Tesla Coil Tuner and 555

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Paul B. Brodie" <pbbrodie@xxxxxxxxxxxxx>

Steve,
It's actually Terry 99, Paul 0! I was confused. Terry's schematic shows pin 
#4 connected to the power supply. It's the example on the Internet that 
lacks a connection on pin #4.

You must have missed Terry's post. The new CMOS 555 can indeed produce a 
true 50% duty cycle square wave with it wired like Terry's. The 
manufacturer's data sheet for the LMC555 shows this method clearly.

You are absolutely correct about the bipolar 555 not being able to produce 
a 50% duty cycle  when wired in the astable multivibrator mode. There is an 
example on one of the data sheets I located (there are more than a half a 
dozen different ones, at least that shows how to get a 50% duty cycle but 
it requires a lot of additional components.

You are also correct about the output required for what the Tesla Coil 
Tuner is trying to accomplish and that was one of the things that had me so 
confused. I couldn't understand why it wasn't wired in the "traditional 
manner" as an astable multivibrator. I kept thinking that there was 
something it was supposed to do that I was missing. As it turns out, I was 
way over analyzing the thing. Terry actually used the easiest, simplest, 
least number of parts solution for what the TCT is designed to do and it 
works like a champ.

I have built over a dozen 555 circuits and looked and looked at over 6 
different data sheets. I had never seen this particular way to accomplish 
the 50% duty cycle before now. I don't understand why it is only included 
on this one data sheet. One thing I do know is that the 555 is one of the 
most versatile and useful IC's I have ever seen. Regards.
Paul
Think Positive

----- Original Message -----
From: "Tesla list" <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
Sent: Saturday, May 21, 2005 2:31 PM
Subject: TCT was Re: good scopes for coiling

> Original poster: Steve Conner 
<<mailto:steve@xxxxxxxxxxxx>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 :)
>
>
>