Question

4. Read Part 2 (a) and sketch the circuit you think will work. Also draw the modified circuit as described in part 2 (b). You may change your design as you work through the lab, however. (You do not need to draw the Optional Design Circuits 1 and 2) You are now finished with Part 1. Part 2: Designing an automatic night light (a) You will need a light-sensitive resistor for this design. A cadmium sulfide (Cds) photocell has a low resistance when exposed to light (typically 1-4 k) and a high resistance in darkness (typ 50k-100 k). Using the Cds cell provided, measure and record the light and dark resistances in your lab notebook. Now design a BJT circuit which turns a red LED on when the room lights are off. Conversely, the LED should extinguish when the room is lit again. (Cup your hands around the circuit to simulate darkness.) (b) Once your circuit is working, you will notice that it is susceptible to flicker when the light is momentarily interrupted (for example, if auto headlights were to occasionally illuminate the sensor of an automated street light). This is definitely undesirable. Modify your circuit so that the illumination must remain either light or dark for about 1 second before a change in the LED status occurs. Hint: You will need to use a capacitor. Shop Note: Electrolytic capacitors are polarized that is, they have + and - terminals. If you connect the capacitor with reversed polarity, it will "leak" a small but devastating DC current through its terminals. Always discharge the capacitor before inserting it into the circuit. This prevents stored charge in the capacitor from destroying your work. When your design is working, draw a schematic of the circuit, demonstrate it to the instructor and get the instructor's design approval on your schematic. In your lab notebook, describe the operation of your design, the rationale of picking the devices, and include a schematic with the instructor's design approval. All designs will be graded when you hand-in your notebook at the end of the semester. Optional Designl: Modify your circuit so that the Cds photocell must be illuminated for 1 second before the light emitting diode goes off AND the photocell must be in the dark for 1 second before the LED turns on. Optional Design2: Add a BJT inverter to your circuit so that an additional green LED is lit when the room lights are "on" but extinguished when the room is darkened. Make certain the red LED still functions as specified in the original design

Answer 1

An automatic night light circuit using a CdS photocell and a BJT can be designed to turn an LED on or off depending on the light level. A capacitor is added to introduce a time constant and reduce flickering, requiring consistent light levels for about 1 second before the LED status changes. The design must be documented and approved by an instructor.

Step-by-step explanation:

Designing an Automatic Night Light Circuit

To design an automatic night light using a BJT (Bipolar Junction Transistor) and a light-sensitive resistor (CdS photocell), you start by measuring the resistance of the photocell in light and dark conditions. Based on these values, you can design a BJT circuit where the BJT switches on the LED when the light level is low (high resistance on the photocell) and turns the LED off when there is sufficient light (low resistance on the photocell).

Modifying the Circuit to Reduce Flicker

To address flickering due to transient changes in light, you can add a capacitor to the circuit. This incorporates a time constant (τ), calculated by multiplying the resistance (R) by the capacitance (C). The time constant determines how quickly the circuit responds to changes in light level. By setting an appropriate value for the time constant, you ensure that the LED status changes only when the light level is stable for about 1 second.

Remember to pay attention to the polarity of the electrolytic capacitors, as incorrect connection can lead to circuit damage. After designing and testing the circuit, you will document the schematic and operational principles in your lab notebook and get your instructor's approval for the design.