Nuts and Volts - February 2011

www.nutsvolts.com/index.php?/magazine/article/february2011_Sullivan

■ FIGURE 2. Zoomed in view of burst at eight milliseconds. Burst is comprised of 28 flashes of infrared light at 56 kHz. Each flash is about nine microseconds long.

the receiver separate out IR code information from background light.

IR Code Sources

A universal remote control contains an abundance of IR code data. These are available for about $9 at department and electronic appliance stores. Look for a remote that stores many codes with a manual code search to scroll one-by-one through the codes. This functionality allows for easy code capture. For example, more than 200 codes are easily captured from a GE 24991 three-Device remote control.

Capturing Codes

The circuit used to capture IR codes centers on an IR receiver module such as RadioShack part number 276- 640. The circuit is shown in Figure 3 and it uses an Arduino Duemilanove to collect and send the codes to the PC. The IR receiver in this example was taken from a broken DVD player.

Figure 4 is the schematic for the circuit. It works by holding pin 2 high until an IR signal is detected, then pin 2 goes low. The receiver outputs a constant signal when it detects the modulated carrier signal. Or put another way, the signal will appear like Figure 1 with the rapid flashes shown in Figure 2 removed. This is an efficient way to capture data because each flash is not signaled which greatly reduces the amount of data to manage.

A concern with this method is that the IR receiver module is designed to capture a given modulation frequency. For example, 38 kHz in the case of the RadioShack module. Signals at very different frequencies may not be reliably detected. A validation check showed good results with a signal at 55 kHz, so the large majority of codes will be captured effectively with this method.

The Arduino program for capturing codes named “Code_Collector” is short and is provided in the downloads section for this article. It utilizes interrupts to scan for signal changes from the receiver module, records the time in microseconds, then stores the data in an array. When the code is collected, the program automatically calculates burst durations and outputs the data as an Arduino statement. This makes for easy copy and paste of the data into the program that transmits the codes. Here’s a short example output:

PROGMEM prog_uint16_t code13[19] =
{38000,17,114,115,341,115,226,2679,114,115,341,11
5,226,2679,114,114,341,115,226};

This statement will store the code in Arduino Flash memory. As an example of how to decipher the code, this one is code 13 and has 19 elements. The modulation frequency is 38 kHz. There are 17 pulse durations ( 19 elements minus 1 element that holds the frequency, and minus 1 element that holds the number of pulses). The first pulse is always an “on” pulse and, in this case, it is 1.14 ms long (114/100). There is then an off pulse of 1.15 ms, on for 3. 41 ms, and so on. The pulse durations are multiplied by 100 to avoid the need for decimals.