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Re: TCT was Re: good scopes for coiling



Original poster: "Gerry Reynolds" <gerrytesla@xxxxxxx>

Hi Steve,

Even with old 7400 series TTL, I never have believed in leaving inputs floating. We always tied these pins to Vcc thru a resister (cause they couldn't take input volts much more than 5V. Newer series could take inputs to 7V so it was then OK to tie to +5V. Open TTL inputs would float at 1.5V (right at the switching threshold) and be sensitive to noise. The 0.4V input spec was for a guaranteed low level, but they would typically switch at 1.5V. BTW, with CMOS inputs, they more often float at 0V but could be pulled high internally by a leaky oxide.

For those that want good 50% duty cycles, it is often done by creating a 2x frequency and using a D flipflop to do a divide by 2. Often, just plain oscillator chips only guarantee their duty cycles to within 40/60.

Gerry R

Original poster: Steve Conner <<mailto:steve@xxxxxxxxxxxx>steve@xxxxxxxxxxxx>
However, IME, the chip works just fine with it floating. It's the same kind
of input as on the old 74 series TTL- you need to pull it right down to .4v
above ground and sink some current out of it before it actually does anything.


> "Inaccurate 50% Duty Cycle schematic."

I think they just mean that the duty cycle isn't going to be quite 50%.
This circuit always worked fine for me using either bipolar or CMOS 555s,
but I never used it in any application that needs 50.00000000% duty cycle.
Terry's tuner only needs "a sort of square wave looking thing"- the duty
cycle is not at all critical- so it is a total non-issue.

For 555 fetishists- there is an alternative circuit with steering diodes
that is claimed to give a more accurate 50%. There is also a variation with
a pot to adjust the duty cycle from almost 0% to almost 100% without
changing the frequency much. It's great for PWM motor drives and such like.

Steve Conner.. down to two 555s per coil now :)