RC-5 36kHz carrier absent??

[AmitK]

We tried to decode RC-5 waveform from an RC5 remote on a DSO using a circuitry consisting of a photo transistor (TIL 78)amplifiers and comparators..it is a pretty neat and working circuit and we actually managed to see perfect 14-bit manchester pattern for various controls on the remote, as can be seen from picture 1-remote waveform.

But the problem is, where is 36 kHz carrier??? We are not using a TSOP IC, its a general phototransistor, so it should decode all the frequency components AS IT DID when we actually generated our own RC5 waveform using 8051 and 555. In the wave we generated and then decoded using the SAME circuitry, we could clearly see the 36kHz carrier inside the information bits, as in Picture 2-generated waveform.

we kept the same DSO controls in both the cases but actual RC5 remote is not showing the 36kHz carrier but the waveform we generated as per theory of RC-5 is showing the carrier on DSO using same receiver circuitry!! We are confused! Do actual RC-5 remotes use 36kHZ carrier?? Some one kindly help us....!

 

[Nigel Goodwin]

Yes, RC5 remotes use 36KHz modulation - they wouldn't work otherwise.

BTW, why use a 555 along with your 8051?, it's absolutely trivial (and more accurate) to generate the 36KHz (or whatever) directly in the software.

 

[mneary]

I would guess that the "real" RC5 remote is much stronger than your 8051 version and that it overloads your home made receiver. The overloaded photo detector is taking more than 14 microseconds (1/2 cycle 36kHz) to recover from saturation and does not "see" the dark periods of the modulation. Try greater distances or indirect/reflections.