LEDS and the 555

I am designing a project for a freind of mine and I am making 3 LEDS blink at at 3 different rates. I am using 555's to do this. The problem I am having is the LEDS are not bright enough when I drive them with the 555 directly. I have used high brigtness LEDS and they are a little brighter but not bright enough. I am using 9 vdc and fiquirng 20ma to run the LEDS. When I drive them directly without the

555's they are plenty bright enough. My question is how do I get the LEDS brighter. Is there a driver or something I can us.

Your help is greatly appreciated.

Thank you,

Randy

Ironically, I just finished a version of this using a 556 this weekend for IR LED's. You probably already know this but the voltage drop across a regular LED is pretty large, I think my reds were 3.7 volts (while the IR's are only 1.3 volts) and the 555 output as a current source drops atleast 1 volt from 5-15 v Vcc with currents from 15-100 mA. So the 9 volt battery puts out 9.3 volts but the current limiting resistor only drops 4.6 volts, so your current calculations may be double the actual current. See the previous post "voltage drop from a 555/556".

The standard 555 has a max rated output of about 150 mA if I remember correctly. I bought the 556-CN for a few cents more which has a rated max continuous output of 1.6 A! I've used them to drive 1A on several occasions reliably, I try not to push harder than 50% max for most things.

Lastly, the lens at the end of LED's varies tremendously from model to model. This is useful as some applications require a spread beam while others require a tight beam. So the LED with the spread beam looks dimmer but actually puts out the same total amount of light.

good luck Lyle (just a hobbyist)

Check the voltage/condition of your 9 vdc source, especially if it is a

9V battery.

If you did not use resistors with the LEDs, you may have cooked the LEDs.

Try higher brightness LEDs.

If you are using resistors, use ones that allow 30 mA through the LEDs. Check available output voltage from the 555 as suggested by John Fields in his response - the 555 will drop some voltage.

If the 555 is the TLC555 or LMC555 or another one with FET output, the pullup output transistor has significant resistance - maybe a few hundred ohms - and is not suitable for sourcing large amounts of current. But the pulldown transistor in these is not as bad, with resistance generally well under 100 ohms. Check actual LED current by putting a milliammeter in series with the LED or measuring the voltage across the LED's dropping resistor, with the

555's trigger pin connected to whichever supply rail forces the LED to be continuously on. And I would not demand the pullup transistor in an FET-output IC to source enough current to experience power dissipation anywhere near the rated limit of the IC.

- Don Klipstein ( snipped-for-privacy@misty.com)

Hi Lyle,

I have been looking for 'lensed led' suppliers for ages, i have seen led's in products that project a nice 'square' onto a surface 10cm away (although faint at that distance). I need to use them for 'aiming guides' for allignment of an optical device.

I have asked many people about this, including posting here but everone just argues that they must be laser diodes. They are not, most curtainly they are LED's, i just cant find a supplier probably because i dont know what they are called exactly, any idea ? :)

Alex.

9V | [330] | |A [LED] 555 | +-----+ C | OUT|---[3k]-+--B 2N4401 +-----+ | E [1k] | | | GND GND

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John Fields
Professional Circuit Designer

I tried your 3rd option and the LED is bright enough but it stays on all the time when I remove the 3k resistor from the base it goes off. The timing circuit works well without the 2N4401 but like I said earlier its not bright enough. How do I get the 555 timing circuit to work again using the 2N4401. I think I am close but need a nudge.

Thank you,

Randy

Thanks Don, thats interesting.

Specifically the ones i have seen and want project only the outline of a square (not filled) with very little ambient light around that. Correct me if i am wrong, but i imagine the die would cast a solid 'filled' square by just using a narrow angle ?

Alex.

These don't have special notation other than very narrow "viewing angle" around 8 degrees or less. And not all with "viewing angle" in the upper part of this range have nice square focused images of the "chip" (die).

A few with such nice nice narrow focused beams even claim a much wider "viewing angle" as wide as 15 degrees for parts with bodies 5 mm in diameter and 30 degrees for parts with bodies 3 mm in diameter due to claiming the width of the image of the "die cup".

- Don Klipstein ( snipped-for-privacy@misty.com)

The image of the die includes the image of the connection(s). With most LEDs, there is a contact in the center, usually round in shape and of diameter around something like 1/3 of the width of the die. This has the image of the die having a significant hole in it.

LEDs whose dice are made with nonconductive substrates have two contacts. The image of the die will show two holes, notches, "bites taken out" or whatever.

- Don Klipstein ( snipped-for-privacy@misty.com)

I am still having problems with this circuit. It has really thrown me. I can get the LEDS to a usable brightness with the LED Driver circuit below it just when I hook it up the blinking stops of the 555 and the led stays on all the time. I have no problems until I hook up the 2n4401. I must be missing something on the 555 though because when I measure the output of pin 3 I get a constant 7 volts and when I hook it up to the base of the 2N4401 it casues the transistor to stay on. The parts that is throwing me is the LED blinks without the

2N4401 hooked up. Should not the output of pin 3 change states from 7V to roughly OV, is this not what casues the LED to turn on and off. This is not hapenning with my measurements.

Your help appreciated.

Randy

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It sounds to me like you may have a wiring error.  How do you have the
555 hooked up?