Thebighat99
New Member
Hi all,
I am trying to build a night light for a glass etching I did. The 4 LEDs will but up against the glass and illuminate the pain. The problem I am having is that the LEDs stay on weather its dark or light. I turn the potentiometer and led will get brighter but will not turn off. When the voltage is 9v instead of 18v it seems to work but with just two LEDs.
I used info from these pages for this circuit.
http://www.elxevilgenius.com/02ONLINE/Part_I/Lsn11/ANims/ec4eg.htm
**broken link removed**
Schematic
**broken link removed**
Using this formila to find R1.
R1 = Supply Voltage/(Maximum Current Required /Minimum HFE * (minimum base current+30%))
R1 = 18v/(.12/100*1.56)
R1 = 18v/(.0012*1.56)
R1 = 18v/.001872
R1 = 9615.384 Ohm or 10k for nearest standard value.
18v is supplied buy two 9v batteries
D1,D2,D3,D4 LEDS are blue 3.5 voltage forward, forward current 30mA
LDR Dark 500k ohm, light 15k ohm
Q1 is NPN 2N3904 transistor 100hfe Ic 200mA.
I been working on this for a while. I think it is because the current is over powering the transistor. When I tried a higher rated transistor Ic 600mA it had same affect. I been reading everything on the web that I can find about transistors but I am really not grasping how to apply the information to fix the problem. This is the first circuit that I have tryed to modify to suit my needs. Maybe I am doing it all rong. I am sure there is somthing I missing like a engineering degree. Please any help will be apretiated.
Thank you,
I am trying to build a night light for a glass etching I did. The 4 LEDs will but up against the glass and illuminate the pain. The problem I am having is that the LEDs stay on weather its dark or light. I turn the potentiometer and led will get brighter but will not turn off. When the voltage is 9v instead of 18v it seems to work but with just two LEDs.
I used info from these pages for this circuit.
http://www.elxevilgenius.com/02ONLINE/Part_I/Lsn11/ANims/ec4eg.htm
**broken link removed**
Schematic
**broken link removed**
Using this formila to find R1.
R1 = Supply Voltage/(Maximum Current Required /Minimum HFE * (minimum base current+30%))
R1 = 18v/(.12/100*1.56)
R1 = 18v/(.0012*1.56)
R1 = 18v/.001872
R1 = 9615.384 Ohm or 10k for nearest standard value.
18v is supplied buy two 9v batteries
D1,D2,D3,D4 LEDS are blue 3.5 voltage forward, forward current 30mA
LDR Dark 500k ohm, light 15k ohm
Q1 is NPN 2N3904 transistor 100hfe Ic 200mA.
I been working on this for a while. I think it is because the current is over powering the transistor. When I tried a higher rated transistor Ic 600mA it had same affect. I been reading everything on the web that I can find about transistors but I am really not grasping how to apply the information to fix the problem. This is the first circuit that I have tryed to modify to suit my needs. Maybe I am doing it all rong. I am sure there is somthing I missing like a engineering degree. Please any help will be apretiated.
Thank you,