IR TRANSMITTER

IR CONTROL

Using pulsed IR (InfraRed) is a way to provide short distance command and control to a variety of applications. Some advantages to IR control are low cost, freedom from most types of interference and the ability to make secure the operation by designing a proprietary transmission code. An operational distance of up to 25 feet is easy to achieve and longer distances (>100 feet) are possible using lenses. Line of sight between the transmitter and receiver is necessary for operation.

Infrared radiation can be broadly described as electromagnetic radiation that starts at the limit of visible light at the red end of the spectrum and extends to the upper limit of the radio spectrum. The infrared spectrum, which has a wavelength spanning  from about 700 nanometers (0.7 micron) to about 1000 microns, can be further divided into four different bands; Near Infrared, Middle Infrared, Far Infrared and Extreme Infrared.

IR TRANSMITTER DESIGN

This is a simple IR transmitter that uses a FREESCALE MC9S08SH4 microcontroller and has three selectable transmission channels. Each channel has an indicator LED to indicate the active channel. It is possible to have up to a total of eight channels by lighting more than one LED at a time to indicate channel selection. A Bourns PEL12S encoder is used to select channels. This encoder has an LED lighted shaft that is used as a power on indicator. An encoder was chosen since it is cheaper and more versatile than a miniature rotary switch and can be used to create any number of functions through MCU programming.

Using the MC9S08SH4’s TIMER (TMR1, Channel 0) in output compare mode, the timer is programmed to create a burst of exactly fifty pulses at a precise period (frequency) with a duty cycle of 50%. Channel one has a period of 250 microseconds (4000 Hz). Channel two has a period of 500 microseconds (2000 Hz). Channel three has a period of 1000 microseconds (1000 Hz).

A hardware interrupt controlled by a normally open push button switch is used to send the pulse data. Each time the pushbutton is pressed, a burst of fifty pulses of the selected frequency is sent to the IR LED’s. A red LED provides a visual indicator that the pulses are being sent. Using this method, the IR receiver would be configured to measure and average the period of the pulses.

Three Vishay VSLY5080 Hi-Power IR LED’s in series are used as the optical output. These LED’s have a peak output wavelength of 850 nm and are suitable for high pulse current operation. The LED’s are switched on and off to ground by a 2N2222 transistor. A 30 ohm resistor limits the LED current to about 100 mA at 8 VDC.

The power supply consists of a 9VDC battery with a 78M05 regulator used to provide +5VDC to the microcontroller. A 9V battery was used because of its reasonably small size and also allows series operation of the IR LED’s.

IR TRANSMITTER SCHEMATIC (PDF)

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IR TRANSMITTER PCB (PDF)



FINISHED TRANSMITTER PCB