Circuit Project: A Handy Pen Torch

This easy to construct
“Handy pen torch” electronic circuit and low component count, uses two
power white LEDs for lighting. Low volt (4.8V
dc) supply available from the built in rechargeable Ni-Cd battery pack
is first converted into two channel (independent) constant current
sources by two pieces of the renowned precision adjustable shunt
regulator chip LM334 (IC1 and IC2). Around 25mA at 3.6 volt dc is
available at the output of these ICs.

This regulated dc supply is used to drive two power white LEDs
D4 and D6. Resistors R3 and R5 limits the output current (and hence the
light output) of IC1 and IC2 circuits respectively. Besides these
components, one red color LED (D2) is included
in the main circuit which works as a battery charging supply input
indicator. Resistor R1 limits the operating current of this LED.

Pen Torch Electronic Circuit Schematic

Diode D1 works as an input polarity guard cum reverse current flow
preventer. Capacitor C1 is a simple buffer for circuit stabilization.
After succesful construction, preferably on a small piece of general
purpose PCB, enclose the whole circuit in a
suitable and attractive pen torch cabinet. If necessary, drill suitable
holes in the cabinet to attatch the dc socket, on/off switch and the
input indicator etc. In prototype,commonly available 4.8 volt/500mah
Ni-Cd battery pack (for cordless telephones) is used.

One very simple but reliable ac mains powered battery charger
circuit for the handy pen torch is also included here. Basically the pen
torch circuit is a constant current charger wired around Transistor T1
(BC636), powered by a 12v/350mA step down transformer and associated
componentsD1, D2 and C1.

AC mains powered battery charger for the pen torch

Unregulated 12 volt dc available from the input power convereter circuit, comprising step down transformer(TRF), rectifier diodes (D1,D2) and filter capacitor (C1), is fed to T1 through a current limiting resistor R1. Grounded base PNP
transistor T1 here works as a constant current generator. With 22 ohm
resistor for R1, the charging current available at the output of the
charger is near 50mA.

Red LED (D3) provides a fixed voltage
reference to the base of T1, with the help of resistor R2. (During
charging process, Diode D1 in the main circuit prevent reverse current
flow from the battery pack when charging input supply is absent.) After
construction of the pen torch circuit, fit the assembled unit inside a
small plastic enclosure for safety and convenience.

Circuit Source: DIY Electronics Projects

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