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Offline 6666Topic starter
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« on: March 25, 2010, 02:00:03 am »
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There are many PI ccts

some have one 555 providing the timing pulses to the coil

some have two 555's providing the timing pulses to the coil
(one driving the next)

is there any real advantage in using two 555's over only one

what are two giving ? IE: better pulse control or shaping or what

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Offline Homefire
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« Reply #1 on: March 25, 2010, 07:11:29 am »
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The only circuits I've seen using two 555 the first provided the basic timing.

The secound provided the delay for samples.

 Idea

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Offline xavier
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« Reply #2 on: March 31, 2010, 03:39:23 pm »
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Gary's pulse uses five one for the audio and as far as I can remember one is used for the basic frequency one for the dead time one for the on time and the other for the pulse width Like Homefire said when there are two 555 the one is for the frequency and the other is the thresh hold or rather the on time, can you confirm that Homefire.

Regards Xavier

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So many questions so little time

Offline Homefire
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« Reply #3 on: March 31, 2010, 04:39:13 pm »
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Don't Remember the Exact but for sure one was for basic timing, next for Delay, and Other for Audio.


As they all Drift together there was not Problems.

Jitter could be a problem.

I'm thinking there was yet another used for Intigration.   Wise

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Offline 6666Topic starter
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« Reply #4 on: March 31, 2010, 10:16:00 pm »
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hi thanks for the reply
garys has 5 555's, four for timing and one for audio

but what I am trying to ask is what is the advantage
of useing two 555's (or a 556 configured as two 555's)
to provide the pulse that turns the coil fet, on and off
not the rest of the circuit timing, for this question

What is the theory of useing two 555's like in the XR-71, ( it uses a 556 configured as two 555's)
most ccts are able to provide a suitable pulse from just one 555
like in the sandbanks circuit, or whites surf PI.

some ccts that use just one 555 , have pots
to adjust the pulse width (power ?) and Freq.

 all I am asking is, does anybody have a theory why two 555 are possibly technically better to fire the coil FET?
one as an oscillator, one as a one shot, I think that's what the second 555 would be called ?
the designer must have some reason, does two give a better shorter, sharper, squarer, more accurate pulse by being able to alter some characteristic of the pulse that you cannot do with just one 555.

thanks
 


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« Last Edit: March 31, 2010, 10:41:22 pm by 6666 »
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Offline xavier
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« Reply #5 on: April 01, 2010, 06:17:06 pm »
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Like we told you the second is for the on time the first for frequency and it's a very simple way of doing it.

regards Xavier

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So many questions so little time

Offline 6666Topic starter
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« Reply #6 on: April 01, 2010, 07:19:57 pm »
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ok thanks
I will build both ccts now and see what the difference in characteristics there are with the on time

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« Reply #7 on: April 26, 2010, 08:07:27 pm »
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Hello 6666,
Better use the timing like the Goldscan  with 4538 Ic's  more precise timing that 555....
Alexis.

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Offline DOOLEY1972
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« Reply #8 on: May 30, 2011, 01:46:05 pm »
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hello ,the 555 question , there are many ways of doing things,

a 555 can provide all the frequencys we want BUT , duty cycle or on time can only be varied between 50 % and 99% with a standard circuit , so on gary's PI and many others , the first 555 makes the frequency and the second is triggered to provide an output duration of aprox 160us on time (+output), it uses 2x555 to do this.

option 2 :
set the 555 to have 160us OFF time and use the zero volt output to drive a p-channel mosfet or pnp transistor,
this only needs 1X555

option 3:this is what i have done.
setup a 555 to do the frequency you require , then use a diode allowing current from pin 7 to the pin 2 and pin 6 join ,
this charges the capacitor fast , and the resistor discharges it slow , which restults in a short on time which can be varied with the pin 7 feed resistor to + whilst the pin 7 to 6/2 resistor varies the frequency.

the 555 chip is very versatile , and can be made to do anything from 5% duty to 95% duty.

the other 2x555's on garys PI are used in this way:
1st one gets triggered from the outut END of the TX timming output,waits for the delay period,
then triggers the 2nd 555 , which has a high output for the desired sample time.
all straight forward stuff , but there is no shortcut to using 2x555 for delay and sample,
but the 2x555's for TX pulse can be reduced to using only one by adding a 1n4148 or similar diode as mentioned above.

still , dam fine design , and using one more 555 than needed is neither here or there.

search for "blind squirrel" on the net , now that timming method i like simple , easy to play with , uncomplicated , when you get your head round it , only 2 chips used to do the lot TX ,delay and sample,and not many external components,
 i like it.





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Offline 6666Topic starter
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« Reply #9 on: June 01, 2011, 11:34:09 pm »
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Thanks dooley
I will have a read and digest


something like this ? pins 6/7/2




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« Last Edit: June 02, 2011, 12:18:27 am by 6666 »
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