The remainder of this article explains how to achieve extended operating range with your Fire-Stick by selecting compatible components, pulsing the LED at currents well beyond its normal operating limits, selecting an appropriate infrared LED, and infrared detector module.
The manufacturers data sheet for the HT-12A encoder IC includes a schematic for using the HT-12A in an infrared application. Here's a screen capture of the schematic.
A Few Changes:
The majority of the circuit was left intact for the finished Fire-Stick design, but a few small changes were made to increase the overall operating distance.
 | The 8050 NPN transistor has been replaced with the ZTX603. |
| The 100-ohm current limiting resistor for the infrared LED has been replaced with a 5.1 ohm. |
| The infrared LED isn't connected to Vdd, but directly to +9VDC. |
Doesn't really sound like much, but the overall effect is an extended operating range of up to 100'. This is quite an improvement over what you'll normally find available with off-the-shelf infrared equipment.
The ZTX603:
The ZTX603 is an NPN silicon planar medium power darlington transistor capable of handling a continuous collector current of 1-amp. The peak-pulsed current can reach as high as 4-amps maximum. That's pretty incredible when you consider that the ZTX603 is no larger than an ordinary NPN transistor that blows-up at a mere fraction over 100mA.
A standard NPN transistor such as the commonly used 2N2222 with a maximum collector current of 100mA will definitely not cut-the-mustard for pumping the infrared LED up for maximum operating distance.
Pulse-Driving The LED:
The LED forward voltage drop is 1.7V. The ZTX603 transistor C-E voltage drop is 1V. If we use 5V we have 5V - 2.7V = 2.3V. Take 2.3V/5.1-ohms, and you have peak LED current of 451mA. For most purposes this will work fine, but what if we want to really push the envelope here and go for maximum distance..? Take 9V - 2.7V = 6.7V. Now take 6.7V/5.1-ohms for a peak current of 1.3-Amps...! Are we going to blow-up some infrared LEDs here or what..?
Because we are never actually turning the LED on continuously, but instead pulsing the LED at 38KHZ, thirty eight thousand cycles per-second, we can push the envelope, bend the rules a little, and crank-out some high-powered infrared energy.
The infrared LED used for the Fire-Stick is just a plain infrared LED with a 940nm wavelength. There's nothing really special about this LED whatsoever. It's just pulsed to operate well beyond normal limits, and matches the wavelength of the IR detector module used (in this case 940nm).
You can change the value of the LED series current limiting resistor to alter the operating distance for whatever your application requires. Using the Fire-Stick for momentary pulsed operation you can reach very long distances, but it's important that you only briefly pulse each data input pin on the HT-12A to logic 0. Holding the transmitter in a continuous transmission state will over-load the resistor, the LED, and eventually destroy your Fire-Stick altogether. Remember this, or buy plenty of extra components....
The Trade-Off:
The original Fire-Stick was designed primarily for use as a hand-held unit. Normal operation of hand-held infrared transmitters consists of pushing the button briefly and releasing it. If your application only requires an operating distance of 30 foot, you can tune-down the output power by adjusting the LED current limiting resistor, using Vdd as the source voltage, or both.
The trade-off for the ultimate operating distance is very short-burst operation with the transmitter. If you need extended distance, and short burst operation is OK, then go-for-it. Short burst operation would normally be less than 1-second ON time for at least 4-seconds OFF. This gives the LED and other components time to cool before being turned on again. Just like any other high-powered device, if you push it too hard, something's going to give...
The first Fire-Sticks we originally put together for our first prototype tests here in our shop have been going strong for well over 1 1/2 years with no ill effects or apparent degradation to any of the Fire-Stick transmitter components. The overall range is still up to 100', and we use them almost daily.
Critical Components:
Careful consideration should be given when selecting some of the components for any infrared remote control system. Three components that are critical to the overall performance & operation of your system will be:
| The drive transistor. |
| The infrared LED. (must match the IR detector module wavelength) |
| The infrared detector (receiver) module. |
The Drive Transistor:
The drive transistor should be selected for:
| The ability to operate continuously at the target frequency. |
| The ability to handle your intended power requirements for the IRLED. |
| Low, or acceptable C-E voltage drop at saturation. |
If your data carrier is operating at 38KHz, the transistor must be able to operate at this frequency and provide sufficient current to the LED during peak demands. The low C-E drop will waste less power thereby leaving more for the LED.
The Infrared LED:
Select an infrared LED that:
| Provides a beam angle sufficient for your application. |
| Operates at the same wavelength as the infrared detector module you select. |
The beam angle is important. Some LEDs provide very narrow beam angles, and are best suited for applications such as detecting objects in a narrow path or precision focusing of emitted IR energy. Wide beam angles are preferable for operation where the IR energy of the emitter (LED) needs maximum coverage such as large rooms, etc,,.
If you need a simple LED/detector arrangement to sense objects close to the detector, and the IRLED will be located in close proximity to the detector, choose an LED with a narrow beam angle. If you want maximum coverage, and detection, select an LED with a wide beam angle.
A Bit On Wavelength:
Light is both a wave and a particle, and travels through space as electromagnetic waves. Because of this wave motion, each different color has its own unique wavelength. Light wavelengths are very short, and are expressed in nanometers. The abbreviation for this is nm. When you examine a data sheet for a particular infrared LED, you'll see its wavelength expressed as 940nm or 880nm.
To ensure optimal operation of your infrared system it's very important to match the LED characteristics as closely as possible to the infrared detectors.
Example: You have decided on an infrared LED because it's inexpensive, easy to find, and the LED provides a 90 degree beam angle that will work perfectly for your new robot sensor array. You download the data sheet for this LED from the manufacturers web site and notice that the LED operates at 940nm. The next step is to choose an infrared detector module that has an optimum spectral sensitivity to the 940nm infrared wavelength of your selected infrared LED.
Note: Using the chart above locate where a wavelength of 880nm falls. This is how far off this particular infrared detectors sensitivity will be with just a simple mismatch of components, and shows the importance of selecting the correct components for your infrared remote control system for maximum effectiveness. The LED you selected that has a 940nm wavelength will be perfect for this detector....
If you find an infrared detector module that matches the wavelength of your LED, you're going to finish your project with an infrared remote control system that works at optimum efficiency. At least as far as the detection circuit is concerned. The picture above was captured from a data sheet for the PNA4601M series Panasonic infrared detector module. This is a bipolar integrated circuit with photo detection, specifically designed to work with infrared in the 940nm wavelength.
Notice from the chart that the maximum sensitivity of the detection circuit falls within the region of 940nm. Match these two up, and you're in business. More often than not, people will overlook simple design considerations such as this. It will still function even if these parameters are not matched, but not at optimum efficiency.
This is just one of the critical design considerations that ultimately makes the difference between an infrared remote control system with an operating distance of only 20 foot, and the one that goes well beyond the normal limits.
The Infrared Detector Module:
Select an infrared detector module that:
| Has maximum spectral sensitivity at the same wavelength of the infrared LED you have selected. |
| Operates at the exact frequency of your modulated data carrier. |
| Has a visible light cutoff resin over the detection pin diode to block visible light. |
| Offers the maximum reception distance required by your application. |
| Requires little or no external components for operation. |
The lens element should be impregnated with a visible light cutoff resin that helps eliminate interference from external visible light sources. This is an option that's highly desirable with an infrared detector. Most detectors have this feature, but it's definitely possible to find some at bargain-basement prices without it. You're better off paying an extra 20-cents for a quality component....
What's Going On Inside..?
Selecting a detector module that operates at the same frequency and wavelength as the data transmitter is another critical part of the design. The screen capture above has been reduced in size to fit into the tables here, but is sufficient to illustrate how the detector works internally.
I won't go to great length here to explain how each individual stage inside the detector module works, but essentially what happens is this. The PIN diode detects the incoming signal, the amplifier pumps-up the signal then passes it on to the band-pass filter stage. From here on the carrier is stripped and the remaining data is passed to the output transistor. The effect is that the incoming data is separated from the carrier frequency, and delivered to the output.
Since peak efficiency is obtained when the carrier is at the same frequency as the demodulation/conversion circuit, it's obvious why matching the frequency of the carrier and detector is important. Yes. A 40KHz detector module will work with a 38KHz carrier, but not at optimum efficiency and operating distance will be reduced considerably.
The Fire-Stick Receiver:
Note: This schematic (above) shows the pin-out for the Liteon 38KHz infrared detector module. This IR detector is no longer available, but the Panasonic detector module [shown below] is widely available, and works equally well. The pin-outs of the Liteon and Panasonic IR detector modules vary as can be seen by the schematic using the PNA4602M below, but both IR detectors perform essentially the same function.
The incoming data signal is fed to the base of the 2N3904 NPN transistor. The signal coming in has been inverted by the transmitter driving the base of the ZTX603 on the transmitter, so it is inverted once again by the NPN transistor on the receiver to restore the data to its original format and present this data to the HT12D decoder. Since the data has been inverted twice, it's back to its original state.
Some people have tried to build their own versions of the Fire-Stick once they received the schematics we supplied with each KIT. This was one area that many people had trouble with. If you build your own Fire-Stick on the bread board take your time and check this connection. If it's wrong, it simply won't work.
The receiver section is incredibly simple, but you need to remember that you can't drive just any load directly from the output pins of the HT-12D. Quite a few people have tried to directly connect relays, LEDs, solenoids, and just about everything else you could imagine to the data outputs directly, and found out the hard-way that it simply doesn't work.
The data output pins of the HT-12D decoder IC, D0 to D3 will source/sink only 1.6mA @5V. While not quite enough to drive an LED or relay, this is more than enough to drive a transistor or the ULN2803 high-current darlington transistor array.
Tip: One thing to remember about the HT-12D data outputs is that when the receiver circuit is first powered-up, the data output pins default logic state will be logic 0. When the HT-12D receives its first valid transmission each data output will then go to whatever logic state is on the encoder IC (HT-12A) data pins.
Using The Encoder/Decoder With Microcontrollers:
The HT-12A, and the HT-12D data pins can be directly connected to microcontrollers such as the Basic Stamp, PIC and 8051. Setting up a complete control system with a microcontroller instead of switches is relatively straight-forward. Use the controller I/O-pins to control the data input pins of the HT-12A encoder for the transmitter section.
Note: If you're a beginner, it's a good idea to use series resistors between the data pins of the encoder/decoder ICs and the microcontroller to avoid shorting conditions. In the event your code goes totally hay-wire, and you have one pin at logic 0 while another is logic 1. With the series resistors to limit current you'll just waist a little power instead of smoking up the house. Just slap a couple 1K resistors in series between your microcontroller I/O-pins and the encoder/decoder data pins. This will limit current to around 5mA per connection and keep you in the game without replacing too many parts. --- Just In Case ---
On the receiver the easiest approach is to use the VT output to generate interrupts, or have a routine that will occasionally test the logic state of the VT pin. For simple applications the micro can sit & wait for input from the decoder output pins, and then proceed to process data once it has arrived. There's an endless number of ways to integrate this infrared system into just about any application you can think of.
Have fun.......:o]
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