[sdiy] help with basic circuit

[Dave Manley]

db wrote:
>> Are you driving 1 LED or 3 (or 14?) from this circuit.  If three, as 
>> the schematic shows, the transistor collector current is going to be 
>> split between the three LEDs.  Also the 1K collector resistor is 
>> going to limit the current to less than 5 mA (roughly 5V divided by 
>> 1K, assume the transistor is saturated  at 0V, and there's no voltage 
>> drop across the LEDs).  The Beta of this transistor is spec'd at a 
>> min of 300.  Ib = Ic/Beta = 5mA/300 = 16uA, while your circuit could 
>> provide Ib = (5V-0.6V)/1K = 4.3mA, which is way more than you really 
>> need.  Try reducing the collector resistor, or remove some of the 
>> paralleled LEDs so that the current isn't divided between multiple LEDs.
> I don't quite follow all that you said, but the LEDs are actually a 
> cheaply purchased flashlight that has 16 LEDs in it, and is wired just 
> as shown.. I also used on with 24 LEDs. In each case one resistor is 
> used in it to drive all the LEDs.
>
> Does that have some effect on the resistor used on the base of the 
> transistor?
Try reducing the collector's 1K resistor feeding the LEDs and see if 
that works better for you.  When you get the LEDs as bright as you want, 
you can then start increasing the base resistor.  Watch out making the 
collector resistor too small, at some point the transistor will start to 
get warm if you draw too much current through it.  The datasheet says it 
can handle 50 mA (100mA absolute max), but I didn't run through any 
thermal calc to see how hot the case would get.

-Dav