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September 11, 2007

IR Detector

Besides my POV display, I'm trying to do a robot that detects obstacles using IR light. In order to make it easy to work with IR light, I built a very simple IR detector based on this article.

General Idea
An IR signal is "seen" by a reverse biased IR diode which produces an electrical signal according to the light intensity. This signal is then amplified trough a transistor which powers an ordinary LED. This means that the LED will light up when the IR diodes receives some IR light (from a TV remote for instance).

  • An IR Diode (I got mine from an old TV set)
  • An LED (a yellow one in my case)
  • A 1k Ohms resistor
  • A 50 Ohms resistor (two 100 Ohms resistors in parallel)
  • A BJT (I used the 2N3904, but any common BJT should work with the right biasing)
  • An old 9V battery (for the casing)
  • A new 9V battery (for power)
The Circuit

Construction Notes
I built the circuit by soldering the components together without using a PCB and enclosed it in an old 9V battery.

I used a battery as the casing because it is free, it looks cool, and it includes a battery clip.
The battery's cap is actually a battery clip that can be soldered to the circuit in order to connect it to another 9V battery.
When using a battery as a case, keep in mind that:
  • the inside of the battery should be insulated (with blue masking take in my case)
  • it is a good idea keep one or two of the AAAA batteries (that can be found inside the 9V battery) and use them to keep the top and bottom caps in place (make sure they are insulated two).


Unknown said...

Not sure how I would use this, but can you use this to make an IR Repeater? I purchsed a repeater from Radio Shack, but it is a peice o'crap. I was thinking of making something that would transmit/send through CAT5. Do you know how it wouldeb done?

Carlos said...

if your idea is to have an IR detector at one end of a cable and an IR transmitter at the other, then this circuit could certainly work.

But this circuit is set so to have a very low range (so it doesn't pick up ambient noise). In order to adjust it to a decent range you need to tweak the resistors, specially the lKOms one.

Finally if you want to transmit IR, simply replace the yellow LED with an IR LED.

About the cable, I think you could use pretty much any cable, since the IR signals for remotes are usually in the kHz range so the frequency response of the cable is not an issue.

Anonymous said...

Have you thought about keep some cells of the 9V batery, and use the to provide energy to the circuit?

Carlos said...

stendall, thats a very interesting idea! I never thought of that.

Doing so would require:
1- not using a dead battery (obviously).
2- modifying the bjt's biasing or maybe even replacing it altogether.
3- modifying the form factor of the circuit so it fits in the battery along with as many cells as possible.

Overall, it could be a bit difficult but definitely feasible.

Anonymous said...

Using an LM567 Tone Decoder is a better idea. With a small handful of parts you can have a modulated signal. I've built a few of these (Jameco apparently sells a kit, which fortunately has the schematic online), and they work great:


Anonymous said...

I'm a little confused. In the picture of the bread board circuit, the Clear LED appears to be indicating led and the Blue the IR, because of the way they are in the circuit. However in the last picture you show the blue led facing out as if it is the indicating LED. What am I missing here?

Carlos said...

I see why you're confused Anonymous. The blue LED is used to detect the IR light and the clear LED is a standard yellow LED.

However, in the last picture the LED sticking out from the top of the battery case seems to be blue. This is caused (I guess) by something blue on the environment reflecting on the clear LED.

In short, the LED coming out from the top of the battery case is the clear one but it looks blue for some strange reason.

Phil Winder said...

I often find it easier to just use a TSOP variety of IR demodulators. This is the one I use in the UK:

All it does it receive a 38kHz IR signal in, like that from a remote, and produce a a logic one output. Obviously much more noise resistant than a simple photodiode that react's to all sorts! (There are other frequencies available)


P.s. On a simple robot I i did a while ago, I did exactly the same thing you are about to. Take a look

Carlos said...

Thanks for the tip Phill. Now I finally know how those three-legged "IR diodes" work. I think I have some of those laying around so I'll give that a try for sure.

Anonymous said...

I was curious about the first picture, the one with the remote. What are the two prongs sticking out of the sides of the project battery?

Carlos said...

Anonymous, on the fist picture, I placed one of the AAAA batteries underneath the detector so it would be at the same height as the remote (which is thicker than most remotes)

Anonymous said...

Thankyou very much. Early today I started this project, I have my project battery ready. After a trip to Radio Shack tomorrow I'll be ready to finish it up. It's a very neat project.

Anonymous said...

dude i want to know.....wat will happen if i use a forward biased ir diode....cn u just explain me why not we use the forward biased case...sorry for the trouble

Carlos said...

The IR diode is basically an IR light. If you forward bias it, it will light up (of course you won't be able to see the light with the naked eye)

Anonymous said...

hey if that is so....then cn an ir diode detect ir rays too?????i mean u r using it for detecting ir rays right....but a diode cn only emit right not detect....i mean is ir diode and ir sensor the same????

Anonymous said...

hey dude help me here...i think i've lost my basics here...i just wanna ask howcome an led meant to emit rays absorbs them to generate electric signals....pls reply ...cn i get ur yahoo or orkut id..maybe we cn chat sumtime

Carlos said...

LED emit light when a current passes trough them in a certain way. Similarly, they allow current to pass trough them when when an appropriate light is shined to them.

Unknown said...

Hi there. I'm interested in using this ciruit to power a small hobby motor (1.5-3v) in addition to the L.E.D. Due to the nature of the project, I also need to power the circuit with 3 volts only instead of 9. Can you tell me what I'd need to do for these modifications? Thanks!

Carlos said...

You will have to build a similar circuit (that is, one that uses a transistor as a switch) but you need to use a transistor that will work under your voltage requirements and you'll have to figure the correct biasing for your circuit to work.

Here you can find a simple guide about using a transistor as a switch.

Good luck with your project.

haha said...

hi there,
do u have a circuit or project ( IR photo detector circuit that can function when the dark)using 24 VAC?do u have any idea?thank you..