Hi Fellow B&O Enthusiasts,

I was wondering if anybody knew the infrared wavelength of the transmitters/receivers used in the Terminals for the Beomaster 8000/6000? The service manual does not seem to have any specific information about the IR LEDs etc...

Thanks!

Rudy

Are you really after the wavelength, as opposed to the carrier frequency the transmitters are driven with? The wavelength usually doesn't matter - although it can vary on the data sheets, the receiver components have quite wide response curves and generally all IR photodiodes / transistors will be sensitive to the light from any IR LED.

The carrier frequency is a different thing. All remotes with current Beolink codes use 455 kHz but I believe the earlier remotes, including 6000/8000 are much lower. You would need to put a scope or some kind of an "IR spy" on the remote to measure it, but I suspect it will be between 35-50 kHz as most other manufacturers are still using.

It is annoying that the service manuals don't mention it... I once tried to calculate it from the resonant circuit component values of a BM5000 receiver, but can't remember if the result made any sense.

Anyway, I think the old remotes might be well served with a generic, non-B&O ready learning remote if that's what you're getting at!

Hi Mika,

I am looking for the actual light wavelength emitted by the IR diodes in the Terminal. The carrier frequency can be inferred from the service manual where is states that each command consisting out of 8 bit and an equal-length "pause" period takes 48ms, i.e. one could say that each bit has a duration of 48ms/16=3ms, which would make it a carrier frequency of 330Hz...or am I on the wrong track about the meaning of 'carrier frequency'.

Anyway, my current goal is to use the Arduino microcontroller platform to act as a smart remote control, which could have additional functions like alarm clock etc...So I need to know what infrared LED to buy as a transmitter, so the BM8000 can understand the signals...

Thanks!

Rudy

HarryPierce:

or am I on the wrong track about the meaning of 'carrier frequency'.

I'm afraid you are - as I said, the wavelength of the IR led isn't too critical here. Any IR led will do.

The thing is that the command codes you see described in the service manual are low frequency (as you calculated) pulse trains, which are then used to amplitude modulate (simply on/off) an oscillator (running at the carrier frequency), and the resulting signal is then used to drive the transmitter leds on and off. Here's a web site that describes this with pictures:

http://www.sbprojects.com/knowledge/ir/index.php

Think of it as an AM radio transmitter / receiver pair if you wish - the pulse train you want to send corresponds to the audio signal, not the electromagnetic radiation leaving the antenna. The discrete IR receivers in the newer B&O devices actually closely resemble a direct mode, fixed frequency 455 kHz AM receiver, with a photo diode in place of an antenna!

There are some remote control protocols that don't use a carrier and directly drive the IR light output (such as the ITT protocol described on the site above), but B&O never used any of those.

I took a look at the BM8000 service manual which includes the internal schematics of the Terminal. It uses an SN 76831 transmitter IC; apparently by Texas Instruments, but it's too old to have any documents online... luckily the internal block diagram is shown, and it has first a 455 kHz main oscillator, followed by apparently a 1:11 divider block, which is then used to gate the LED driver. This would give a carrier frequency of 455/11 kHz ~= 41 kHz, which does make sense.

With an Arduino, you can set up one of the timer/oscillators of the AVR controller to run at the required carrier frequency and simply gate it on/off with the pulse train you want to transmit, and get the signal to drive the transmitter LED out of an I/O pin. That way you don't need an external hardware oscillator nor do you have to worry about generating the carrier in software, and the timing for the much lower frequency IR command codes becomes quite easy. After the setup phase, you can simply toggle a register bit on and off to control the IR carrier sent to the led, and from the programmer's point of view the problem is reduced to what you anticipated at first.

The Arduino web site and forums have plenty of examples on IR remote control including ready libraries you might be able to use. Good luck, it's a fun project

Hi Mika,

Outstanding!! Thanks much for the insight! I did not understand that the data signal to the LED is run on a carrier signal (the website you cited is a great intro). I understand from your discussion that all IR LEDs are compatible with any IR receivers in terms of emitted light wavelength, i.e. it would not matter if I used an IR LED with 850nm output or one with 940nm? The IR receiver in the BM8000 would be able to see these signals?On a different note: What radiant intensity and emission angle would you think is best/sufficient for a remote control application? Thanks!

Rudy

I wonder what this guy is doing:

http://www.youtube.com/watch?v=WN4dyvegNNs

5.4 on RL60s...he is doing something dangerous, I would say...;-).

HarryPierce:

I understand from your discussion that all IR LEDs are compatible with any IR receivers in terms of emitted light wavelength, i.e. it would not matter if I used an IR LED with 850nm output or one with 940nm?

Yes, it will work - the sensitivity might be reduced a bit if the peaks are far apart, but as long as it is a regular IR LED, it will be fine. The carrier frequency doesn't need to be exactly spot on either, within +/-5% should be fine.

I have never paid much attention to the LEDs, I've used LD 271 as they were readily available... and what do you know, the Terminal seems to have those as well

Remember that if you simply drive the LED(s) directly from the Arduino I/O pins, the range will be rather short. With a series resistor that keeps the max current at a safe 10 mA or so, you can expect perhaps one meter. The real remotes use all kinds of circuit tricks to send shorter but larger pulses through the LED to increase the range. The two-way transmitters in the stackable Beomasters for example use four LEDs driven from a 40V supply!

Test the circuit with a regular red LED first. Once you get it flashing, you can replace it with an IR LED and keep it close to the receiver. Once the control actually works, you can start tweaking to increase the range if needed.

This is becoming an increasingly interesting project...;-). I studied the LD271 datasheet. Do I understand it right (pg. 5 "Permissible pulse handling capability" graph) that if I run the diode at 40 kHz, that I can push it to ~2A pulse current at a <0.05 duty cycle? Is this the way they do it in 'professional' remotes? Though: Why do they need 40V in the newer Beomasters? The I/V curve in the datasheet suggests that about 2.5V should yield about 2A...?? Thanks!

Rudy

HarryPierce:

The I/V curve in the datasheet suggests that about 2.5V should yield about 2A...??

In principle, yes... but once you start to design & implement a 2A @ 2.5V power supply that can be used to source 1 / 40 kHz * 0.05 = 1.25 µs pulses (not to mention the required rise & fall times) you realize it's not going to be the way to go - and even less at a carrier frequency 11 times that!

At these frequencies you can mostly forget simple Ohm's law calculations, as stray inductances etc will come into play with force. The defining thing will be the energy (current integrated over time) you get into the LED in a single pulse, and it will be much easier to achieve with a higher voltage. Handheld remotes typically charge a cap for a single pulse to get a large initial current (decaying very quickly) into the LED. It's obviously a compromise between battery life & range.

I believe the motivation for a hefty transmitter in Beomasters was that it is easy for the user to understand the limited range of a remote for sending, but he would be highly annoyed if he had to worry (or even realize) that the return direction could be limited as well.

Keep it simple. A 100 ohm series resistor and an IR LED off an Arduino I/O pin, with no calculations at all, will get you started