Nuts and Volts - January 2008

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.