Infra red receivers

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi, i got a bunch of infra red receivers (photo resistors) that has a minimum resistance of ~270 ohm and maximum not too sure about the value but it showed me 9.5 on the multimeter when the dial was at 200k.

I made a little test program that reads analog values from the analog pin which is connected to the ground and to the v3. there is a resistor of 100k ohm that connects to the v3 and the photo resistor is connected to the ground.

my program displays the values it receives from the analog pin through the serial port and if a value is low than usual (means the ir led is seeing another ir-emitting led its resistance drops) it stops showing the numbers.
so far so good, but the numbers stop showing only when the ir-emitting led is around like 10 inches. is there a way of increasing that distance?

thanks very much.

2

is there a way of increasing that distance

  1. Increase the light output of the emitter by reducing the resistor to get the maximum current you can pass through it.

  2. Use two or more IR LEDs to get more light.

  3. Amplify the signal of the sensor by using an operational amplifier.

  4. Use a sensor that is more sensitive than a photo resistor, like a photo diode or photo transistor and follow it with an amplifier.

3

i actually have a photodiode, how is this different than a photo resistor? (except how it looks) and can i use the methods you gave for a photodiode too?

thanks

4

i actually have a photodiode, how is this different than a photo resistor?

All the above techniques (except 4 obviously) will work. A photo resistor uses a bulk material and the photo electric effect to liberate electrons for conduction, one photon one electron. Where as a photo diode uses a NP silicon junction and converts every photon it captures into current, an electron and hole are created for each photon. Therefore it is more sensitive. Also a photo resistor is slow in responding (about 100mS) where as a photo diode is much faster (5uS or so).

5

whats the difference between the operational amplifier than a transistor? they both can amplify the current

6

An operational amplifier contains about 50 transistors designed to provide an idealised configurable differential input amplifier. The gain can be configured from zero to tens of thousand of times. With that complexity and flexibility comes some limitations. Most of the cheaper op-amps won't output the full rail swing being short of the rails by as much as 1.5v.

A transistor is just that, and it's up to you to design it to do what you want. For using it as a switch, which is what most embedded users do with it, is cheap and simple. They can also handle more current than an op-amp, but the gain is limited typically to 300 for a small signal transistor right down to 10 for a big power transistor.