Electronic – 555 astable operation not getting stable frequency

555

I needed a 38 kHz wave no matter the shape. So I started off with a 555 timer to produce a 38 kHz rectangular wave using its astable mode. Here's the circuit:
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For that I used R1=9.8kohm, R2=17.6kohm, and C=820pF. At the beginning, the frequency I measured was right at 38.2 kHz but as time passes by the output frequency started scaling up for as much as 2 kHz. I mean, the frequency increased by 2 kHz in less than half an hour. I tried to change the type of the capacitor just to make sure that the capacitor was ok. But I didn't get a constant freq. either. Maybe it's with 555 timer that makes it behave like that?

Best Answer

If you're building an oscillator for communication purposes, C-charge based oscillators won't do – aside from a very few speciality devices, capacitors are rated for 5%, 10% or even 20% tolerance. So I'd say your circuit operates well within the physical boundaries of what your components offer.

I'd strongly recommend just getting something that has a trimmed internal oscillator – that something might actually be an 38 kHz oscillator, or, even simpler, a microcontroller generating 38 kHz with e.g. a PWM unit. This option sounds twice as reasonable considering that when communicating over IR, you most probably already have some digital logic!