More  LED Circuits

Modified 4 D Cell LED Lantern (July 29, 2017 )
designed by David A. Johnson, P.E.

While shopping at a sporting goods store one day, I spotted a compact LED lantern.  It looked like the kind of lantern I could modify. The lantern LEDs were grouped into three sections of 7 LEDs each, oriented into 120 degree angles.  The LEDs were standard T 1-3/4 epoxy types.  The lantern had a three mode switch which selected between off, full power and half power modes.  At the half power mode, only half of the 21 LEDs were illuminated.  At full power all LEDs were illuminated.    All the LEDs were wired in parallel. The lantern was powered by four 1.5v D alkaline cells, making for a 6 volt supply.  I wanted to turn the lantern into one more useful, by converting the push button switch into a rotary dial to control the LED intensity.  I also wanted to power the light from four D size 10 amp hour NiMH
rechargeable cells.

The first thing I did was dismantle the lantern and remove the control circuit.  I also replaced the 4 alkaline cells with rechargeable 10 amp-hour 1.2v NiMH
cells, forming a 4.8v supply.  I designed a new control circuit, which used a variable resistor containing a switch.  When rotated fully counter-clockwise, the switch would click, turning off the LED.  When rotated fully clockwise, the light would be at full power.  I set the LED current control range from a low 1ma to a full 400ma setting.  The low setting would be great for a night light feature for inside a camping tent.  When fully charged the lantern’s battery should operate the LEDs for 24 hours at full power and a year at the lowest power setting.  The circuit I used is shown below.