■ PHOTO 1.
Driver circuit on
In order to fit the pulsed driver circuit into the battery holder of the flashlight,
surface-mount parts are a must. I used a
SMT-400 surface-mount "breadboard"
made by BoardworX, trimmed to slide into
the three AAA battery holder (Photos 1
The board is pretty easy to use —
place each part where you want it, hold it
in place with a toothpick, and tack the
leads to the tinned bumps on the board
with a fine-tipped soldering iron.
The LED connections for the circuit
tap into the positive and negative terminals
of the battery holder, so installing the
driver circuit into the flashlight is
The wires to the external power
source pass through a small hole in the
side of the flashlight which can be
sealed (if needed) to preserve the
flashlight's water resistance.
The stock on-off switch in the end
of the flashlight still works to turn the
LEDs on and off, but does not cut
power to the pulsed driver circuit. The
circuit draws about 4 mA with the
LEDs turned off when powered by a
two-cell lithium-ion battery pack ( 7.4V
The pulsed driver circuit (Figure 1)
is based on the venerable LM555 timer wired as an
astable multivibrator and drives an N-channel power
MOSFET transistor to switch the LEDs on and off.
Don't substitute a CMOS 555 in this circuit, as it
doesn't have the "oomph" to effectively drive the MOSFET
(see the article Power MOSFETs, Part 1: Theory, in the
January 2009 Nuts & Volts for an explanation).
Diode D1 allows the circuit to have a duty cycle
below 50% for better power efficiency; with the
component values shown, the duty cycle is about 10%
(Photo 3). Diode D2 is for reverse voltage protection.
Capacitor C3 smooths voltage spikes across the LEDs due
to the MOSFET switching and may further increase their
lifespan. Switch SW1 is an external power switch (if
desired), not the flashlight's stock on-off switch.
■ PHOTO 2. SMT breadboard installed in
the flashlight's battery holder.
■ FIGURE 1. Pulsed driver circuit schematic.
April 2013 39