Interesting IR 38kHz observation

(Well, it interested me, anyway)

I found this site where the guy made a 38kHz carrier wave transmitter with a 555. Nothing surprising, just a matter of speccing the components to get the frequency, comes out at 37.6kHz in fact.

But, with the components given, it’s very asymmetric as shown in the blue 'scope trace below: about 85% high (22/26). (I actually have a 4k7 instead of the 10k variable.)

Yet, as shown by the yellow trace, the TSOP goes low as it should for a 38kHz carrier. I had just assumed the carrier needed to be 50/50, but on reflection I guess there’s no reason it should be: all the TSOP looks for is the edge of the next cycle I suppose.

Next test is to find another 555 and use that to trigger the first one, to superimpose a message on the carrier. I think I should be able to that on its /reset pin.

38kHz 555 IR.jpg

Where is the Arduino ?

Two 555's at Christmas? That is one scary Christmas tale.

Perhaps an ATmega or ATtiny chip might be cheaper than a 555 with a few components.

Arduino not in the loop yet: just reading the TSOP standalone- I was just trying to get my mind round a few things from (more or less) first principles. I'll probably have a bash at reading the TSOP with pulsein() later on.

Yep, no expense spared here, 2x 555s, pretty frivolous actually. Next time I buy any I might get 556s instead.

Watch this space for a simple tutorial on this whole modulation thing: it seems to confuse folks, that there are really "two signals in one", namely the message and the carrier.

Currently formulating it in my head and capturing rough 'scope shots, will write it up tomorrow. What are holidays for anyway?....

(Just in case you're wondering, Johannesburg in the shade right now at 14h00 on 25th December, 30C and a few clouds. Probably going to get a scary thunderstorm later.)

Indeed a carrier. It is like an AM/ASK on/off transmitter. The 38kHz is the carrier, and the modulation is on/off. So the receiver can tune in at 38kHz and that makes it possible to detect the signal between the noise. That shouldn't be so hard.