I am today talking led LCD tv card IR Remote Control for Electronics device | IR Sensor in Bangla
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IR Remote Signals
Now we know that the sensor works, we want to figure out whats being sent right? But before we do that let's first examine exactly how data is being sent from the IR remote (in your hand) to the IR receiving sensor (on the breadboard)
For this example, we will use the Sony power on/off IR code from a Sony TV remote. It is very simple and commonly documented!
Let's pretend we have a Sony remote, and we can look at exactly what light is being blasted out of the IR LED. We'll hook up a basic light sensor (like a basic photocell!) and listen in. We won't use a decoder like a PNA4602 (just yet) because we want to see the undecoded signal. What we see is the following,
Basically, we see pulses or IR signal. the yellow 'blocks' are when the IR LED is transmitting and when there is only a line, the IR LED is off. (Note that the voltage being at 3VDC is just because of the way I hooked up the sensor if I had swapped the pullup for a pulldown it would be at ground.)
The first 'block' is about 2.5ms long (see the cursors and the measurement on the side)
If you zoom into one of those blocks
You can measure the frequency of the IR pulses. As you can tell by the cursors and the measurements on the side, the frequency is about 37.04KHz
OK so now we can understand how IR codes are sent. The IR transmitter LED is quickly pulsed (PWM - pulse width modulated) at a high frequency of 38KHz and then that PWM is likewise pulsed on and off much slower, at times that are about 1-3 ms long.
Why not have the LED just on and off? Why have PWM 'carrier' pulsing? Many reasons!
One reason is that this lets the LED cool off. IR LEDs can take up to 1 Amp (1000 milliamps!) of current. Most LEDs only take 20mA or so. This means IR LEDs are designed for high-power blasting BUT they can only take it for a few microseconds. By PWM'ing it, you let the LED cool off half the time
Another reason is that the TV will only listen to certain frequencies of PWM. So a Sony remote at 37KHz won't be able to work with a JVC DVD player that only wants say 50KHz.
Finally, the most important reason is that by pulsing a carrier wave, you reduce the effects of ambient lighting. The TV only looks for changes in light levels that clock in around 37KHz. Just like it's easier for us to tell differences between audio tones than to pin down the precise pitch of a tone (well, for most people at least)
OK so now we know the carrier frequency. It's 37KHz. Next, let's find the pulse widths!
The first pulse is 2.5ms. We can use the cursors to measure the remaining pulses. I'll spare you the 12 images and let you know that the pulses are:
PWM ON OFF
2.4 ms 0.6 ms
1.2 ms 0.6 ms
0.6 ms 0.6 ms
1.2 ms 0.6 ms
0.6 ms 0.6 ms
1.2 ms 0.6 ms
0.6 ms 0.6 ms
0.6 ms 0.6 ms
1.2 ms 0.6 ms
0.6 ms 0.6 ms
0.6 ms 0.6 ms
0.6 ms 0.6 ms
0.6 ms 270 ms
So let's say you don't have a $1000 oscilloscope, how else can you read these signals? Well, the IR decoder such as the PNA4602 does us one favour, it 'filters out' the 38KHz signal so that we only get the big chunks of the signal in the millisecond range. This is much easier for a microcontroller to handle. That's what we'll do in the next section!
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Using an IR Sensor
The good news is that it is very easy to hook up this sensor. Just connect the output to a digital pin. The bad news is that the Arduino's friendly digitalRead() procedure is a tad too slow to reliably read the fast signal as its coming in. Thus we use the hardware pin reading function directly from pin D2, that's what the line "IRpin_PIN & BV(IRpin))" does.
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