havent used them for a couple of months, I usually have to read the basics through again. They always catch me out

[Soeren]

Hi,

Quote from wikipedia
NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter.

So provided there is some positive signal on the base (bringing it higher than the grounded emitter), the transistor is always on (the voltages example above is true for 1 particular type of npn transistor but will obviously change with the type of transistor that you have).

When corrected, you can either learn from the experience or try to defend the wrong assumption and stay ignorant - your choice.

Even though Wikipedia is often wrong (hey, anybody can write articles there - don't take it as Gospel), they are correct in this quote. You postulate something entirely different.

A BjT is a current amplifier - a small current through the base-emitter controls a larger current through the collector-emitter.
If you just input a current to the base-emitter and measure the voltage of the collector-emitter, you will see absolutely 0V, since it just opens more or less, enabling it to pass a current - and you don't "send n.n volt to the base" but rather some magnitude of current and you don't get any voltage output either - current in, current out, nothing more to it.

Wrong. Phototransistors are made with the base junction exposed to light

True, but the makeup is exactly the same as that of an ir detector led. When you consider that a transistor is made of the same thing and in the same way as diodes, its not surprising to find that the part exposed to light is an ir led and a base junction of a transistor at the same time.

Well, I for one would be very surprised if I found an IR LED inside the base junction of a photo transistor.

The light sensitive base of a photo transistor is NOT an IR LED, neither separate, nor as part of it and they are NOT made in the same way. an analogy would be to postulate that a drill bit and an anvil is the same, since they both contain iron.
Where do you pick up such stuff? Seems you even confuse emitters with detectors LED is an acronym for Light Emitting Diode and can not be used to describe a detector.

You could invent whatever model for your own struggle to understand something, but giving advice to others, you really should check your facts, rather than just second guessing and expediting incorrect assumptions - or you are going to be spreading misinformation.

All the same, I usually forget how to use transistors properly. If I havent used them for a couple of months, I usually have to read the basics through again. They always catch me out

All that more of a reason to also reread the basics before giving advice to others, since someone may actually base their further learning on such info and this person may later on pass that info on - how many persons will be affected by a single piece of wrong advice is hard to say, but if you wanna spread info, please stick to what you know or at least tell when you are just guessing.


This is not meant as a personal attack, but rather as a call to rigid correctness in the info contributed by anyone attempting to educate (or at least to adding a disclaimer when guessing or lacking the exact knowledge))
I personally don't give advice on neither Cancer treatment, nor sweater knitting, since I only know enough of either to really foul things up

[paulstreats]

I hate to say this, but after researching, i can confirm that an ir phototransistor is infact made up of a transistor and a Gaas photodiode(without which it couldnt filter infrared from).

BJT transistors are commonly referred to as being nothing but 3 common diodes. The internal make up is the same as 3 common diodes polarised correctly http://ourworld.compuserve.com/homepages/Andrew_Wylie/homemade.HTM- you could even use an led as one of these diodes. Diodes are photosensitive, just led's are more geared towards producing light of 1 particular wave length.

I havent confused led's with detectors. Leds are quite commonly used to detect light(being made of photosensitive material), and quite successfully aswell they are used in many colour detecting circuits there is even a tutorial about it on this site.

I do admit to being wrong about the voltage controlled part of the transistor (which I wholeheartedly apologise about ), it being the current that alters not the voltage which I do apologise about, though it is voltage controlled as is described in a lot of detail here http://amasci.com/amateur/transis.html basically explaining why it is the base voltage that controls the charge. So we were both wrong really .
I usually use them in conjunction with a voltage dividing resistor to provide the varying voltage for me

[Webbot]

Look at http://www.jaycar.com.au and search on 'resistors' to find some light dependent resistors (LDR).

[Admin]

Sometimes I'm too lazy to double check information . . . and when I am, I usually write:

"I think X is true, but I'm too lazy to look it up so you should double check to be sure"

That way if it turns out you are wrong, you have worming room

Whats great about a forum is there are many people checking for incorrect info!