TCT was Re: good scopes for coiling

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Original poster: Steve Conner <steve@xxxxxxxxxxxx>

At 19:25 20/05/05 -0600, you wrote:
> I still don't understand why you left off the supply voltage connection to 
> pin #4.

Woo! Terry is getting his a** kicked! Pin 4 is the "Reset" pin and sure 
enough, all 555 datasheets say you're supposed to connect it to Vcc if it's 
not in use.

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.

I haven't used a CMOS 555 in ages though, they may well be more sensitive.


>  "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 :)