Re: LEDS and the 555

On Tue, 07 Jun 2005 16:51:23 -0400, Randy Joseph
<joseph2017@xxxxxxxxxx> wrote:

>On Mon, 06 Jun 2005 12:02:38 -0500, John Fields
><jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
>>On Mon, 06 Jun 2005 12:24:19 -0400, Randy Joseph
>><joseph2017@xxxxxxxxxx> wrote:
>>
>>>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.
>>
>>---
>>There are a few things you can do:
>>
>>1. Assuming that you're using the 555 to source current into the LED,
>> Pull the 555's TRIGGER input to ground so that the LED stays lit
>> all the time, then measure the output voltage at pin 3 and use that
>> voltage to calculate the value of the current-limiting resistor.
>>
>>2. If you're using the 555's output as a sink, do the same thing with
>> the TRIGGER input pulled up to Vcc.
>>
>>
>>3. Or, do this:
>>
>> 9V
>> |
>> [330]
>> |
>> |A
>> [LED]
>> 555 |
>> +-----+ C
>> | OUT|---[3k]---B 2N4401
>> +-----+ E
>> |
>> GND
>
>
>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.


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


--
John Fields
Professional Circuit Designer
.

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