556 timer help...

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dacflyer
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556 timer help...

Post by dacflyer » Sat Dec 03, 2011 9:58 pm

i been looking for a circuit. i caught a glimpse of it and have been trying to duplicate it. but have had little luck. i'll try to explain it, and see if someone might have something similar to it.
its supposed to be a muscle massager of sorts. i am wanting to use it to play with neon flicker bulbs,
different frequencies make them flicker more and make them look more realistically.

the duration pulses (on/off) can be from 0% to 100%
and the frequency i think was from 0Hz to maybe 500Hz.
and both had a common output thru the audio transformer..
something along them lines.. does not have to be exact, once its working i can expiriment with caps etc to get it to do what i want.


anyway it uses a simple 556 timer, a small audio transformer.
2 control pots. 1 controls on/off time ( duration ) the 2nd one controls frequency.

i did see that the output pin-9 goes to a cap and then to the primary of the transformer.
pin 10 and 5 are connected together and go to a 1k resister and then to the base of a transistor.
the transistor drives a led as a visual indicator.

1st pot connects to pins 1 and 2
2nd pot connects to 13 and 14
pins 2 & 6 are jumped together
pins 8 & 12 are jumped together
also seen...
resistor on pins 1 & 14 = 43K
resistor on pins 12 & 13 = 33K
a non polar cap on pins 7 & 8

that's about all i was able to gather.
i thought i could make it work. but when i did the output on pins 5 & 6 would not completely go on/off, thus the transistor would stay full on or dim. but never off..
it was a 2N2222 PNP collector went to ground. emitter went to the led.

i probably could with 2 single 555's but i do not know how to make the frequency and duration work together.
anyone have any ideas on this ?

tried to look for something similar in the radio shack 555 cookbook. but nothing close..

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Joseph
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Re: 556 timer help...

Post by Joseph » Sun Dec 04, 2011 7:29 am

It maybe that if you drive the transistor (2N2222 is NPN) with a voltage divider, it will be able to turn off. I'm wondering if the base-emitter is getting more than about .6v, when that voltage should be closer to zero to turn a bjt off.

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dacflyer
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Re: 556 timer help...

Post by dacflyer » Sun Dec 04, 2011 9:04 am

well the pin 5 & 10 should either be on or off ,nothing in between i thought ?
most 555's i play with, pin 3 is either on or off..
i am thinking something is amiss in the 556 circuit. Hmmm..

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reloadron
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Re: 556 timer help...

Post by reloadron » Sun Dec 04, 2011 2:42 pm

Hi Ya DAC

Would it be like the TENS2 circuit linked on the bottom of this page? They use a pair of 555s driving a MOSFET and get about 120 volts. The circuits for muscle stimulation are called TENS Simulators or TENS machines. I believe they also can be had relatively inexpensive at drug stores.

Ron

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Joseph
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Re: 556 timer help...

Post by Joseph » Sun Dec 04, 2011 3:05 pm

Ideally, the outputs are either completely at one rail or another. CMOS often isn't specified as being able to pass full rail voltage to the output pin, especially under a load.

Also, what you say might be right, the chip could have gotten damaged. If the IC has gotten stressed, it might even have lost some of its original conduction ability.

Normally, the high impedance of the outputs is a trade-off involving economy in manufacture s well as overload protection. Going to the pair of separate 555 ICs, if you can, might help that, since then the output stage is bjt instead of CMOS.

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dacflyer
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Re: 556 timer help...

Post by dacflyer » Sun Dec 04, 2011 5:29 pm

well thinking the ic might be damaged, i put it into a home made 556 tester i built, ya 1 output is leaky.
i replaced it.
its a bit better but not fully..now it is off, but turns on 1/2 way then to full...depending on control settings.
the original device was either on or off,. nothing in between ..
where is Forrest Mimms when i need him :P

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Re: 556 timer help...

Post by Robert Reed » Sun Dec 04, 2011 5:48 pm

" i probably could with 2 single 555's but i do not know how to make the frequency and duration work together.
anyone have any ideas on this ?"

First 555 configured as a one shot of relatively long duration. This in turn controls the second 555 configured as an astable multivibrator. Duration and frequency are your choice with selected RC values. Also try Googling 555 as there is a ton of info on this IC. Some sites are souly dedicated to schematics of these chips with hundreds of circuit diagrams.

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Re: 556 timer help...

Post by k7elp60 » Mon Dec 05, 2011 5:52 am

I used to build tens units. I see if I can find the schematic. It used a 556, both of them were configured as astable MV's. The first MV drove a 2N3904 to drive a 2N2222. A pot to control drive. The 2N2222 drove a 1k/8ohm transformer in reverse to control the output voltage. As I recall the voltage ouput could be as high as 700V. The second MV was a timer to turn on/off the other MV in an adjustable time. All from a 9V battery.
Ned

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Re: 556 timer help...

Post by dacflyer » Mon Dec 05, 2011 8:09 am

that would be excellent if you could share that schematic :)
the unit i looked at was similar, but only 1 transistor driving the leds. the audio transformer was driven directly from the 556 with a cap in between.

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Re: 556 timer help...

Post by k7elp60 » Tue Dec 06, 2011 8:24 am

Here is a direct link to the schematic.
http://s1030.photobucket.com/albums/y365/k7elp601/

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Re: 556 timer help...

Post by dacflyer » Tue Dec 06, 2011 1:45 pm

thanks :D

looking at what i see..
what indication is the green led ? output pulse indicator ?
and on pin 4 & 9 of the 556.. looks like a switch.. what does this do ?

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Re: 556 timer help...

Post by sghioto » Tue Dec 06, 2011 3:10 pm

They're using the output of the duration timer (pin#9) through the switch to turn the variable frequency oscillator ON and OFF by way of the RESET (pin #4). The green LED is on when the oscillator is running.

Steve G

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