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Dark Activated Transistor Switch

Setup: Voltage divider using a potentiometer and a cds photoresistor
controlling the base current to a MPS2222 NPN transistor. 12v
supply. Common emitter. Load is a cold cathode light driver circuit.

Problem: When the photoresistor is put into darkness, the cathode
tube lights up only half-way. I can adjust the pot to get the cathode
tube to light up all the way but when the photoresistor is brought
back into brightness, the cathode tube only turns off half-way.

I'm guessing the transistor is not fully saturated. Is there a way to
increase the range of the photoresistor (without getting a new one) or
some other way to turn this fully on when the lights go out?
 
E

Ecnerwal

Jan 1, 1970
0
Problem: When the photoresistor is put into darkness, the cathode
tube lights up only half-way. I can adjust the pot to get the cathode
tube to light up all the way but when the photoresistor is brought
back into brightness, the cathode tube only turns off half-way.

Dumb question before you get too far into the driver electronics - is
the light sensor adequately shielded from the light it turns on? What
happens if you put your hand, or something else opaque, over the sensor?
 
J

John Larkin

Jan 1, 1970
0
Setup: Voltage divider using a potentiometer and a cds photoresistor
controlling the base current to a MPS2222 NPN transistor. 12v
supply. Common emitter. Load is a cold cathode light driver circuit.

Problem: When the photoresistor is put into darkness, the cathode
tube lights up only half-way. I can adjust the pot to get the cathode
tube to light up all the way but when the photoresistor is brought
back into brightness, the cathode tube only turns off half-way.

I'm guessing the transistor is not fully saturated. Is there a way to
increase the range of the photoresistor (without getting a new one) or
some other way to turn this fully on when the lights go out?

Use a mosfet instead of the 2222. You'll have a lot more margin for
tweaking.

A schmitt trigger might be nice, too, so the fet snaps on or off.

John
 
J

Jan Panteltje

Jan 1, 1970
0
Setup: Voltage divider using a potentiometer and a cds photoresistor
controlling the base current to a MPS2222 NPN transistor. 12v
supply. Common emitter. Load is a cold cathode light driver circuit.

Problem: When the photoresistor is put into darkness, the cathode
tube lights up only half-way. I can adjust the pot to get the cathode
tube to light up all the way but when the photoresistor is brought
back into brightness, the cathode tube only turns off half-way.

I'm guessing the transistor is not fully saturated. Is there a way to
increase the range of the photoresistor (without getting a new one) or
some other way to turn this fully on when the lights go out?

Try using 2 transistors (darlington).
 
J

Jamie

Jan 1, 1970
0
Setup: Voltage divider using a potentiometer and a cds photoresistor
controlling the base current to a MPS2222 NPN transistor. 12v
supply. Common emitter. Load is a cold cathode light driver circuit.

Problem: When the photoresistor is put into darkness, the cathode
tube lights up only half-way. I can adjust the pot to get the cathode
tube to light up all the way but when the photoresistor is brought
back into brightness, the cathode tube only turns off half-way.

I'm guessing the transistor is not fully saturated. Is there a way to
increase the range of the photoresistor (without getting a new one) or
some other way to turn this fully on when the lights go out?
cascade another transistor behind that one.
look up darlington transistors. What you need is more gain.
I'm assuming you're trying to keep the part count down and
advancement to a minimum ? Other wise, I'd use voltage
comparator IC.. or an OP-AMP with a little schmitt trigger on
it..
 
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