Path: utzoo!utgpu!watserv1!watmath!att!oucsace!olbers
From: olbers@bobcat.ent.ohiou.edu (Robert Olbers ece)
Newsgroups: sci.electronics
Subject: Re: LED brightness, colors
Message-ID: <1507@oucsace.cs.OHIOU.EDU>
Date: 3 Jul 90 20:19:05 GMT
References: <7644@jarthur.Claremont.EDU> <1990Jun29.215920.4003@ddsw1.MCS.COM>
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In article <1990Jun29.215920.4003@ddsw1.MCS.COM> whos@ddsw1.MCS.COM (Ben Feen) writes:
>In article <7644@jarthur.Claremont.EDU> rspangle@jarthur.Claremont.EDU (Randy Spangler) writes:
>>
>...
>>Also, what's the easiest way to blink a high-current LED at say 2-10 Hz?
>>I've heard there's some way to do it with a 555 timer, but electronics isn't
>>my strong point.  Blinking LED's (that have the driver built in) seem to be
>>a lot dimmer than non-blinking ones.
>An RC circuit should do it:
>
>+  Resistor       LED
>-------------------
>       \    |    |
>       /   ---  \v/ 
>       \   ---  --- 
>-      /    |    |
>-------------------
>        Capacitor
>
>
>To calculate frequency - 
>Ohms*farads = 1 time constant (In seconds)
>Multiply TC by 5 to find frequency. More I can't remember, but others should
>be able to build on this.
>

This circuit won't blink the LED from a dc source.  The LED needs a
current-limiting
resistor in series to prevent burnout, and the capacitor will simply
charge to the
supply voltage and stay there.

The simplest way to blink an LED (for those who are not
electronically inclined)
would probably be the LM3909 IC which is specially designed for this purpose.
These are even available at Radio Shack.

Here is an example of a 555 LED-flasher circuit:

     (+)    ---------------------------------------------------------
                     |     |           |                |           |
                     |    /           ---               |           |
                     |    \ R1        --- 0.1uF         |           |
                     |    /         NC |                |           |
                 NC  |    \      ----- | -------        |           /
             ------- | ----|     |     |       |        |           \
             |    --------------------------   |        |           / R3
             |    |  8     7     6     5   |   |        |           \
             /    |                        |   |        |           |
        R2   \    |          555           |   |        |           |
             /    |                        |   |        |          ---
             \    |  1     2     3     4   |   |        |          \ /
             |    --------------------------   |        |           v LED
             |       |     |     |     |       |NC      |          ---
             ------- | ----|     |     ------------------           |
                  NC |     |     |NC           |                    |
                     |     |---- | ------------                     |
                     |     |     |                                  |
                     |    ---    ------------------------------------
                     |    ---
                     |     | C
     (-)      --------------

(Excuse the 'professional quality' schematic 8) )
NC = no connection

The LED will be on* for time T1 = 0.693 * ( R1 + R2 ) * C
   and will be off* for time T2 = 0.693 * R1 * C

*(I may have the 'on' and 'off' reversed)

R3 is used to set the LED current; e.g., assuming 9V input, 20 mA desired
on-state LED current, and an LED voltage of 1.7 V (red LED),

             R3 = ( 9 - 1.7 ) / 0.02 = 365 ohms


For high levels of current, the 555 may need to drive the LED(s) through a
transistor -- check the 555 specs.

                                    R. L. Olbers
                                    Ohio University ECE Department