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Offline DOOLEY1972
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« Reply #10 on: June 02, 2011, 01:58:29 am »
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yes , thats it , see there is an output transistor attatched , depending on pnp or npn , you can alter with transistor and playing with resistors to get desired duty and frequency.

or just use diode , and connect directly to a mosfet.

   if you have an android phone or ipad , you can download "electrdroid" it's free , and has recently added to all its very usefull functions a 555 calculator , including a calculator for a 555 with a diode in it to have duty below 50%.

555 is a brilliant , universal chip , but , if you find the circuit on the net for the "blind squirrel" pulse induction metal detector , look at the way he has done all the timing with just 2 chips and minimum components , tx pulse with adjustable width , dip switch adjustable frequency , and delay and sample width in just 2 chips insted of using 3 or 4 555's , i was very impressed to the point where i think i'll incorperate that idea into a design intead of 555's just for compact simplicity.

have fun with this chip , oh by the way , duty can also be controled by voltage on pin 5 , usualy pin 5 has a capacitor to  zero v to reduce noise , but can be held at + v with say a 33k and then use an npn to zero v to vary the voltage from say 12 to 0v which can alter duty from say 50% down to totaly off at zero volts , very useful when using this chip for voltage doubler , so can be used to regulate output voltage via resistor voltage divider set to required output.

thought i'd through that bit in ,

getting the idea of what to build is hard , working out how to build it can be harder , get's easyer with time and plenty of playing.

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Offline 6666Topic starter
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« Reply #11 on: August 05, 2011, 07:51:04 am »
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Hey Dooley
I got around to trying the diode between pins 7 and 6/2
while looking at the output wave on pin 3 I did not see
any difference, just a very tiny amount of wobble in the signal
comments ?

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« Last Edit: August 05, 2011, 08:05:46 am by 6666 »
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Offline DOOLEY1972
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« Reply #12 on: August 05, 2011, 10:02:40 am »
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hello , had a look at the schematic posted earlier , and thats how it should be for duty below 50%

double check the diode is pointing FROM pin 7 TO pins 6/2 join, if its the wrong way round it will not lower duty,

also check diode not open circuit , a simple 1n4148 will be fine.

when you have it right , pin 3 can be used to drive an n-channel mosfet (IRF740,840 etc..etc) direct , which is better than using a transistor drive with resistor pull down , using pin 3 direct will switch off mosfet alot quicker.

below is the setup i use on my current home brew PI ,

the 100k pot varies freq. from 200HZ to 400HZ approx,

and the 10K pot varies pulse width ,

play with R1 , R2 and C1 for your own requirements.



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Offline 6666Topic starter
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« Reply #13 on: August 05, 2011, 05:55:36 pm »
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Hi
thanks for that cct, I will rejig
and retry, the diode is ok ,
I was just adding it to the strip board cct i knocked up based on
typical 555 pi
cheers

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Offline haikel774
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« Reply #14 on: August 05, 2011, 06:08:58 pm »
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hi6666 Grin


thar'is not pictures for schematic and not pcb?Huh?Huh?Huh?Huh???




thank you

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Offline 6666Topic starter
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« Reply #15 on: August 06, 2011, 06:23:07 am »
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Hi haikel774

I am just using experimenters bread board for 555 circuit
like what dooley showed in his attached drawing
is that the answer to your question ?
cheers

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« Last Edit: August 06, 2011, 06:26:10 am by 6666 »
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