Re: TCT was Re: good scopes for coiling

["Tesla list" <tesla@xxxxxxxxxx>]

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 :)