Bpw41n problem

[bouvett]

hi
i want to design an infrared communication between two modules. the frequency of communication doesn't need to be very high... the range on the other hand i need it to be of the order of at least 2 meters preferably more...

the communication is always in one way, ie i only have one receiver and one transmitter. the transmitter can only be driven with a supply of 3v.. whilst the receiver has up to 5V...

just to have a general idea of m design it is like the infrared communication in car alarms...

now, i bought a bpw41n which is said to be used for TV sets, and an infrared led (i have two type one with the blue package and another with the transparent package)..

receiver: using a 7805 i fed the Bpw41n using a 1.2Mega ohm (i also tried the 3Mohm and the 620k resistors, the difference is that the voltage on the bpw41n when in the dark condition is less as the resistance increases) voltage on the Bpw41n is read using a digital multimeter...

transmitter: using a 3v button battery i switched on the IR led using a 75 ohm resistor(i reduced it up to 9.1 ohms also with similar results for each value of resistance, the battrey voltage decreased to1.5V when i connected the resistor and led setup to it ) i also tried the same procedure using a 6V battery, and again the voltage decreased to 4V as i connected the transmitter setup(for this case i also varied the resistance from 220ohms down to 9.1ohms)...

every time the results were the same, the communication can only be sustained to up to 2 inches maximum....

each procedure was tried using another Bpw41n ic and IR led with no difference at all ...

this is certainly not a good result.. any ideas how to properly use the Bpw41n please?

thanks a lot

 

[MikeMl]

I just went through this. I first tried an IR phototransistor and an IR LED. Just using DC detection, the best I got was about 20cm, and it would not work in sunlight. I then tried a 40KHz IR receiver like the ones used for remote control in a TV set. I had to build a 40Khz oscillator to pulse the IR LED, but as a beam-break detector, the range is about 3m in sunlight. Look here and here.

 

[audioguru]

A photo-diode has a very low sensitivity. It needs a high gain amplifier that will be overloaded if the signal is not modulated.

TVs, DVD players and some MP3 players use a IR receiver IC that has the photo-diode, a bandpass filter and a high gain amplifier with AGC.

 

[bouvett]

hi, i took your advice and i have ordered the SFH 5110 ir detector ic....

now this is rated for 28Khz... so how can i provide this frequency? can i use a simple 555 timer ic to generate the oscillation? the duty cycle of the oscillation must be more exactly 50%? and should it be exactly 28Khz? 29 or 27Khz due to manufacturing errors in the resistors etc, would still do?


thanks regards

 

[bouvett]

still a problem since i am using 3V supply only for the transmitter...
there exist low voltage oscillators (helpful so to reduce the load from my pic12f629), but if i do it in software i will lose a I/o pin but possibly more accurate and also takes less space i suppose...

any ideas please submit

 

[MikeMl]

The unit I used needed 40kHz, +-1Khz. Further, to get the 3m range in sunlight, I had to pulse the IR LED with a 50% duty cycle, where the peak current through the LED was set to about 65mA (the max rating for the LED is 100mA). Indoors, out of the sun, I could get 3m range with the LED current set to 20mA peak.

The forward drop across the LED is about 2.4V, so it will be tough to do this at a supply of only 3V. I used 12V. However, IR transmitters used with TV remotes use just two AA or AAA batteries, so it can be done.

I used a high-speed, high-performance opamp (OP37) which I had on hand to make the 40kHz oscillator. I'm attaching a schematic so you can see what I did, but because I had 12V to power it on, it might not work for you...

 

[audioguru]

The automatic gain control (AGC) in IR receiver ICs reduces the gain when the carrier is continuous. The carrier must be in bursts of pulses for the gain to be at max for very good range.

A Cmos 555 will work from a 3V supply but its output current is very low. A second Cmos 555 can make the bursts.

 

[MikeMl]

The automatic gain control (AGC) in IR receiver ICs reduces the gain when the carrier is continuous. The carrier must be in bursts of pulses for the gain to be at max for very good range.

A Cmos 555 will work from a 3V supply but its output current is very low. A second Cmos 555 can make the bursts.

Are you saying that I would improve range of my beam-break detector by running the TX in bursts? Seems that running it in CW mode primes the RX to detect a short interruption of the IR beam. That is the mode I am using it in.

If I put the TX in burst mode, I would have to distinquish a short interruption due to the burst from a slightly longer interruption due to the IR beam being blocked...

 

[audioguru]

Are you saying that I would improve range of my beam-break detector by running the TX in bursts?

Certainly.
Vishay (TSOP series) IR receiver datasheets show that the AGC reduces the gain a lot when it receives continuous IR pulses. The gain is reduced to reduce interference from compact fluorescent light bulbs that flicker IR at about 40kHz.
The datasheets also show the length of the bursts and the length of the pauses between bursts. The lengths are different for the different receiver ICs. Remote controls normally send bursts of data.

 

[bouvett]

by bursts you mean you use manchester code for example on the 40khz signal in order to transfer code right?

so you mean the carrier frequency needs to be 50% duty cycle only?


thanks a lot

 

[audioguru]

A burst of the 40kHz carrier is a certain number of 40kHz pulses as explained in the datasheet for the IR receiver IC. The datasheet also explains the duration of the pause that is needed between bursts of pulses.

Remote control codes make the bursts and pauses as part of the code.

The duty-cycle of the carrier has nothing to do with the bursts and pauses.

 

[MikeMl]

Certainly.
Vishay (TSOP series) IR receiver datasheets show that the AGC reduces the gain a lot when it receives continuous IR pulses. The gain is reduced to reduce interference from compact fluorescent light bulbs that flicker IR at about 40kHz.
The datasheets also show the length of the bursts and the length of the pauses between bursts. The lengths are different for the different receiver ICs. Remote controls normally send bursts of data.

I tried a second oscillator to gate the 40kHz On for 600us/Off for 600us and got a reduction in range. I'm using the IR detector in a mode not covered on the data sheet!

It is able to discriminate between a weak continuous IR 40kHz signal and the residual background IR radiation from the sun (what is left when the IR beam is blocked) better if the signal prior to the interruption of the beam is continuous 40Khz.

 

[audioguru]

I tried a second oscillator to gate the 40kHz On for 600us/Off for 600us and got a reduction in range.

Each TSOP has a different baud rate and a different required number of 40kHz pulses per burst and a different required length of pause between bursts. What does its datasheet say?

 

[MikeMl]

Each TSOP has a different baud rate and a different required number of 40kHz pulses per burst and a different required length of pause between bursts. What does its datasheet say?

40kHz carrier, 600usec ON/600usec Off. I got better range when the carrier is continuous for a long time (many seconds), and then break the beam for a couple of seconds.