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IR Transmitter

Main README

1. Hardware Requirements

The transmitter requires a Pyboard 1.x (not Lite), a Pyboard D or an ESP32. Output is via an IR LED which needs a simple circuit to provide sufficient current. Typically these need 50-100mA of drive to achieve reasonable range and data integrity. A suitable 940nm LED is this one.

On the Pyboard the transmitter test script assumes pin X1 for IR output. It can be changed, but it must support Timer 2 channel 1. Pins for pushbutton inputs are arbitrary: X3 and X4 are used. The driver uses timers 2 and 5.

On ESP32 the demo uses pin 23 for IR output and pins 18 and 19 for pushbuttons. These pins may be changed. The only device resource used is RMT(0).

On Raspberry Pi Pico the demo uses pin 17 for IR output and pins 18 and 19 for pushbuttons. These pins may be changed. The driver uses the PIO to emulate a device similar to the ESP32 RMT. The device driver is documented here; this is for experimenters and those wanting to use the library in conjunction with their own PIO assembler code.

1.1 Pyboard Wiring

I use the following circuit which delivers just under 40mA to the diode. R2 may be reduced for higher current.
Image

This alternative delivers a constant current of about 53mA if a higher voltage than 5V is available. R4 determines the current value and may be reduced to increase power.
Image

The transistor type is not critical.

The driver assumes circuits as shown. Here the carrier "off" state is 0V, which is the driver default. If using an alternative circuit where "off" is required to be 3.3V, the class variable active_high should be set False.

1.2 ESP32 Wiring

The ESP32 RMT device now supports the carrier option, and this driver has been updated to use it. The same circuits as above may be used to connect to pin 23 (or other pin, if the code has been adapted). The active_high option is not available on the ESP32 RMT object, so any alternative circuit must illuminate the LED if the pin state is high.

1.3 RP2 Wiring

There is no active_high option so the circuit must illuminate the LED if the pin state is high, as per the above drivers. Test programs use pin 17, but this can be reassigned.

2. Dependencies and installation

2.1 Dependencies

The device driver has no dependencies.

On ESP32 a firmware version >= V1.14 is required. The Loboris port is not supported owing to the need for the RMT device and other issues.

The demo program uses uasyncio primitives from this repo. Clone the repo to a directory on your PC:

$ git clone https://github.com/peterhinch/micropython-async

move to its v3 directory, and copy the primitives directory with its contents to the filesystem.

2.2 Installation

The transmitter is a Python package. This minimises RAM usage: applications only import the device driver for the protocol in use. Clone the repository to the current directory of your PC with:

$ git clone https://github.com/peterhinch/micropython_ir

Copy the following to the target filesystem:

  1. ir_tx Directory and contents.

The demo is of a 2-button remote controller with auto-repeat. It may be run by issuing:

from ir_tx.test import test

Instructions will be displayed at the REPL.

3. The driver

This is specific to Pyboard D, Pyboard 1.x (not Lite), ESP32 and Raspberry Pi Pico (RP2 architecture chip).

It implements a class for each supported protocol, namely NEC, SONY_12, SONY_15, SONY_20, RC5 and RC6_M0. Each class is subclassed from a common abstract base class in __init__.py. The application instantiates the appropriate class and calls the transmit method to send data.

Basic usage on a Pyboard:

from machine import Pin
from ir_tx.nec import NEC
nec = NEC(Pin('X1'))
nec.transmit(1, 2)  # address == 1, data == 2

Basic usage on ESP32:

from machine import Pin
from ir_tx.nec import NEC
nec = NEC(Pin(23, Pin.OUT, value = 0))
nec.transmit(1, 2)  # address == 1, data == 2

Basic usage on Pico:

from machine import Pin
from ir_tx.nec import NEC
nec = NEC(Pin(17, Pin.OUT, value = 0))
nec.transmit(1, 2)  # address == 1, data == 2

Common to all classes

Constructor args:

  1. pin A Pin instance instantiated as an output. On a Pyboard this is a pyb.Pin instance supporting Timer 2 channel 1: X1 is employed by the test script. On ESP32 any machine.Pin may be used. Must be connected to the IR diode as described below.
  2. freq=default The carrier frequency in Hz. The default for NEC is 38000, Sony is 40000 and Philips is 36000.
  3. verbose=False If True emits (a lot of) debug output.

Method:

  1. transmit(addr, data, toggle=0, validate=False) Args addr, data and toggle are positive integers. The maximum vaues are protocol dependent. If validate is True passed values are checked and a ValueError raised if they are out of range. If validate is false invalid bits are silently discarded. For example if an address of 0x11 is passed to MCE.transmit, the address sent will be 1 because that protocol supports only a four bit address field. The toggle field is unused by some protocols when 0 should be passed.

Class method:

  1. active_low No args. Pyboard only. A ValueError will be thrown on ESP32. The IR LED drive circuit is usually designed to turn the LED on if the driver pin is high. If it has opposite polarity the method must be called before instantiating the class - it will be ineffective if called later.

Class varaible:

  1. timeit=False If True the .transmit method times itself and prints the result in μs.

The transmit method is synchronous with rapid return. Actual transmission occurs as a background process, on the Pyboard controlled by timers 2 and 5. On ESP32 the RMT class is used. Execution times were measured on a Pyboard 1.1 and the ESP32 reference board without SPIRAM. Tests were done at stock frequency and with validate=True, verbose=False. A small saving could be achieved by skipping validation.

Protocol ESP32 Pyboard
NEC 7.8ms 3.2ms
SONY12 3.2ms 1.3ms
SONY15 3.6ms 1.5ms
SONY20 4.5ms 1.9ms
RC5 4.9ms 1.5ms
RC6_M0 6.0ms 2.0ms
MCE 6.7ms 2.0ms

NEC class

Class NEC. Example invocation:

from ir_tx.nec import NEC

This has an additional method .repeat (no args). This causes a repeat code to be transmitted. Should be called every 108ms if a button is held down.

The NEC protocol accepts 8 or 16 bit addresses. In the former case, a 16 bit value is transmitted comprising the 8 bit address and its one's complement, enabling the receiver to perform a simple error check. The NEC class supports these modes by checking the value of addr passed to .transmit and sending the complement for values < 256.

A value passed in toggle is ignored.

Sony classes

Classes SONY_12, SONY_15 and SONY_20. Example invocation:

from ir_tx.sony import SONY_15

The SIRC protocol supports three sizes, supported by the following classes:

  1. 12 bit (7 data, 5 address) SONY_12
  2. 15 bit (7 data, 8 address) SONY_15
  3. 20 bit (7 data, 5 addresss, 8 extended) SONY_20

The .transmit method masks addr and data values to the widths listed above. toggle is ignored except by SONY_20 which treats it as the extended value.

Philips classes

Classes RC5 and RC6_M0. Example invocation:

from ir_tx.philips import RC5

The RC-5 protocol supports a 5 bit address and 6 or 7 bit (RC5X) data. The driver uses the appropriate mode depending on the data value provided.

The RC-6 protocol accepts 8 bit address and data values.

Both send a toggle bit which remains constant if a button is held down, but changes when the button is released. The application should implement this behaviour, setting the toggle arg of .transmit to 0 or 1 as required.

Microsoft MCE class

Class MCE. Example invocation:

from ir_tx.mce import MCE
# MCE.init_cs = 3

There is a separate demo for the MCE class because of the need to send a message on key release. It is run by issuing:

from ir_tx.mcetest import test

Instructions will be displayed at the REPL.

I have been unable to locate a definitive specification: the protocol was analysed by a mixture of googling and experiment. Behaviour may change if I acquire new information. The protocol is known as OrtekMCE and the remote control is sold on eBay as VRC-1100.

The remote was designed for Microsoft Media Center and is used to control Kodi on boxes such as the Raspberry Pi. With a suitable PC driver it can emulate a PC keyboard and mouse. The mouse emulation uses a different protocol: the class does not currently support it. Pressing mouse buttons and pad will cause the error function (if provided) to be called.

This supports a 4 bit address, 6 bit data and 2 bit toggle. The latter should have a value of 0 for the first message, 1 for repeat messages, and 2 for a final message sent on button release.

The remaining four bits are a checksum which the driver creates. The algorithm requires an initial 'seed' value which my testing proved to be 4. However the only documentation I could find stated that the value should be 3. I implemented this as a class variable MCE.init_cs=4. This enables it to be changed if some receivers require 3.

4. Principle of operation

The classes inherit from the abstract base class IR. This has an array .arr to contain the duration (in μs) of each carrier on or off period. The transmit method calls a tx method of the subclass which populates this array. This is done by two methods of the base class, .append and .add. The former takes a list of times (in ) and appends them to the array. A bound variable .carrier keeps track of the notional on/off state of the carrier: this is required for bi-phase (manchester) codings.

The .add method takes a single μs time value and adds it to the last value in the array: this pulse lengthening is used in bi-phase encodings.

On completion of the subclass .tx, .transmit calls .trigger which initiates transmission as a background process. Its behaviour is platform dependent.

4.1 Pyboard

Tramsmission is performed by two hardware timers initiated in the constructor. Timer 2, channel 1 is used to configure the output pin as a PWM channel. Its frequency is set in the constructor. The OOK is performed by dynamically changing the duty ratio using the timer channel's pulse_width_percent method: this varies the pulse width from 0 to the duty ratio passed to the constructor.

The duty ratio is changed by the Timer 5 callback ._cb. This retrieves the next duration from the array. If it is not STOP it toggles the duty cycle and re-initialises T5 for the new duration. If it is STOP it ensures that the duty ratio is set to the _SPACE

Here .trigger appends a special STOP value and initiates physical transmission by calling the Timer5 callback.

4.2 ESP32

The RMT class now supports carrier_freq and carrier_duty_percent constructor args, so the base class IR (in __init__.py) uses these to enable the OOK (on-off keying) waveform.

The .trigger method calls RMT.write_pulses and returns with RMT operating in the background.

4.3 Duty ratio

In every case where I could find a specified figure it was 30%. I measured that from a variety of remotes, and in every case it was close to that figure.

5. Unsupported protocols

You can use the receiver module to capture an IR burst and replay it with the transmitter. This enables limited support for unknown protocols. This is strictly for experimenters and I haven't documented it in detail.

There are two limitations. The first is timing accuracy: both receiving and transmitting processes introduce some timing uncertainty. This is only likely to be a practical problem with fast protocols. In brief testing with a known protocol the scripts below worked.

The more tricky problem is handling repeat keys: different protocols use widely varying approaches. If repeat keys are to be supported some experimentation and coding is likely to be required.

The following captures a single burst and saves it to a file:

from ir_rx.acquire import test
import ujson

lst = test()  # May report unsupported or unknown protocol
with open('burst.py', 'w') as f:
    ujson.dump(lst, f)

This replays it:

from ir_tx import Player
from sys import platform
import ujson

if platform == 'esp32':
    from machine import Pin
    pin = (Pin(23, Pin.OUT, value = 0), Pin(21, Pin.OUT, value = 0))
else:
    from pyb import Pin, LED
    pin = Pin('X1')
with open('burst.py', 'r') as f:
    lst = ujson.load(f)
ir = Player(pin)
ir.play(lst)

The ir_tx.Player class is a minimal subclass supporting only the .play method. This takes as an arg an iterable comprising time values of successive mark and space periods (in μs).

The ir_rx.acquire.test function makes assumptions about the likely maximum length and maximum duration of a burst. In some cases this may require some modification e.g. to instantiate IR_GET with different args.