IR Question

[vbSteve]

Hi,

I'm new to electronics, well new, I mean I know the basic theory of electricity. You guys don't mind if I ask noob questions here not? I'm actually better at coding in Visual C++ and Visual Basic :)

I'd like to know what influence an IR beam has on a photodiode, a diode with 3 pins.

I'd like to light a green LED whenever the IR receiver receives. I have other intentions with it, but I guess this is the simple way of explaining.

Eventually it will be used in a computer controlled doorlock, where when the IR beam is established, it returns "1" (= "closed/connected") to the parallel port.

So what do I need to do in order to light the LED. Nothing to fancy stuff please. I search the net, but there is just so little information about it (Or maybe i was looking at the wrong sites)


Here is image that might help:



Thanks in advance.

[Sukhbinder]

Hi vbSteve,
                  wellcome to the world of electronics, its not too difficult to learn electronics, if u know some basic hardware concepts well then with ur current knowldge of C++ and VB things will be piece of cake. back to ur query, u said that u have a three terminal photodiode, well in ur case u must be refering to a photo transistor as a diode (di= 2) is a 2 terminal device. a photodiode is used in the reverse bias mode and when IR light falls on the junction minority carriers are generated which cause the diode current for detection. a photodiode will work in any visible wavelength also but the case is made such that it filters out all radiation except IR ( this prevents the device from being activated in ambient light). u are using a phototransistor which is like a normal transistir except for the case that it is switches on by the light and not the base current (u may have a little base bias to allow for proper switching. u can connect the green led between  the phototransistor  emitter and ground. and remember to connect the phototransistor collector to the positive supply through a current limit resistor. when the IR falls on the phottransistor it will switch on and start conducting and the LED will glow. ;)

[vbSteve]

hmm, sounds complicated.

So I have to treat it as a transistor? How many Ohms does the resistor need to be?

And how does a receiver with 2 pens work? like a simple switch?


Ah, sorry,  just experimenting,see whats the easiest ;)

[Sukhbinder]

use the equation R=(Vcc-Vled)/Iled. Vcc is ur supply voltage, Vled is the led forward voltage drop (usually 2-2.2 volts for green leds), Iled is the green led current u need (check up its data sheet for the recommended value, usually normal leds work at 2-5mA and high efficiency leds can work at 10-20mA). i suggest that u stick to the phototransistor as its easier for u to use. a photodiode is connected in reverse bias with a limiting resistor, it works best with a pulsed signal but it cannot give enough current to drive ur LED. a photodiode must be used with a buffer circuit to boost the current (or u may say that the diode is the sensor but the transistor can act as a switch). the advantage of a transistor is its higher current switching and higher sensitivity but it has a lower bandwidth (upto a few hundred KHz) while a diode has a very low current but has a very high bandwidth (few MHz). u can also use the IR detectors used in the TV remotes like the TSOP1738, u just need to connect to Vcc and ground and the 3rd pin for the o/p goes low when it detects an IR signal. u can connect a transistor to invet its output and drive the LED. u'll need a pulsed signal at about 38Khz for the transmitter. here is the data sheet for it check out its detection frequency range

TSOP1738.pdf

[vbSteve]

Ah of cource, the Ohms Law, how could I forget that! :P
Well I guess I will be using the phototransistor then :)

Another thing, when I bundle the IR beam (using a lens or something) , is it possible to increase the range between the IR LED and the diode? Probably not relevant to my project now, but it's good to know :)

[Sukhbinder]

generally all IR diode packages are made such that the field of emission is limited, a lens may help collimate the beam but won't improve the range much. to improve the range u'll need to use two IR diodes. ( as seen in some older TV remotes, it improved the range and also the field of emission so the remote could be used from different angles from the detector) ;D

[vbSteve]

Cool it worked! Thanks!  ;D