556 (dual 555) PWM for LED design

[Michael Shannon]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

 

[John Popelish]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

I haven't gone over the circuit with a fine tooth comb, but I see a
couple problems.

1. Both trigger inputs are grounded. Move them to the top of their
capacitors.

2. Both caps are pulled up to within one diode drop of the 9 volt
supply all the time. You need some timing resistance in series with
the pull up diodes.

 

[Rich Grise]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

Yes. First, redraw the schematic so that it's actually a schematic.
What you've given us is a "pictorial", that shows us the pin connections,
yes, but doesn't really impart any sort of sense of what the circuit
is trying to do, if you get my point.

In other words, there should be two separate rectangles, to represent
the two separate timer functions, even though they're in one package.

My lazy part wants to just issue the RTFM directive, but my nurturing
inner mommy wants to go find you a good schematic:
[quick web search]
Yeah, I decided lazy - you'll have to sift through these for yourself:
http://www.google.com/search?q=556+dual+timer+circuits
Just for S&G, I entered this by hand - try it:
http://www.google.com/search?q=555+timer+circuits

Have Fun!
Rich

 

[Michael Shannon]

I posted a new circuit (just on one half, for simplicities sake) with
a slightly different configuration....

But it still doesn't quite work right. =]

-Michael

 

[Terry Pinnell]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

Whatever else may be wrong with it, both the LEDs are the wrong way
around! Their cathodes, not their anodes, should be at 0V.

 

[Terry Pinnell]

I posted a new circuit (just on one half, for simplicities sake) with
a slightly different configuration....

But it still doesn't quite work right. =]

-Michael

(I don't see a link to a new circuit?)

Anyway, if you make all the corrections shown here (for the left hand
section)
http://www.terrypin.dial.pipex.com/Images/556VariableDCAstable.PDF
it will work OK.

Altogether there were 4 errors:
1. LEDs wrong polarity
2. Trigger inputs grounded
3. Diode D2 wired wrongly
4. Need a limiting resistor R1a.

 

[John Fields]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

1. The 9V supply is shorted to ground
2. RESET2 is wired low
3. The LEDs are backwards
4. The TRIGGER inputs are wired low
5. You've got too many diodes in there
6. The diodes are hooked up wrong
7. You need a series resistor between the top of the pot and +9V



Here's what you need to do:


+9V>-----+----------------+
| |
[R1] +---+---+
| |_ Vcc |
[POT]<--+-----O|D OUT|--------->OUT
| |A |_ |
| [CR1] +-O|T C|---+
| | | | | |
+-----+---+--|TH | |
| +---+---+ [C]
[Ct] | |
| | |
GND>-----+----------------+-------+----->GND

 

[John Fields]

Yes. First, redraw the schematic so that it's actually a schematic.
What you've given us is a "pictorial", that shows us the pin connections,
yes, but doesn't really impart any sort of sense of what the circuit
is trying to do, if you get my point.

 

[Fred Bloggs]

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

View in a fixed-width font such as Courier.



+-----+
| |
+---+--/\/\/\/\----+
| |
| |
| ---
--- \ / ~~
- ---
| |
| |
+------------------+

 

[Michael Shannon]

My thanks to everyone who is helping me with this. =]

Yes, I have the LEDs the right way on the board, I just put them
backwards on the diagram. =]

Are the diodes on either side of the pot necessary? One diagram I
found said yes, the other said no, so I don't know if they are
necessary. I fixed the shorted +9v, and the grounded trigger...my only
other question is what size resistor I need.

New diagram is up at the same link as the last:
http://terralthra.net/LEDPWM555.jpg

Hello...I just need some help with a circuit design I'm working on...

http://terralthra.net/LEDPWM556.jpg

That's the basic design I'm working with...it's a 556 chip in the
center, basically two 555s with a common Vcc and GND. It's supposed to
be in astable oscillator config, with pots instead of Ra/Rb, for LED
dimming via PWM.

Unfortunately, I can't get it to work on the breadboard.

Suggestions?

1. The 9V supply is shorted to ground
2. RESET2 is wired low
3. The LEDs are backwards
4. The TRIGGER inputs are wired low
5. You've got too many diodes in there
6. The diodes are hooked up wrong
7. You need a series resistor between the top of the pot and +9V



Here's what you need to do:


+9V>-----+----------------+
| |
[R1] +---+---+
| |_ Vcc |
[POT]<--+-----O|D OUT|--------->OUT
| |A |_ |
| [CR1] +-O|T C|---+
| | | | | |
+-----+---+--|TH | |
| +---+---+ [C]
[Ct] | |
| | |
GND>-----+----------------+-------+----->GND

 

[Terry Pinnell]

My thanks to everyone who is helping me with this. =]

Yes, I have the LEDs the right way on the board, I just put them
backwards on the diagram. =]

Are the diodes on either side of the pot necessary? One diagram I
found said yes, the other said no, so I don't know if they are
necessary. I fixed the shorted +9v, and the grounded trigger...my only
other question is what size resistor I need.

New diagram is up at the same link as the last:
http://terralthra.net/LEDPWM555.jpg

Details, details... The link is indeed the same as before, i.e.
http://terralthra.net/LEDPWM556.jpg

If you remove the diodes, you lose the full range PWM control. Its
range will be halved, covering only 50%-99% duty cycle.

You've not added the limiting resistor John Fields and I suggested.
What do you believe will happen when your pot is at the end of its
travel, so that there is zero resistance from its upper end and +V?

I got the impression from your original post that this circuit came
from an existing design?

 

[Michael Shannon]

My thanks to everyone who is helping me with this. =]

Yes, I have the LEDs the right way on the board, I just put them
backwards on the diagram. =]

Are the diodes on either side of the pot necessary? One diagram I
found said yes, the other said no, so I don't know if they are
necessary. I fixed the shorted +9v, and the grounded trigger...my only
other question is what size resistor I need.

New diagram is up at the same link as the last:
http://terralthra.net/LEDPWM555.jpg

Details, details... The link is indeed the same as before, i.e.
http://terralthra.net/LEDPWM556.jpg

If you remove the diodes, you lose the full range PWM control. Its
range will be halved, covering only 50%-99% duty cycle.

You've not added the limiting resistor John Fields and I suggested.
What do you believe will happen when your pot is at the end of its
travel, so that there is zero resistance from its upper end and +V?

I got the impression from your original post that this circuit came
from an existing design?

The designs around are not consistent in what they include or do not
include, unfortunately. I had a 555 I was using as a monostable
oscillator, and I've looked at more schematics online than I care to
remember, but most of them are long on circuit and short on
explanation. =[

I do see the need for the limiting resistor, I'm just unsure of what
resistance it needs to be.

-Michael