Hope the schematic proves useful JEBster. It should provide a better demonstration of how a transistor can act as an electronically-controlled switch. You may wish to replace R2 with a resistor on each branch of the LEDs though, as otherwise damage to D1 may cause D4 and D5 to be damaged by too much current, for example.

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Good riddance, ETO .

 

Not for a collector current of only 20mA.
The "forced beta" for a 2N3904 little transistor is 10 and the forced beta for a BC547 little transistor is 20 on their datasheets.

A 2N3055 power transistor has a forced beta of only 3 on its datasheet (10A collector current) and it still saturates poorly.

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Uncle $crooge

 

Wow. This is great! I apologize for the poor example, but I'm very glad I put it out there. I learned a great deal... well, I'm sure I will after some more quality time with the breadboard!

Thank you all!

 

Rebuilt my Fisher Price "My First Circuit" as per RCinFLA suggestions (no offense giftiger... just afraid parallel anything might make my head asplode). New schematic is attached.

Didn't have exact components, but now have R1 @ 2.2K and R2 @ 33. And 4 red LEDs.

I'm measuring .9V across R2 and 1.7V @ base, so I believe I'm on reasonably close to RCinFLA's specs. But best I can figure, there is no minimum current that needs to be applied to the base to close the transistor collector to emitter (I used a 100K R1 providing .07mA to base and still the LEDs light.) Is this correct? I'm afraid I don't understand RCinFLA's post.

And here's another thing thats got me confused: the voltage measured at the base with R1 @ 2.2K is 1.7V. How is it that the resistor is dropping 7.3V? If this is a result of a two resistor voltage drop on the circuit, the math doesn't work out: drop across R1=R1/(R1+R2)*V=2200/2233*9=8.9. There's still 1.6V unaccounted for..?

Attached Thumbnails

 

 

R2 is in the wrong place, it needs to be in the collector, the emitter should go directly to the battery.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

A transistor with a certain part number can be very sensitive and turn on with a low base current but another transistor with the same part number and made by the same company could be weak and not sensitive so it needs the amount of base current that is shown on its datasheet: 1/10th the collector current.

If you make every circuit use a base current 1/10th the collector current then all your circuits will work, not just the ones that have very sensitive transistors. Maybe you won't have any very sensitive transistors.

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Uncle $crooge

 

Indeed, the schematic I suggested had the same problem, as well as the base current having to flow through the LEDs as well which probably wouldn't have been possible; hadn't considered that.

Never mind, after moving the resistor your schematic should be fine JEB. (Better than my hastily-constructed one )

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Good riddance, ETO .