Quote:
Originally Posted by kellys_eye
The 7555 is powered via a 10k resistor (R5)?
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Thanks for reading the thread and for your well observed comment expressing surprised at the 10K resistor in the supply line to pins 4&8 of the 7555 IC.
I just built it as designed, which shows a 10K resistor (R5) to pins 4 & 8, and a further 10k (R4) from there to pins 6&7.
When the originally specified 555 IC was substituted with an LM7555 it worked fine, but I don't pretend to understand how the circuit works! I'd agree that ordinarily, pins 4 & 8 of 555 circuits as often as not receive the full supply rail Voltage, as in the circuit I've added at pic 3 of a little 555 tester I built to test a batch of 555s I'd acquired back in 2015. Similarly, the square wave section of the simple square/sine wave function generator at pic 4 below using a 555 shows pins 4 & 8 receiving the full supply Voltage.
I don't like posting indistinct, scrappy or hand-drawn circuits on the forum, which is why I re-drew the original Sawtooth Generator circuit as supplied to me by Colin, but for the sake of completeness, I've attached the original circuit below, alongside the one that I re-drew. As will be seen, the original used the U.S. symbol for fixed capacitors and values for some components that are now non-standard (.02, not .022, 500k not 470k) so I updated those aspects. I also used rectangles for resistors - not zig-zags and added the 78L12 regulator, which is on the PCB layout, but not on the original circuit.
As earlier explained, the sawtooth generator was used by Colin, myself and maybe a few other constructors as an add-on to the RF Haigh Wobbulator design originally published in Radio Bygones #82 in April/May 2003, and some time later, in the BVWS Bulletin using long since unobtainable TOKO coils. Obviously it could be used with other Wobbulator designs if desired.