Author Topic: Duty cycle of 38KHz IR carrier  (Read 2693 times)

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Peabody

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Duty cycle of 38KHz IR carrier
« on: April 01, 2018, 07:11:57 pm »
I'm working on a controller to automate several things for my Sharp TV, and have found pretty much all the specs for the IR remote controller to accomplish that.  I just assumed the duty cyle of the 38KHz carrier should be 50%, but then I found a Sharp protocol description that says the recommended duty cycle is 25-33%.

I also found a Vishay PDF that has the following:

"The duty cycle of the carrier frequency can be between 50 % and 5 %.  A remote control system using a Vishay IR receiver is more efficient regarding battery power consumption on the emitter side if the carrier duty cycle is low. This is shown in the following example:

• Carrier duty cycle 50 %, peak current of emitter IF = 200 mA, the resulting transmission distance is 25 m

• Carrier duty cycle 10 %, peak current of emitter IF = 800 mA, the resulting transmission distance is 29 m"

So you increase the transmit power four-fold, but reduce the duty cycle by 80% and the average current by 20%, and you get 16% more distance.  Not bad.  I don't need to know for my project, but just wondered what the math is behind this.  Presumably the current limit of the transmitting IR LED would limit the distance improvement at some point, and I wonder if having the duty cycle too short might effect the performance of the receiver.  I have no information on the Vishay receivers other than this PDF:

https://www.vishay.com/docs/80069/circuit.pdf

So what is this all about?

Benta

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Re: Duty cycle of 38KHz IR carrier
« Reply #1 on: April 01, 2018, 07:19:53 pm »
Simple.
Switching speed of the IRLED and IR-receiver is waaaay faster than 38 kHz, so only the emitter power plays a role. By reducing the duty cycle, you can increase the peak power without increasing power dissipation in the IRLED.

Smf