555 timer how-to

T

The Green Potato

Guest
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!

TGP
 
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!
Click to expand...
What's the voltage drop per LED at its operating current?

What's your supply voltage?
 
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999d1e9$0$22977$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is
giving me fits!

Thanks!

What's the voltage drop per LED at its operating current?

What's your supply voltage?
Click to expand...
supply votage varies from 3-4.5vdc depending on desired brightness. haven't
scoped the voltage drop...
 
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999d1e9$0$22977$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is
giving me fits!

Thanks!

What's the voltage drop per LED at its operating current?

What's your supply voltage?


supply votage varies from 3-4.5vdc depending on desired brightness.
haven't
scoped the voltage drop...
Click to expand...
Well, what I was getting at was, a typical LED forward voltage
is around 2V (It can be higher for LEDs of colors other than
the standard red). With 27 of them in series you're looking at a
total drop of at least 54V.

Supposing that you've got a supply voltage of more than 54V handy,
what's the maximum recommended supply voltage for your 555?
 
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999dac5$0$22989$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999d1e9$0$22977$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is
giving me fits!

Thanks!

What's the voltage drop per LED at its operating current?

What's your supply voltage?


supply votage varies from 3-4.5vdc depending on desired brightness.
haven't
scoped the voltage drop...

Well, what I was getting at was, a typical LED forward voltage
is around 2V (It can be higher for LEDs of colors other than
the standard red). With 27 of them in series you're looking at a
total drop of at least 54V.

Supposing that you've got a supply voltage of more than 54V handy,
what's the maximum recommended supply voltage for your 555?
Click to expand...
the specs say 2-18v. i suspect this is the problem? should i switch to a PIC
for so many leds?
 
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!
Click to expand...
---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF
 
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999dac5$0$22989$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999d1e9$0$22977$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to
blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone
is
giving me fits!

Thanks!

What's the voltage drop per LED at its operating current?

What's your supply voltage?


supply votage varies from 3-4.5vdc depending on desired brightness.
haven't
scoped the voltage drop...

Well, what I was getting at was, a typical LED forward voltage
is around 2V (It can be higher for LEDs of colors other than
the standard red). With 27 of them in series you're looking at a
total drop of at least 54V.

Supposing that you've got a supply voltage of more than 54V handy,
what's the maximum recommended supply voltage for your 555?


the specs say 2-18v. i suspect this is the problem? should i switch to a
PIC
for so many leds?
Click to expand...
I think you may have to look at reconfiguring your LEDs into
several strings (each string with its own resistor) to keep the
required voltage down below your available supply, and then
drive those strings in parallel with something beefy enough
to handle the current. That "something" can be switched by
the 555. It might be a transistor, for example. Or perhaps
one transistor per string if you're driving high current LEDs.

You'll have to add up the current requirements to determine
what sort of switching "something" you'll need. Do you have
the LED part specifications?
 
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999e2c1$0$23006$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999dac5$0$22989$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
"Greg Neill" <gneillRE@MOVEsympatico.ca> wrote in message
news:4999d1e9$0$22977$9a6e19ea@news.newshosting.com...
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to
blink
using a 555 timer with 2 capacitors and 2 resistors but it won't
work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone
is
giving me fits!

Thanks!

What's the voltage drop per LED at its operating current?

What's your supply voltage?


supply votage varies from 3-4.5vdc depending on desired brightness.
haven't
scoped the voltage drop...

Well, what I was getting at was, a typical LED forward voltage
is around 2V (It can be higher for LEDs of colors other than
the standard red). With 27 of them in series you're looking at a
total drop of at least 54V.

Supposing that you've got a supply voltage of more than 54V handy,
what's the maximum recommended supply voltage for your 555?


the specs say 2-18v. i suspect this is the problem? should i switch to a
PIC
for so many leds?

I think you may have to look at reconfiguring your LEDs into
several strings (each string with its own resistor) to keep the
required voltage down below your available supply, and then
drive those strings in parallel with something beefy enough
to handle the current. That "something" can be switched by
the 555. It might be a transistor, for example. Or perhaps
one transistor per string if you're driving high current LEDs.

You'll have to add up the current requirements to determine
what sort of switching "something" you'll need. Do you have
the LED part specifications?
Click to expand...

Electrical-Optical Characteristic (Ta=25oC)

Item
Symbol
Condtitions
Min.
Typ.
Max.
Unit

Forward Voltage
VF
If=20mA
/
3.2
3.6
V

Reverse Current
IR
VR=5V
/
/
10
uA

Dominant Wavelength
/
IF=20mA
465
/
470
nm

Luminous Intensity
IV
IF=20mA
/
40,000
/
mcd

50% Power Angle
/
IF=20mA
/
15
/
degree


Absolute Maximum Ratings (Ta = 25oC)

Item
Symbol
Absolute Max. Rating
Unit

Power Dissipation
PD
100
mW

Forward Current (DC)
IF
30
mA

Peak Forward Current
IFP
100
mA

Reverse Voltage
VR
5
V

Operation Temperature
Topr
-40~85oC
/

Storage Temperature
Tstg
-10~100oC
/

Lead Soldering Temperature
Tsol
Max. 260oC for 5 sec.

(3mm from the base of the epoxy bulb)
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF
Click to expand...
thanks! this looks simple but i do not understand parts of it... the leds
are driven by pin 3, and pin 1 is ground and 8 is voltage in -- correct?

what am i missing?
 
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?
Click to expand...
Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum (at
room temperature -- they'll need more when they're cold) to operate. To
drive these directly from a chip requires a chip that can withstand 54V.
Your 555 can't do that. Your proposed alternative, the PIC, can't do
that either.

So you need to either (a) reconfigure the LED portion of your circuit so
it can run off of some voltage that a 555 can handle (and then cope with
the fact that a 555 probably can't deliver all the current you need,
unless you want 27 dimly lit LEDS), or (b) add some additional circuitry
to your circuit to switch the voltage that the LEDs need with the voltage
that a 555 has.

John suggested using an N-channel FET, which would be high on my list of
candidates, along with a big enough voltage source (probably much bigger
than 54V), and some device to limit the current to the LED string
(because LED's tend to want to be constant-voltage devices, but they need
to be constant current devices if you want them to light up and not burn
up).

Does it make more sense now?

--
http://www.wescottdesign.com
 
On Feb 16, 2:42 pm, "The Green Potato" <2...@2.2> wrote:
"John Fields" <jfie...@austininstruments.com> wrote in message

news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...



On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2...@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!

---

      [SOME POSITIVE VOLTAGE]
                |
     [CONSTANT CURRENT SOURCE]
                |
        [27 LEDs IN SERIES]
                |
                D
555PIN3>-------G NCH
                S
                |
               GND

JF

thanks! this looks simple but i do not understand parts of it... the leds
are driven by pin 3, and pin 1 is ground and 8 is voltage in -- correct?

what am i missing?
Click to expand...

Use the 555 to power the gate of an N-channel MOSFET, which is rated
to withstand the higher (54V?) voltage necessary to drive all those
LEDs in series.

Or use a relay. Up to you

Michael
 
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone
is giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?

Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum (at
room temperature -- they'll need more when they're cold) to operate. To
drive these directly from a chip requires a chip that can withstand 54V.
Your 555 can't do that. Your proposed alternative, the PIC, can't do
that either.

So you need to either (a) reconfigure the LED portion of your circuit so
it can run off of some voltage that a 555 can handle (and then cope with
the fact that a 555 probably can't deliver all the current you need,
unless you want 27 dimly lit LEDS), or (b) add some additional circuitry
to your circuit to switch the voltage that the LEDs need with the voltage
that a 555 has.

John suggested using an N-channel FET, which would be high on my list of
candidates, along with a big enough voltage source (probably much bigger
than 54V), and some device to limit the current to the LED string
(because LED's tend to want to be constant-voltage devices, but they need
to be constant current devices if you want them to light up and not burn
up).

Does it make more sense now?

--
http://www.wescottdesign.com
Click to expand...
yes... i didn't consider the 555 couldn't handle that much voltage -- or
even needed to. i thought 3v from the source translated into 3v total on the
board minus the drops...

electronics for dummies
 
The Green Potato wrote:
I have wired 27 10mm leds in series and I'm trying to get them to blink
using a 555 timer with 2 capacitors and 2 resistors but it won't work!

Could someone please lead me to some helpful information? I've successfully
used the 555 with a 4011 to create a sequencer, but using the 555 alone is
giving me fits!

Thanks!

TGP


I think it's safe to say that wiring 27 LED's in series will require
Click to expand...
some voltage beyond what the timer can deliver.

http://webpages.charter.net/jamie_5"
 
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:

yes... i didn't consider the 555 couldn't handle that much voltage -- or
even needed to. i thought 3v from the source translated into 3v total on the
board minus the drops...
Click to expand...
If you connected them in parallel rather than in series, you would only
need 3.2-3.6V, but then you'd need to switch 27 times the current (e.g.
540mA, assuming 20mA each).

You could get by without an external transistor if you used e.g. 9 strings
of 3 LEDs each. 3 LEDs in series @ 3.6V = 10.8V, 9 strings in parallel @
20mA = 180mA.
 
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to
blink using a 555 timer with 2 capacitors and 2 resistors but it
won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone is giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?

Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum
(at room temperature -- they'll need more when they're cold) to
operate. To drive these directly from a chip requires a chip that can
withstand 54V. Your 555 can't do that. Your proposed alternative, the
PIC, can't do that either.

So you need to either (a) reconfigure the LED portion of your circuit
so it can run off of some voltage that a 555 can handle (and then cope
with the fact that a 555 probably can't deliver all the current you
need, unless you want 27 dimly lit LEDS), or (b) add some additional
circuitry to your circuit to switch the voltage that the LEDs need with
the voltage that a 555 has.

John suggested using an N-channel FET, which would be high on my list
of candidates, along with a big enough voltage source (probably much
bigger than 54V), and some device to limit the current to the LED
string (because LED's tend to want to be constant-voltage devices, but
they need to be constant current devices if you want them to light up
and not burn up).

Does it make more sense now?

--
http://www.wescottdesign.com

yes... i didn't consider the 555 couldn't handle that much voltage -- or
even needed to. i thought 3v from the source translated into 3v total on
the board minus the drops...

electronics for dummies
Click to expand...
The 555 only needs to handle 3V if that's what you give it -- but the
LEDs _NEED_ at least 54V if you're going to connect them in series, plus
a circuit to limit the current.

Putting 27 LEDs in series across 3V will give you a whole lot of darkness.

--
http://www.wescottdesign.com
 
On Feb 16, 9:43 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:
"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfie...@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2...@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to
blink using a 555 timer with 2 capacitors and 2 resistors but it
won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone is giving me fits!

Thanks!

---

      [SOME POSITIVE VOLTAGE]
                |
     [CONSTANT CURRENT SOURCE]
                |
        [27 LEDs IN SERIES]
                |
                D
555PIN3>-------G NCH
                S
                |
               GND

JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?

Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum
(at room temperature -- they'll need more when they're cold) to
operate.  To drive these directly from a chip requires a chip that can
withstand 54V. Your 555 can't do that.  Your proposed alternative, the
PIC, can't do that either.

So you need to either (a) reconfigure the LED portion of your circuit
so it can run off of some voltage that a 555 can handle (and then cope
with the fact that a 555 probably can't deliver all the current you
need, unless you want 27 dimly lit LEDS), or (b) add some additional
circuitry to your circuit to switch the voltage that the LEDs need with
the voltage that a 555 has.

John suggested using an N-channel FET, which would be high on my list
of candidates, along with a big enough voltage source (probably much
bigger than 54V), and some device to limit the current to the LED
string (because LED's tend to want to be constant-voltage devices, but
they need to be constant current devices if you want them to light up
and not burn up).

Does it make more sense now?

--
http://www.wescottdesign.com

yes... i didn't consider the 555 couldn't handle that much voltage -- or
even needed to. i thought 3v from the source translated into 3v total on
the board minus the drops...

electronics for dummies

The 555 only needs to handle 3V if that's what you give it -- but the
LEDs _NEED_ at least 54V if you're going to connect them in series, plus
a circuit to limit the current.

Putting 27 LEDs in series across 3V will give you a whole lot of darkness..

--http://www.wescottdesign.com- Hide quoted text -

- Show quoted text -
Click to expand...
you need a resister between each led and 27 leds has quite a voltage
drop with resisters inbetween each one if the max v+ on your 555 is 18
volts use leds with milli amp ratings at less than one volt per led
becuase your cant have more one volt leds than your original b+supply
with any real brightness you will have to supply a higher B+ of
greater than the number of combined led lights in your circuit
 
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:KZudnayrAtLy3QfUnZ2dnUVZ_q_inZ2d@web-ster.com...
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2> wrote:

I have wired 27 10mm leds in series and I'm trying to get them to
blink using a 555 timer with 2 capacitors and 2 resistors but it
won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone is giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?

Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum
(at room temperature -- they'll need more when they're cold) to
operate. To drive these directly from a chip requires a chip that can
withstand 54V. Your 555 can't do that. Your proposed alternative, the
PIC, can't do that either.

So you need to either (a) reconfigure the LED portion of your circuit
so it can run off of some voltage that a 555 can handle (and then cope
with the fact that a 555 probably can't deliver all the current you
need, unless you want 27 dimly lit LEDS), or (b) add some additional
circuitry to your circuit to switch the voltage that the LEDs need with
the voltage that a 555 has.

John suggested using an N-channel FET, which would be high on my list
of candidates, along with a big enough voltage source (probably much
bigger than 54V), and some device to limit the current to the LED
string (because LED's tend to want to be constant-voltage devices, but
they need to be constant current devices if you want them to light up
and not burn up).

Does it make more sense now?

--
http://www.wescottdesign.com

yes... i didn't consider the 555 couldn't handle that much voltage -- or
even needed to. i thought 3v from the source translated into 3v total on
the board minus the drops...

electronics for dummies

The 555 only needs to handle 3V if that's what you give it -- but the
LEDs _NEED_ at least 54V if you're going to connect them in series, plus
a circuit to limit the current.

Putting 27 LEDs in series across 3V will give you a whole lot of darkness.

--
http://www.wescottdesign.com
Click to expand...
i have reconfigured the 27 leds in parallel. 3v makes them bright, 4.5 makes
them very bright.

is there now an option i have [or didn't explore] that i didn't with the
leds in series?

blinking lights are more complicated than first thought!
 
The Green Potato wrote:

i have reconfigured the 27 leds in parallel. 3v makes them bright, 4.5
makes
them very bright.
Click to expand...
Well it should. The data sheet information you provided indicates
a typical forward voltage of 3 V, max of 3.6 V.

Putting 4.5V across them will make them very bright indeed, for a
short while...

The data sheet also indicates forwward current 20 mA typical, 30 mA
max. With 27 of them in parallel you need to supply between 540 mA
and 810 mA. What's the maximum driving current capability of your
555?

is there now an option i have [or didn't explore] that i didn't with the
leds in series?
Click to expand...
Look at series-parallel configurations:

.----/\/\/\---[LED]---[LED]---[LED]---...---[LED]-----------.
| |
+----/\/\/\---[LED]---[LED]---[LED]---...---[LED]-----------+
| |
+----/\/\/\---[LED]---[LED]---[LED]---...---[LED]-----------+
| |
+----/\/\/\---[LED]---[LED]---[LED]---...---[LED]-----------+
| |
----- |
- -----
----- ---
- -
|
-----
---
-

You can make the individual series string required voltage add up
to something manageable and balance the voltage versus current
requirements. You'll still need some kind of switching device
to handle the power; the 555 alone probably won't be able to
handle the power dissipation.
 
On Mon, 16 Feb 2009 23:56:14 -0800, extremesoundandlight@yahoo.com wrote:

On Feb 16, 9:43 pm, Tim Wescott <t...@seemywebsite.com> wrote:
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:
"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
-- snip --

The 555 only needs to handle 3V if that's what you give it -- but the
LEDs _NEED_ at least 54V if you're going to connect them in series,
plus a circuit to limit the current.

Putting 27 LEDs in series across 3V will give you a whole lot of
darkness.

--http://www.wescottdesign.com- Hide quoted text -

- Show quoted text -

you need a resister between each led and 27 leds has quite a voltage
drop with resisters inbetween each one if the max v+ on your 555 is 18
volts use leds with milli amp ratings at less than one volt per led
becuase your cant have more one volt leds than your original b+supply
with any real brightness you will have to supply a higher B+ of greater
than the number of combined led lights in your circuit
Click to expand...
Uh -- no.

If it's all in series, you do _not_ need more than one resistor -- one
resistor with a value of 27 * R will have the same effect as 27 resistors
each with a value of R, except that it'll be more reliable because of
fewer solder joints.

_If_ you use a resistor to establish the current then you will need a
considerable voltage overhead. If you use something like an active
current source you can get by with much less supply voltage, at the cost
of greater circuit complexity (but still less than 27 resistor's worth).

--
http://www.wescottdesign.com
 
On Tue, 17 Feb 2009 08:38:40 -0600, The Green Potato wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:KZudnayrAtLy3QfUnZ2dnUVZ_q_inZ2d@web-ster.com...
On Mon, 16 Feb 2009 17:45:55 -0600, The Green Potato wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:9tydnXyDjegpZQTUnZ2dnUVZ_rDinZ2d@web-ster.com...
On Mon, 16 Feb 2009 16:42:07 -0600, The Green Potato wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:rlojp4lttv6lkv0ppl702dmf9hpda3jfht@4ax.com...
On Mon, 16 Feb 2009 14:45:20 -0600, "The Green Potato" <2@2.2
wrote:

I have wired 27 10mm leds in series and I'm trying to get them to
blink using a 555 timer with 2 capacitors and 2 resistors but it
won't work!

Could someone please lead me to some helpful information? I've
successfully
used the 555 with a 4011 to create a sequencer, but using the 555
alone is giving me fits!

Thanks!

---


[SOME POSITIVE VOLTAGE]
|
[CONSTANT CURRENT SOURCE]
|
[27 LEDs IN SERIES]
|
D
555PIN3>-------G NCH
S
|
GND



JF

thanks! this looks simple but i do not understand parts of it... the
leds are driven by pin 3, and pin 1 is ground and 8 is voltage in --
correct?

what am i missing?

Some basic knowledge of electronics, apparently.

As already pointed out, your 27 LEDs need 54V at the absolute minimum
(at room temperature -- they'll need more when they're cold) to
operate. To drive these directly from a chip requires a chip that
can withstand 54V. Your 555 can't do that. Your proposed
alternative, the PIC, can't do that either.

So you need to either (a) reconfigure the LED portion of your circuit
so it can run off of some voltage that a 555 can handle (and then
cope with the fact that a 555 probably can't deliver all the current
you need, unless you want 27 dimly lit LEDS), or (b) add some
additional circuitry to your circuit to switch the voltage that the
LEDs need with the voltage that a 555 has.

John suggested using an N-channel FET, which would be high on my list
of candidates, along with a big enough voltage source (probably much
bigger than 54V), and some device to limit the current to the LED
string (because LED's tend to want to be constant-voltage devices,
but they need to be constant current devices if you want them to
light up and not burn up).

Does it make more sense now?

--
http://www.wescottdesign.com

yes... i didn't consider the 555 couldn't handle that much voltage --
or even needed to. i thought 3v from the source translated into 3v
total on the board minus the drops...

electronics for dummies

The 555 only needs to handle 3V if that's what you give it -- but the
LEDs _NEED_ at least 54V if you're going to connect them in series,
plus a circuit to limit the current.

Putting 27 LEDs in series across 3V will give you a whole lot of
darkness.

--
http://www.wescottdesign.com

i have reconfigured the 27 leds in parallel. 3v makes them bright, 4.5
makes them very bright.

is there now an option i have [or didn't explore] that i didn't with the
leds in series?

blinking lights are more complicated than first thought!
Click to expand...
You _do_ have a series current limit resistor for each LED, yes?

You need to separate your tasks -- a pulsing output from a 555 is easy,
controlling 27 LEDs from one low-power 3V output is only moderately hard,
and connecting the 555 to the controllable light array is dead simple.

--
http://www.wescottdesign.com
 
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