12v Light/Dark Switch
Often, for certain low voltage lighting systems; you would like to turn off the lights during the bright light of the day.  Most commercial day/night switches are designed for AC lighting.  The circuit below was designed for a 12v DC system.  But, it could be modified for other voltage as well.  It uses an inexpensive phototransistor as the light detector.  An n-channel FET is used to switch power to the lights.  A transistor circuit is included to provide some hysteresis. This keeps the circuit from fluttering the light during the transition from day to night and night to day.  It is recommended that a plastic tube be placed over the transistor to prevent it from being illuminated by the lights it is controlling.  By selecting the appropriate power FET, the circuit could control over 100 watts worth of 12v lighting. (July 27, 2008) ….

1W White LED Night Light
As many of you know, I have a pet peeve with poorly made LED night lights.  Often, the light from the LED quickly fades, so within months, the light is useless.  I have posted several versions of modified night lights using higher quality components. This circuit is yet another version, which produces much more light than those other designs. The circuit brings together two high power white LEDs made by Cree with a compact AC to DC power supply from Bias Power.

5W Fluorescent Lamp Intensity Modulator
The circuit was designed to experiment with using small fluorescent lamps as a broad pattern source of modulated light. The circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is a bit crude but the circuit does work.

60 Watt Lamp with Auxiliary 14v LED Lamp
This circuit controls power to a standard 60 watt lamp plus generates an auxiliary 14v DC supply which can power an automotive style 12 LED array.  (added 7/06) ….

Closet Light with Automatic 3 Min Timeout
The circuit below is powered by three 1.5v alkaline AA cells.  With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off.  The circuit’s standby current is only a few microamps.  A set of fresh batteries should last at least 200 light applications.  The circuit uses a Schmitt trigger inverter and two transistors.  When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged. When the switch is released, the capacitor is charged through R1. With the capacitor C1 discharged, the Schmitt trigger output swings to 4.5v, which turns on transistor Q2. The combination of R4 and Q1 form a constant current control circuit.  The current is set about 15ma per LED.  When the capacitor charges up to about 2.3v, the Schmitt trigger changes state, turning off the LEDs. [circuit of the week]

Incandescent Lamp Inrush Current Limiter
This circuit limits the large inrush current often associated with large incandescent lamps.  With the components shown the current is limited to 1 amp, but it could be scaled to any desired current.

Pot Controlled Variable LED Intensity Circuit
The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.  A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%.  The beauty of the circuit is that when the pot is all the way toward 0%, very little power is drawn from the battery.  Likewise, when the pot slider is moved toward the 100% end, full power is fed to the LEDs.  In effect, the pot becomes the on/off switch.

Pushbutton Controlled Up/Down Ramping Light
There are times when you want a lamp to gradually change from dark to light and from light to dark, with each push of a pushbutton switch.  The circuit below performs this control function using a flip/flop and a classic pulse width modulation or PWM method.  The circuit uses a common flip/flop IC to control the on and off logic states. A simple pushbutton switch toggles between the two states with each press of the button.

Ramping Lamp Driver
This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion.  Each press of the button toggles the lamp from a fully off mode to a fully on mode.  But, instead of an instantaneous lamp intensity change, the power is slowly ramped up and down.  With the components shown, the ramping action takes two seconds to complete.  By changing the capacitor C3, you can increase or decrease the ramping time.  The circuit will also operate with supply voltages ranging from 15v to 3v. [Hobby circuit designed by Dave Johnson]

Visible Light Blocking Filter
This isn’t an electronic circuit but the drawing below shows the curve for an optical filter made from exposed color film. The material blocks most visible light but is very transparent to 880nM infrared light. Kodak 100ASA is exposed to "cool white" fluorescent light for about 5 seconds. ….