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What's wrong with this ckt

jackorocko

Apr 4, 2010
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Well if that was my circuit, I would put the battery where the LED is(neg tied to the 2n2222 emitter) and move the LED and series resistor and put it in series with the collector of the 2n2222. Then move the LDR and place it between the base of the 2n2222 and emitter.
 

GonzoEngineer

Dec 2, 2011
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The transistor is wired backwards.......where did you get that schematic?

(and let me also add...... Sempr Fi !)
 

GonzoEngineer

Dec 2, 2011
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I forgot the Marinespeak...."Tthe little thingy with the arrow is the emitter!"

(Sorry, jusrt couldn't help myself!):D:D
 

SargeUSMC

Mar 6, 2012
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this is the way her little board is wired up......they etched their own boards and everything......I don't know if this is the actual ckt they were given to build or not.......I reversed the transistor with no joy.......put it back the way it was and now there's nothing at all....maybe I popped it.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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A reversed collector and emitter can destroy a transistor as the BE junction will be reverse biased and it can only withstand a small reverse voltage (the CB junction is much more rugged)
 

SargeUSMC

Mar 6, 2012
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Jack, it looks like what you suggest is basically the same thing......reverse the C - E connections.

The transistor was no good. Thanks, men.
 
Last edited:

john monks

Mar 9, 2012
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The led anode should be connected to the emitter of the transistor. The 10ohm resistor should be much larger, maybe 270ohm and should be placed between the cathode of the led and the negative side of the 9V battery. The photo resistor should be placed between the base of the transistor and the negative side of the battery. The positive side of the battery should be connected to the collector of the transistor. The 22Kohm resistor should be placed between the base and the collector of the transistor. Then a final adjustment should be made to the 22Kohm resistor to get the led to turn off when it should.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Perhaps a picture will speak a 1000 words...

attachment.php


The LED really needs to be connected on the collector side of the transistor, not the emitter side.

John Monks suggests 270R, I suggest 330R for the series resistance for the LED. Really the difference is insignificant, you could use either (a lot depends on the LED anyway)

I have also turned the circuit around so that it's drawn in a conventional manner which makes it easier to understand for those of us that look at these things all the time :)

John's other suggestion about the 22K resistor is correct. LDRs differ a lot, and you could play with this resistor (some value between 1000 and 100,000 (100k) ohms would be the expected range) to get the LED to begin turning on/off at some appropriate light level.

You could place a 100k trimpot in series with a 1k resistor which would allow you to adjust the sensitivity.

Beware that the light from the LED should not reach the LDR or the turn on/off light levels will be affected.
 

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