Thursday, January 7, 2016

Data transfer to Android device over infared link

The three previous parts (here, here and here) of this series introduced the infrared-to-Android gateway. Even though those prototypes captured the signals of an ordinary IR remote, I already hinted that I was aiming for something more exciting. In this part, we will replace the IR remote with our own IR transmitter. Once we have our own IR transmitter, we will be able to transfer our own data over IR light. This data link is not reliable enough to transfer large amount of data but in lot of the cases that's not required. E.g. to transfer the data of a temperature/humidity sensor, 32 bit is more than enough.

So what can we expect from IR-based data transfer with respect to more popular, radio-based transfer? There are advantages and disadvantages.

  • Advantage for the IR solution is that it is extremely cheap and also extremely power-efficient.
  • Advantage for radio is that IR-based solution always requires line of sight, while radio waves can - to some extent - traverse walls, etc.
  • Advantage for radio is that the IR transmitter has to be aimed at the receiver, if for some reason the receiver and the transmitter move with respect to each other, they lose contact very easily.
There's also the question of range. Ordinary TV remotes work up to 3-4 meters of distances which is nice for an inexpensive consumer device but is not enough even for indoor sensor network use cases. In this part I try to figure out what the distance limit may be.

For starter, this question is not defined precisely. With sophisticated optics, high-powered transmitters and careful targeting, IR data transfer can be accomplished over significant distances. But we said that we are looking for cheap hardware so we can't rely on sophisticated devices. We need some sort of optics but this should be simple and cheap. That's why I went to a second-hand toy shop and bingo, I found the IR transmitter of Thinkway Toys' Lazer Stunt Chaser. The small toy car has long been lost, but the handgun-like IR transmitter somehow made its way to Hungary. This cheap, plastic toy is a marvelous device. It promises 12 meters of effective range and even though the mounting of the IR light source and the plastic optics is made of cheap materials, it is surprisingly efficient. It even has a normal (red) LED emitting its light through the transparent housing of the infrared LED which produces visible red light circle of about 10 cm of diameter facilitating the targeting of the IR transmitter.

So how far can it transmit our IR codes? In order to try it out, I took apart Thinkway's IR transmitter and replaced the circuitry driving the IR LED with mine. The new emitter circuit is based on an Arduino Pro Mini 3.3V/8MHz and the schematics looks like this:

The software works on any Atmel ATmega328p-based boards, e.g. on Arduino Uno. If your MCU uses power source with higher voltage than 3.3V, adapt R2 accordingly. E.g. for an 5V Arduino Uno board, you need about 160 Ohm.

So this is how I hacked my circuit into the IR transmitter.

Note the two LEDs: the IR led in the tube-like mounting and the ordinary red LED behind it that is used for targeting. Also, note how the IR LED is connected to the ATmega328p's PD3 pin - IRLib which is the software used to construct the transmitter selects OC2B PWM output so the primary Timer2 PWM pin (PB3) would not work.

Click here to download the source code of the IR transmitter.

Open sketch/Makefile and update the ARDUINO_DIR according to your installation. Also, update ISP_PROG according to the programming tool you use to deploy the code. I used USBtinyISP, the Makefile is set accordingly. If your board has USB programming port (like the Arduino Uno has), then setting ISP_PROG is unnecessary.

In order to deploy the code into the ATmega328p, say:

make ispload

if you use a programming tool or simply

make upload

if you don't need a tool.

About the code. The application emits a 32-bit NEC IR code every 4 seconds. The value of the code is increasing. The application is based on the same IRLib library as the receiver was, this time the only modification necessary was that I disabled all the receiver routines as this device does not receive any data. And that's my response to all the people worrying about the security of the Internet of Things: if the Thing is physically unable to receive any data then it cannot be hacked, period.

Even though this application is not particularly energy-efficient, care was taken to implement the sleeping part with the lowest possible energy consumption. Hence the delay logic does not use the Arduino delay() function but puts the MCU into deep sleep and triggers the watchdog timer to launch a new iteration. Read Nick Gammon's excellent analysis of ATmega328p power saving techniques for further explanation.

So what are my experiences? Using the infrared-to-Android gateway I presented previously which is based on a basic IR receiver circuit (TSOP1738) without any optics and the IR transmitter hacked into a Thinkway toy, which uses cheap plastic optics, I was able to transmit codes to a distance of about 10 meters (and then receive the code on the Android device over BLE). Of course, not every transmission was successful. My experience is that the code needs to be repeated about 5 times if you want to be nearly 100% sure that it arrives (100% can never be achieved). One important take-away is that the targeting LED in the Thinkway toy was not there by chance, in order to target the IR receiver, you really need that aid of visible red light circle. But that's about it: 2 LEDs, a cheap MCU and some optics and you can connect sensors from an entire room to one relatively expensive BLE unit. Also, these IR-connected sensors need very little energy. So it is worth considering the advantages/disadvantages.


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