556 (dual 555) PWM for LED design

M

Michael Shannon

Guest
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?
 
Michael Shannon wrote:
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?
Click to expand...
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.
--
John Popelish
 
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

John Popelish <jpopelish@rica.net> wrote in message news:<419C1711.1493688@rica.net>...
Michael Shannon wrote:

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.
Click to expand...
 
terralthra@terralthra.net (Michael Shannon) wrote:

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?
Click to expand...
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
Hobbyist, West Sussex, UK
 
terralthra@terralthra.net (Michael Shannon) wrote:

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
Click to expand...
(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.

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
On 17 Nov 2004 19:11:43 -0800, terralthra@terralthra.net (Michael
Shannon) wrote:

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?
Click to expand...
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
 
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

John Fields <jfields@austininstruments.com> wrote in message news:<5gapp010nco7cp70sbpvhdoln0lusg79u4@4ax.com>...
On 17 Nov 2004 19:11:43 -0800, terralthra@terralthra.net (Michael
Shannon) wrote:

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
Click to expand...
 
terralthra@terralthra.net (Michael Shannon) wrote:

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
Click to expand...
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?

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
Michael Shannon wrote:
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?
Click to expand...
View in a fixed-width font such as Courier.



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