IR LEDs need more power

[joe61]

I'm trying to work out how to make rangefinders from IR LEDs and IR receivers. I'm working on a robot that's too small for the Sharp receivers so I think I'll have to roll my own.

I can get the emitter to send a 38KHz signal, and I can see it with the receiver when they're pointing at each other, but when I point them both in the same direction, I don't get enough of a return from the emitter to do the job.

So I starting thinking about just giving it more current, but if I'm reading the data sheet for the emitters correctly I don't think I can give it enough. I'm either doing something wrong, or I got the wrong emitters or something. Should I be looking for a different emitter, or am I missing it somewhere else? What's the normal way to do this?

Thanks

Joe

[corrado33]

I'm just speculating here, but I'm guessing you'd need narrow beam IR LEDs to be able to bounce the signal off of anything. 

[Soeren]

Hi,

I'm trying to work out how to make rangefinders from IR LEDs and IR receivers. I'm working on a robot that's too small for the Sharp receivers so I think I'll have to roll my own.

In short... You can't.
The Sharp distance sensors use a linear photo receiver array for getting the angle to triangulate the result from.
If you'll settle for a short range obstacle detector OTOH...

I can get the emitter to send a 38KHz signal, and I can see it with the receiver when they're pointing at each other, but when I point them both in the same direction, I don't get enough of a return from the emitter to do the job.

So I starting thinking about just giving it more current, but if I'm reading the data sheet for the emitters correctly I don't think I can give it enough. I'm either doing something wrong, or I got the wrong emitters or something. Should I be looking for a different emitter, or am I missing it somewhere else? What's the normal way to do this?

I would have thought that you've been on this forum for long enough to know we're not mind readers?
Post links to datasheets for emitter and receiver please.

How do you connect them up?

Are they the same wavelength?

Are you perhaps using an integrated IR receiver (like in a TV)?


eMojo needs info to work.

[waltr]

If you're using a 38kHz IR receiver then the modulated frequency needs to be almost exactly 38kHz. Have you measured the actually frequency that is switching the IR LED?

Many years ago (before the Sharp sensors were available) I made a three level IR distance sensor using a IR LED modulated at three current levels (two PIC pin outputs and two resistors) and a 38kHz remote receiver. This worked rather well. The three current levels would detect at about 3cm, 8cm and 10cm. Not quite what can be done with a Sharp sensor but good enough for a simple Bot to avoid objects and follow a wall.

[Soeren]

Hi,

Many years ago (before the Sharp sensors were available) I made a three level IR distance sensor using a IR LED modulated at three current levels (two PIC pin outputs and two resistors)

I believe Sharp made PSD's way before Arizona Microchip held their first inauguration.

This worked rather well. The three current levels would detect at about 3cm, 8cm and 10cm. Not quite what can be done with a Sharp sensor but good enough for a simple Bot to avoid objects and follow a wall.

The main problem with that method is that, a large dark (to IR) object at close range, will give the same reading as a small light object further away.
(Sharp PSD's and LASER rangers are fairly unchallenged by such, due to their working principles).

[joe61]

If you're using a 38kHz IR receiver then the modulated frequency needs to be almost exactly 38kHz. Have you measured the actually frequency that is switching the IR LED?

Yes, my multimeter can measure frequency, and it's coming out right around 38KHz (can't remember the numbers off the top of my head).

Joe

[joe61]

I would have thought that you've been on this forum for long enough to know we're not mind readers?
Post links to datasheets for emitter and receiver please.

How do you connect them up?

Are they the same wavelength?

Are you perhaps using an integrated IR receiver (like in a TV)?


eMojo needs info to work.

I'll have to look them up when I get home. I was actually wanting to know how people normally go about this, as I can already tell I'm doing it wrong. I see lots of robots on the web with IR emitter/receiver pairs, so I figured that would be a common thing that someone could point me to some information on.

No, I'm not using an integrated IR receiver, as I said I'm using a separate emitter and receiver. They are the same wavelength, as I said the receiver can see the emitter when they're pointing at each other. Right now I've just been trying breadboard setups, and haven't found one that works.

Joe

[Soeren]

Hi,

No, I'm not using an integrated IR receiver, as I said I'm using a separate emitter and receiver.

OK. An integrated IR-receiver would be separate from the transmitter (IR-LED) as well and it was the selection of 38kHz that made me think so, as most discreete builds run with lower frequencies.

They are the same wavelength, as I said the receiver can see the emitter when they're pointing at each other.

You said so indeed, but the wavelength is just the peak and sitting an LED in front of a photo transistor/-diode, it will detect even if their (peak) wavelengths are far from each other. When the distance increases, or you want to use them in reflex mode, the differences shines through (or rather not).

Only if you buy them well spec'd you can be certain that their wavelengths match - or if you have access to measuring equipment like a precise spectroscope that can read IR.

Right now I've just been trying breadboard setups, and haven't found one that works.

Could you post the schematic(s) you have worked from?
It's hard to know what's wrong with a circuit that you cannot see.

The photo receiver that you have, is it a photo diode or a photo transistor?
Diodes are fast,but not very sensitive, while transistors are slow, but have a high output.

About the LEDs... If you decrease the duty cycle, you can increase the peak current (within reason), as it's the average current that determines how hot it gets at the crystal. The upward limit for short pulses is the gold bonding wires - too high a current for even the shortest time and they act as fuses.

A pretty common IR-LED is the LD271. It has an abs. max rating of 130mA continuous/3.5A pulsed. This doesn't suggest you should use them at those ratings, but 100mA continuous wouldn't bother me or 3A for short pulses with a low M/S ratio.
If you really need power, you can get some at around 15W (in a TO220 package looking out its navel).

Most range issues, that doesn't stem from mismatched wavelengths, can be traced to the opto receiver design.

[joe61]

Could you post the schematic(s) you have worked from?
It's hard to know what's wrong with a circuit that you cannot see.

I haven't been working from a schematic, I've just been trying things on a breadboard.

I wasn't trying so much to work on the setup I was using, I was actually looking for information about how people normally do it. I found a tutorial here about it a bit ago. I'll understand that first.

Thanks

Joe

[Soeren]

Hi,

I haven't been working from a schematic, I've just been trying things on a breadboard.

Oh well... Without a plan and some calculations, it's almost bound to be less than optimum.

I wasn't trying so much to work on the setup I was using, I was actually looking for information about how people normally do it. I found a tutorial here about it a bit ago. I'll understand that first.

Back in time (way back), I used 555's to modulate IR LEDs at around 15kHz (low M/S and comparatively high current) and CMOS op-amps (CA3n40) in a tuned circuit for the receiver. I think I used photo transistors (as opposed to photo diodes). I got a range of 50..60cm IIRC.

The last time I made optical proximity detectors (around 2005), I used some pretty nice chips from Hamamatsu, looking like a 4-pin slightly enlarged e-line casing and it only needs a 5..15V supply and a common IR LED (it's got a "constant current" driver/modulator for the LED built-in) and gives a detect/not detect output.
The latest versions of these chips are even better than what I used, working with a background light level of up to 10,000 lx!  This would be my recommendation for a very small and compact (and very reliable) IR proximity detector

To just add some extra range to an existing set-up, try adding 2 or 3 IR-LEDs. 4 LEDs gives twice the distance of a single LED.

[joe61]

Thanks, I'm working on a schematic. Still learning to read data sheets though.

Joe