System overview

UM1546

6/17

Doc ID 023229 Rev 1

2

System overview

2.1

Hardware design description

Figure 2.

Wiring block diagram

2.1.1

STEVAL-IHM037V1

This application uses two timers and two external interrupts, one each for decoding the IR

remote signal and the other pair for detecting the zero crossing and timing the firing angle of

the TRIAC for fan speed control. This application can be easily adapted for remote control of

light dimmers, room heaters, water heaters, window pane/curtain control, home automation

and other similar applications. The TRIAC used in this design is rated at 4 A current. This

may be changed to suit the application requirements while keeping everything else the

same. The same MCU can even control multiple TRIACs depending on the application.

2.1.2

RC5 protocol

RC protocol basics:

The RC5 code is a 14-bit word, it uses bi-phase modulation (also called Manchester coding)

of a 36 kHz IR carrier frequency. All bits have an equal length of 1.778 ms, with half of the bit

time filled with a burst of the 36 kHz carrier and the other half being idle. A logical zero is

represented by a burst in the first half of the bit time. A logical one is represented by a burst

in the second half of the bit time (refer to Figure 1). The duty cycle of the 36 kHz carrier

frequency is 33% or 25%. This reduces power consumption for the IR LED on the

transmitter side and also allows the LED to operate at a higher peak current for a longer

communication range.