[Noisebridge-discuss] Help with Simple Circuit : Blinking LED

jim jim at well.com
Wed Jul 28 05:56:14 UTC 2010


   seems to me the circuit fails because there's no 
discharge path for the capacitor. 
   assume a switch in the circuit. 
   ground the top of the cap (just to get it ready) 
then flip the switch. 
   current flows until the cap has the same potential 
as the battery (and the collector of the transistor) 
and the base has the same potential as ground and no 
more current flows: i.e. the LED turns on until the 
circuit gets static. 
   i'd love to know how to use one cap, one resistor, 
one transistor, and one LED and make the LED blink 
repeatedly (i.e. get the circuit to oscillate). 



On Tue, 2010-07-27 at 16:32 -0700, Mike Chambers wrote:
> I am trying to get a simple blinking LED circuit working, and am
> following the following schematic:
> 
> http://tronixstuff.files.wordpress.com/2010/06/pic23.jpg
> 
> Where:
> 
> C1 = 10uF 50v
> R1 = 18k ohms
> 
> Based on my understanding, that combination of resistor and capacitor
> should cycle every 1 second.
> 
> I am using a 9V battery instead of the 6V.
> 
> When I plug it in, the LED turns on for a flash, and then nothing
> happens. The LED is not burning out (I tested it after on a separate
> circuit). It looks like the transistor (BC548) is failing, as if I
> replace it, the LED will Flash, then nothing.
> 
> You can see the specifications for the BC548 Transistor here:
> 
> http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&productId=254781&catalogId=10001&freeText=BC548&app.products.maxperpage=15&storeId=10001&search_type=jamecoall&ddkey=http:ProductDisplay
> 
> It says the max collector is 30V, but the max emitter is 5V.
> 
> Does anyone see why my circuit is failing? Is it because I using too
> many volts for my transistor (9v instead of 5v)?
> 
> I realize this is a super basic question, but im really trying to
> understand how this works, and how to choose the correct parts for a
> circuit.
> 
> thanks...
> 
> mike
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