Emergency Light
NOR gate. Transistors A and B
function as logical NOT gates for each
input. The two 1N4448 diodes function as the two-input OR gate, and
transistor X functions as the logical
NOT gate on the output. By inverting
the inputs of the NOR gate, we
effectively end up with an AND gate.
I will walk you through all four
possible scenarios that the AND gate
will encounter. In the first three
scenarios, the answer to “Is it dark?” is
NO or the answer to “Has the AC
power failed” is NO. Base-emitter current will not flow through the transistors if their inputs are NO. This keeps
the voltages at their respective collectors high enough to allow current to
flow through one or both diodes and
the base-emitter junction of transistor
X. The collector voltage of transistor X
stays close to 0 volts and keeps the
light switch transistor (Q1) off.
In the final scenario, the answer to
both questions is YES. Both transistor A
and transistor B are conducting, both
collector voltages are close to 0 volts,
and base-emitter current from transistor X cannot flow. The collector voltage
of transistor X is high enough to allow
base-emitter current to flow from the
light switch transistor (Q1) on the main
board and switch on the lights.
PARTS LIST
ITEM/DESCRIPTION PART # QTY
❑ D1 – 2.2V 20 mA 3 mm green LED 253665 1
❑ D2, D3 – 1N4448 diodes 100PIV 0.2A 655242PS 2
❑ R1, R4-R7 – 4.7KΩ 1/4 watt resistors 691024 5
❑ R2 – Cadmium sulfide photoresistor 202403 1
❑ R3 – 10KΩ trimmer potentiometer 254203 1
❑ R8 – 330Ω resistor 1/4 watt resistor 690742 1
❑ IC1 – 741 operational amplifier 24539 1
❑ QA, QB, QX – 2N2222 NPN transistors 28629PS 3
❑ Circuit board standoff mounts 393895 2
❑ Eight-pin DIP socket 683104 1
A 2-1/2” x 2-1/2” circuit board will hold all of the components for this project.
RadioShack may have most of the required parts, but Jameco has a wider
selection and lower prices if you are willing to pay for shipping and wait for the
delivery of the parts. All part numbers listed are from Jameco.
cabinet. The second reason is that I
sealed the circuit board’s components
(with the exception of the CdS
photoresistor, the 10K ohm potentiometer, and the LED) in silicone
caulk for electrical insulation. I used
stiff postcard material to construct
crude molds around the components
that I did not wish to seal off from the
outside world. It took the caulk
between two and three days to set
(see Figure 8). Third, I mounted the
emergency light too high for curious
and untutored hands to reach. Finally,
and most importantly, I wanted to
place my creation on display. NV
Mounting the
Daughterboard
Always disconnect the AC power
before working on this project because
the emergency light lacks a power
isolation transformer. Therefore,
mounting the daughterboard inside
the plastic cabinet of the emergency
light is preferable to mounting it on the
outside of the cabinet.
However, I chose to mount it on
the outside of the cabinet, anyway. I
drilled an offset hole in the circuit
board adjacent to one of the original
mounting holes to allow for the
emergency light’s curvaceous cabinet
(see Figure 7). This reduced the strain
on the circuit board.
My primary reason for mounting
on the exterior was the limited
amount of space available inside the
January 2008 41