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

[Jonathan Foote]

Jim's right, that looks like a "one-shot" timer and not an oscillator.

To get something that oscillates, use a circuit that discharges the
cap when it turns on like a multivibrator
(https://secure.wikimedia.org/wikipedia/en/wiki/Multivibrator) or a
555.

There are one-transistor flashers but they are sort of dodgy as they
rely on negative resistance from avalanche  e.g. see "LED flasher"
midway on this page: http://members.shaw.ca/roma/twenty-three.html

I wouldn't count on it working reliably or for any length of time
unless you use transistors designed to work in avalanche -- try a
multivibrator.





On Tue, Jul 27, 2010 at 10:56 PM, jim <jim at well.com> wrote:
>
>   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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>>
>
>
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