IR LED's

Jay Kickliter

I made a simple Propeller circuit to flash a visible LED when the IR receiver detects IR pulses from my Apple remote. It worked. My next step was to use the IR LED's included in the Prop education kit to see if I can establish a serial connection: computer -> prop -> IR led -> IR receiver -> prop -> computer. But I can't get the IR receiver to detect the light from the IR LED's included with the kit. I tried both the little black one and the clear one. Naturally, I can't see if they are lighting up, but colored LED's in their place do light. Is there some basic concept I'm missing? In the long run I'm trying to get the prop to communicate with my HP 48gx calculator.

Mike Green

Most IR communications involves a modulated 38KHz signal. The IR receiver has a filter in it and won't respond unless the IR signal is close in frequency to what the IR receiver is designed for (usually 38KHz). You will need to produce a modulated 38KHz pulse stream. Download the Frequency Synthesis object or the BS2 Compatibility Library from the Object Exchange. Both can generate a 38KHz pulse stream. You can turn that on and off in your program to make a modulated pulse stream. The HP48gx may be using IRDA which is a protocol for data transmission over an IR link. This is complex and may be better implemented with an external I/O chip designed for this use like the MAX3130.

Jay Kickliter

Thanks, I didn't realize the receiver was anything more than a dumb light meter. I've attached page 7 from HP 48 I/O guide. Looks it transmits just like inverted TTL serial, except that instead but each pulse occupying the entire bit frame, the pulse only occupies the first 52 us of each bit. Is there such thing as a dumb IR sensor?

grasshopper

Also for testing IR-leds use a digital camera and either look through the LCD screen or take a picture of the led. This can show you if the led is working. As far a receiving the signal I like to use a voltage divider type of circuit. Making sure to adjust the resistor value so that when IR is present it is high (or low) and if no IR then the opposite takes place.

Mike Green

An IR phototransistor is a "dumb" IR sensor.

The short (52us) pulses are typical of IrDA. The brief pulse conserves power. The MAX3130 and similar IrDA drivers/receivers are designed to generate these short pulses and to stretch out the short pulses on receive so the microcontroller or included UART can respond to them.