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IR remote/Where it stops working/11. When nothing decodes
Where it stops working · 11 of 11

When nothing decodes

There is no error message anywhere in an infrared link, and half of it is invisible. This sketch makes one board send to itself, so a failure becomes a printed line and two red LEDs rather than a silence you have to guess at.

Updated 2026-09-195 min readIR remote

Why a loop-back

Every other test of an infrared link has two unknowns: did the sender send, and did the receiver hear? Neither board can say, because what is between them is invisible and the protocol has no acknowledgement. With both blocks on one board the sketch knows what it sent and checks what came back. A failure becomes a printed line.

Which end is the problem

Which end is the problem
the receiver
The receiver is not seeing a carrier, or is not powered. Watch the TK15's red LED while you press a button. It is wired into SIGNAL, so it flickers whenever the chip pulls the line low, before any code runs. No flicker means power, wiring or distance, not software.

Serial output cannot tell you where a fault is. The two red LEDs can.

The TK16's red LED is on the same transistor as the infrared LED, so it flickers every time a frame actually leaves.

The TK15's red LED runs into SIGNAL, so it flickers every time the receiver hears a carrier, before any code runs.

Between them they split the problem in a second. Sender's LED dark: look at the sender. Sender's flickering and receiver's dark: look at the air, the aim and the distance. Both flickering and nothing printed: look at the sketch.

Run it

Wire both blocks to one board, stand them face to face about 30 cm apart, parts side to parts side, and upload. A healthy link prints:

ok  1 of 1
ok  2 of 2
ok  3 of 3

heard something, decoded nothing almost always means too close. See how far, and how wide.

Five things that are not faults

The sender's LED looks dead. It is 940 nm; use a phone camera.

One press prints several lines. A held key sends a repeat frame every 110 ms. Check IRDATA_FLAGS_IS_REPEAT.

digitalRead on the receiver's SIGNAL always returns 1. That is the idle level; each low lasts 560 µs, and the library samples on a timer for that reason.

The codes differ from every example you have read. They belong to the remote that sent them.

UNKNOWN from an air-conditioner remote. It sends a hundred bits or more; on an Uno, raise RAW_BUFFER_LENGTH before the include, or replay the raw timings.

The code

One board, both blocks, sending to itself once a second. Point the TK16's lens at the TK15's window about 30 cm apart. It prints a line per frame and whether what came back is what went out.

ir_loopback.ino
/*
  IR loop-back test                         TK15 + TK16 / /p/tbir

  Wiring. Count from the square pad on each TinkerBlock board, parts
  up, header at the bottom:

    TK15 receiver
      GND    -> GND
      VCC    -> 5V on an Uno; 3V3 on an ESP32, ESP32-S3 or Pico
      NC     -> nothing   (unconnected on the board)
      SIGNAL -> D2, GPIO 23, GPIO 9 or GP16

    TK16 sender
      GND    -> GND
      VCC    -> 5V, or VBUS on a Pico (only feeds the LEDs)
      NC     -> nothing   (unconnected on the board)
      SIGNAL -> D3, GPIO 22, GPIO 6 or GP17

  Uno, ESP32, ESP32-S3, Pico, in that order.

  Arduino IDE
    Tools > Board                 your board, e.g. Arduino Uno
    Tools > Port                  the one that appears when you plug in
    Tools > USB CDC On Boot       Enabled   (ESP32-S3 only)
    Tools > Manage Libraries      IRremote by shirriff, z3t0 and
                                  ArminJo, version 4 or later
    Serial Monitor                115200
*/

#include <IRremote.hpp>

// Uno: 2. ESP32: 23. ESP32-S3: 9. Pico: 16.
const int IR_RX_PIN = 2;
// Uno: 3. ESP32: 22. ESP32-S3: 6. Pico: 17.
const int IR_TX_PIN = 3;

// Any pair nothing else in the room is likely to send.
const uint16_t ADDRESS = 0x12;
const uint8_t COMMAND = 0x34;

unsigned long sent = 0, back = 0;

void setup() {
  Serial.begin(115200);
  IrSender.begin(IR_TX_PIN);
  IrReceiver.begin(IR_RX_PIN, DISABLE_LED_FEEDBACK);
  Serial.println("Loop-back: sending once a second.");
}

void loop() {
  Serial.flush();                  // no interrupt mid-frame on an Uno
  IrSender.sendNEC(ADDRESS, COMMAND, 0);
  sent++;

  // The library's own SendAndReceive order: restart the receiver,
  // then wait long enough for it to see that the frame has ended.
  IrReceiver.restartAfterSend();
  delay(RECORD_GAP_MICROS / 1000 + 5);

  if (IrReceiver.decode()) {
    IRData &d = IrReceiver.decodedIRData;
    if (d.protocol == UNKNOWN) {
      Serial.println("heard something, decoded nothing: move apart");
    } else if (d.address == ADDRESS && d.command == COMMAND) {
      back++;
      Serial.print("ok  ");
    } else {
      Serial.print("someone else's frame  ");
    }
    IrReceiver.resume();
  } else {
    Serial.print("nothing came back  ");
  }
  Serial.print(back);
  Serial.print(" of ");
  Serial.println(sent);
  delay(1000);
}

Facing the two boards at each other from a few centimetres is the one arrangement that fails while everything works: the receiver is swamped and the decode breaks. Give them 30 cm, or bounce the light off a pale wall.

When it does not work

Nothing prints, not even a failure

The sketch is not running, or not printing: check the port and the baud rate, and on an ESP32-S3 set USB CDC On Boot to Enabled. Every pass prints a line, whether or not anything came back.

It prints heard something, decoded nothing

The usual first result, and it usually means the boards are too close. Within about 20 cm the receiver is swamped and every mark it measures comes out long. Move them 30 cm apart or bounce the light off a pale wall.

Some frames come back and some do not

Losing the odd frame at range or off to one side is normal for infrared, and it is why remotes repeat. Losing most of them by a window is daylight; losing most of them at any distance is aim.

It reports someone else's frame

A real remote in the room, or something that emits infrared: some motion sensors and camera autofocus lamps do. Not a fault, and a reminder that anything you build can be triggered by a neighbour's television.

It worked yesterday and not today

Check what changes without anyone touching the wiring: the remote's batteries, the light through the window, and anything on the same supply that has started switching hard. The receiver has only 100 nF on its supply.

Alongside this page
Why a noisy supply reaches a sensitive chip
Where this goes next

The sender's specifications, wiring and files in one place.

The TK16 reference page →

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