3 Implementation

The figure below shows the complete schematics for a remote control transmitter. See the table for component descriptions. The 455kHz resonator gives the application a reliable and flexible clock base. The external LED driver circuit provides a constant current for the IR-LED. Resistor R3 determines the driver strength, and is in this application chosen to 7Ω giving a drive capability of approximately 100mA. Higher resistor values will reduce current, and lowering the resistor value will increase driver strength. The diodes, D1 and D2, are present to ensure a close to constant driving current and to compensate for temperature variations in the transistor.

Figure 3-1. RC5 Transmitter
Table 3-1. Components
TypeComment
R13kΩExternal pull-up resistor is present to make the system less susceptible to external noise. Without this component, noise might reset the microcontroller.
R23kΩ
R2⁢= VCC⁢min⁡−1.42ILβ
R37Ω
R3⁢= 0.7IL
C1100pFResonator dependent
C2100pFResonator dependent
D11N4148Small signal diode
D21N4148Small signal diode
Q1BC807-40TD
IMAX=0.5A,       β=250⁢−600
XTAL455kHzResonator