AnalogSampling

AnalogSampling

Medium: light pulses over air or optic-fibre | Pins used: 1 or 2

AnalogSampling strategy or data link complies with PJDLS v2.0, it is designed to communicate data wirelessly using light impulses and its sampling technique is based on analog readings. This strategy is able to use a single LED for both photo-emission and photo-reception phases providing with wireless half-duplex connectivity between devices with a range of up to 5 meters. Most appliances have at least a useless energy consuming LED on board, right?

AnalogSampling can also be used with separate emitter and receiver pins enabling cheap long range wireless communication using standard photo-diodes, light-emitting diodes or laser diodes. The proposed circuit, technique and codebase were originally implemented in the far 2011, see the first video documented experiment. Take a look at the video introduction for a brief showcase of its features.

Compatibility

MCU	Clock	Supported pins	Supported modes
ATmega88/168/328 (Duemilanove, Uno, Nano, Pro)	16MHz	A0, A1, A2, A3, A4, A5	1, 2, 3, 4, 5
ATmega2560 (Mega, Mega nano)	16MHz	A0, A1, A2, A3, A4, A5	1, 2, 3

Performance

AnalogSampling works with the following communication modes:

• 1 runs at 1024Bd or 128B/s (AS_PRESCALE 128)
• 2 runs at 1361Bd or 170B/s (AS_PRESCALE 128)
• 3 runs at 3773Bb or 471B/s (AS_PRESCALE 32)
• 4 runs at 5547Bb or 639B/s (AS_PRESCALE 16)
• 5 runs at 12658Bd or 1528B/s (AS_PRESCALE 8)

Caution, mode 5 sets the ADC clock prescale to a higher rate than the manufacturer recommends as maximum ADC sample rate (prescale 16).

What can be done?

The most basic example is to connect two devices using a couple of visible light LEDs used as wireless transceivers.

Leveraging of the interesting features of LEDs:

• Emit light if powered by electricity
• Emit a small but detectable amount of electricity if hit by light (photo-electric effect)

It is possible to use LEDs as wireless (bidirectional) transceivers. This means that wireless half-duplex connectivity can be provided by a single LED per device. See the LED selection guide to know more about how to choose the right LEDs and keep in mind that is necessary to add a 75K-5MΩ pull-down resistor connecting the pin used with ground to reduce the LED capacitance and externally induced interference and that depending on the voltage level used LEDs could be overpowered, add a current limiting resistor if required.

AnalogSampling can be used to experiment with short range infrared or visible light communication (remote control, robot swarms, data streaming using lighting), medium range using light sources (cars transmitting data through front and backlights) or long range laser communication (data between ground and LEO).

Configuration

Before including PJON.h it is possible to configure AnalogSampling using predefined constants:

Constant	Purpose	Supported value
AS_MODE	Data transmission mode	1, 2, 3, 4, 5
AS_RESPONSE_TIMEOUT	Maximum response time-out	Duration in microseconds (15000 by default)
AS_BACK_OFF_DEGREE	Maximum back-off exponential degree	Numeric value (5 by default)
AS_MAX_ATTEMPTS	Maximum transmission attempts	Numeric value (10 by default)
AS_PRESCALE	Set ADC pre-scaler	8, 16, 32

Pass the AnalogSampling type as PJON template parameter to instantiate a PJON object ready to communicate through this Strategy. All the other necessary information is present in the general Documentation.

#include <PJON.h>

PJON<AnalogSampling> bus;

void setup() {
  // Set the pin A0 as the communication pin
  bus.strategy.set_pin(A0);

  // Set pin A0 as input pin and pin 12 as output pin
  bus.strategy.set_pins(A0, 12);

  // Set threshold (default value AS_THRESHOLD)
  bus.strategy.set_threshold(AS_THRESHOLD);
}

After the PJON object is defined with its strategy it is possible to set the communication pin accessing to the strategy present in the PJON instance.

Known issues

• Direct sunlight or other light sources can affect receiver's sensitivity and maximum communication range
• Long wires can degrade performance
• Depending on the power supply voltage, LEDs could be overpowered, add a current limiting resistor if required
• Oscilloscope's probe acting as a pull down resistor influences results and the required pull down resistor's value
• A pull-down resistor is required to obtain optimal performance, see above

Safety warning

In all cases, when installing or maintaining a PJON network, extreme care must be taken to avoid any danger. When working with an AnalogSampling LED or laser based setup safety glasses must be worn and transceivers must be operated cautiously to avoid potential eye injuries. Consider that with AnalogSampling all LEDs that are physically connected to an ADC may be used maliciously to both download or upload data wirelessly, effectively circumventing many air-gapping techniques.