BY CLIFFORD CHIPMAN, KB3PML
circuit to the emergency
light that disabled the
lamps and prevented the
battery from discharging
during the day.
■ FIGURE 3
Circuit Analysis
Lithonia 6V Emergency
Light (EMEPD00114
Rev.A)
A “Portalac” 6V, 4. 5
AH, sealed valve regulated lead acid battery is
the emergency light’s
primary power source. A
120/277 VAC power
supply maintains the battery’s charge. A parallel RC network in
series with the bridge rectifier drops
the line voltage to 15. 65 VAC. A 470
μF 35V electrolytic capacitor filters the
output from the rectifier, and an
LM317T adjustable voltage regulator
in a TO-220 package regulates the
filtered DC voltage (see Figure 3).
A custom integrated circuit
packaged in a 14-pin DIP performs
multiple functions. The chip’s onboard
battery charger monitors the battery
voltage when AC power is available
and adjusts the output voltage of the
LM317 for optimum charging based
on the battery’s state of charge. A
low-voltage disconnect circuit monitors the battery voltage during a
power failure and shuts off the lamps
to prevent damage to the battery from excessive sulfation
of the plates. A power failure
detection circuit monitors the
presence of AC power and
switches the lamps on and off.
My search for the “light
switch” turned up a TIP41
NPN bipolar transistor in a TO-
220 package that the signal
from pin 7 of the IC controls. It
is in a series circuit with the
lamps and the battery. The
lights switch on when the
output of pin 7 is 6 volts, and
they switch off when the signal
from pin 7 falls to 0 volts. I
LITHONIA’S CONTROL
IC FUNCTIONS
• Pin 5 — 6 volt supply
• Pin 6 — Battery voltage feedback input
• Pin 7 — Light switch control signal output
• Pin 9 — AC power failure detection input
• Pin 11 — Voltage regulator control output
• Pin 12 — Ground
• The other pins are unused or their use
could not be determined
needed to tap into this signal for
my brainchild to work, and this
would involve minor “bypass
surgery” on my part.
First, I cut the circuit board
trace between the base of the
light switch transistor and pin 7 of
the control chip with a 1/16” drill
bit (see Figure 4). Next, I scraped
the light green insulating film
from the copper circuit board
traces and soldered a four-conductor ribbon cable from the
freshly exposed traces, the 6V trace,
and the ground trace to the daughterboard I would construct (see Figure 5).
Electric Eye
The schematic for the Electric Eye
option is shown in Figure 6. The
cadmium-sulfide photoresistor voltage
divider provides an analog signal of
the ambient light level to the non-inverting input of the 741 operational
amplifier. The value of the fixed resistor that forms the other
■ FIGURE 5
■ FIGURE 4
January 2008 39
