Hi,
Its a very ordinary 555 Astable thats been working well for six weeks or more as a 38Khz IR modulator. The noise is only a problem when it is installed in an RC Car and there is no transmitter signal - at which point the car electronics, particularly the servo is very susceptible to noise in the power circuit.
I have run the circuit in the car with its own power and it is fine, I have run the circuit in the car with the 7805 and it is fine, however using passive components - capacitors of all types and values and in various combinations across the power lines I cannot eliminate the noise.
While I would not normally have the car turned on without a transmitter signal its no reason not to eliminate the noise that is clearly there.
So for the sake of clarity my current solution is the 7805 its working perfectly, the inspiration came from here -
http://williamson-labs.com/480_dec.htm
And the fact that my Arduino is powered through a regulator and while switching loads causes no interference where sharing power with the rest of the car.
But back to the original question - while it works is it a legitimate approach or is there something I am missing that could be achieved with passive components ?
Duane B