Serial IR Transmitter

I built this simple infra-red transmitter circuit to control various set top boxes and TV's from my linux media machine.

The website for all things Linux and infra-red is at http://www.lirc.org/ and this circuit is based on the Simple transmitter circuit here.

The schematic is below.

This is essentially the simplest circuit with additional indicator LED's and appropriate resistor values. The dotted line is to indicate that the IR LED is separated by a length of wire (telephone cable in this case) from the rest of the circuit.

The linux infra-red daemon lircd pulls the RTS (ready-to-send) serial line positive (about +12V) as part of its initialization. This circuit uses that to light a green LED indicating that the driver is loaded and basic connections are at least superficially in order.

Lircd transmits IR signals as a series of 38Khz pulses on the DTR (data-terminal-ready) line. Equipment with IR receivers will pick-up the pulses from the IR LED. Humans can see the pulses from the red LED.

This is a low power IR transmitter intended to be attached to the front of or very near (1-2 ft.) the receiver box. I've found it very reliable in this range. Both LED's are visible and the red flash LED easily catches the eye.

If you know nothing else about electronics theory you should at least learn Ohm's law. It easily determines the entire operation of this circuit. Note the following points:

• I measure the positive RTS and DTR voltage at about +11.5V on my machine. It's not recommended that you draw more than say 10-12mA from the serial chip. Using these values and applying Ohm's law to the green LED:
(1)
\begin{align} draw = \frac{11.5 Volts}{4.7 k\Omega} = 2.45mA \end{align}
• RTS and DTR may also be negative and -12Volts would likely damage the LED's. The 1N914 (or 1N4148) diode protects them. There should also be a diode protecting the green LED but I've not found this necessary with my specific hardware.
• 1N914 is the radio shack component number for the diode. Anything reasonable would work here and other circuits suggest 1N4148. The green and red LED's are smaller ones from a radio shack variety pack and the IR LED was removed from an old remote control. The 4.7kΩ resistor is colored yellow/violet/red and the 3.3kΩ orange/orange/red. See http://www.dannyg.com/examples/res2/resistor.htm
page revision: 36, last edited: 15 Dec 2008 22:18