Lantern Dimmer/Flasher

Lantern Dimmer/Flasher Lantern dimmer, schematic

Parts List
R1 = 100K       C1,C3 = 10µF/16V
R2 = 100K          C2 = 0.01µF, ceramic
R3 = 100K          T1 = 2N4401
R4 = 100K          T2 = TIP32
R5 = 3K9           U1 = LM358     
R6 = 3K9           L1 = Lantern Bulb
R7 = 470           S1 = On-Off Switch
R8 = 100
R9 = 220, 1/2 watt
P1 = 5K

Circuit Description:
The electronic lantern control circuit adds high-efficiency dimming and flashing to an existing battery-powered lantern or flashlight or to a custom design. For the car it makes a great lamp for changing a flat tire, back seat reading or emergency engine work. The flasher mode is useful for warning other drivers of your troubles and it may be adjusted to have a very short flash duration for long-term use as when the car must be left on the shoulder over night. When camping it is great as a low-power night light for the tent or the portable 'potty'--you may select only as much light as you need! The flasher mode is useful for finding a boat dock in the dark or even attracting fish. At home, the flasher is a great way to tell guests when they have found the right house or to "jazz up" battery powered holiday decorations. The circuit is intended for 6 or 12 volt lantern batteries but it should work well with supplies from 4.5 to 15 volts without any modifications to the circuit as shown above.

In dimmer mode (switch S1 open), the circuit send rapid variable-width pulses to the bulb to control the brightnes and in flasher mode (switch S1 closed) the pulse rate is about one per second. Very short flashes will give a greatly extended battery life. The TIP32 remains cool since it switches on and off instead of simply dropping the voltage like a power rheostat.

The components are not critical and substitutions are fine. Almost any general purpose op-amps or comparators will work in place of the LM358. The two transistors may be replaced by a power FET if desired simply by connecting the gate to pin 7 of the Op-Amp, the source to ground, and the drain to the bulb. The other end of the bulb connects to the positive terminal of the battery in this case. There is nothing particularly critical about the resistor and capacitor values and the experimentor may change them, if desired. For example, a 10K-pot may be substituted for the 5K by increasing the 3.9K resistors by 2 also (8.2K would be fine). the 100K's in the flash circuit may be a different value if the capacitors are also scaled (inversely--if the resistors are doubled, the 0.1 and 10µF are halved). Try experimenting with whatever you have at hand or combine values to get the desired value you want.

Construction is not critical - the entire circuit may be built on a piece of perf-board and wrapped with electrical tape. An old gas-mantle lantern coulb be converted over to battery power by placing a bulb socket in place of the mantle and building a battery compartment in the fuel tank.

Additional Notes
ALL resistors are 1/4 watt, 5% tolerance, unless otherwise posted.
P1 is the dimmer potentiometer. S1 is an additional switch to activate the 'Flashing' mode. R9 has to be a half-watt type. T1 is a NPN audio amp transistor and can be substituted with a2N3904, PN100, NTE123AP, the BC547, Elector's (Elektuur) TUN, etc. T2 is a PNP power amp and can be substituted with a NTE197. Try others, they also may work.