Pulsed IR Technology For Detecting RCJ Soccer Balls

In mode A, the RCJ ball uses a pulsed technology which is designed to eliminate the effects of spurious sources of IR such as sunlight or ionised vapour lamps. The technology used is the same as employed in commercial IR communications mechanisms such as remote controls for TV and similar devices. See below for one significant difference.

The mode A pulse train is composed of a series of 40kHz pulses starting with 8 pulses of full power, followed by 4 pulses of 1/4 power, 4 pulses of 1/16 power and a final 4 pulses of 1/64 power. This pulse sequence is repeated 1200 times per second, i.e. roughly each 833 microseconds.

Note the difference between this pulse train and common remote control sensors is that the ball uses 40 kHz while most remote control systems use 38 kHz pulse trains. The difference is significant. There are papers suggesting the use of 38 kHz sensors but these will significantly reduce sensitivity.

The pulse trains can be easily detected using readily available inexpensive commercial sensors such as the TSOP1140 IR sensor (datasheet) available from Element14.

The TSOP1140 is designed to detect short bursts of 40 kHz pulses, 6 pulses are the minimum number required. This means it can lock to the first 8 pulses. When pulses are found, the output of the TSOP1140 will drop from 5V to 0V while pulses are seen. This means the number of pulses seen will determine the time the output is at zero volts. If the ball is close, all 20 pulses (8+4+4+4) will be seen. As each pulse is 25 microseconds wide, the low time will be roughly (20 x 25 uS) 500 microseconds. Further out, the weakest pulses will not be seen and the low times will vary to roughly 400 uS, 300uS and finally to around 200 uS at a distance when just the first 8 pulses are seen. At other times, the device should give 0uS low time. These time quantities are readily measured in software.

The TSOP1140 is specified as being able to detect an IR emitting diode at 45 metres in a dark environment. It also has a wide viewing area. In practice it has been found necessary to encase the sensor in a light-proof casing e.g. thick plastic, balsa etc, and to use a collimator to constrain the view. This can be done by using an opaque 4 mm diameter (preferably red) plastic tube about 1 cm long. This is an extremely important aspect to experiment with as these devices are very sensitive.