555 timer how-to

successfully

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...

successfully

haven't

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?

blink

successfully

alone

haven't

PIC

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?

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

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)

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?

is

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

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?

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

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

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.

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 @

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.

ote:

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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

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!

makes

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

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.

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).

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.