Circuit Design Help: Progress & Resources

[Aisu01]

Homework Statement

Circuit Requirements
1. A light dependent resistor (LDR) is to be used to determine ambient light levels.
2. A red light emitting diode (LED) should illuminate whenever the ambient light level is
below the pre-set value.
3. A green LED should be illuminated if the ambient light is above the predetermined level.
Note, the use of two LEDs in this manner serves to indicate that power is applied to the
system, as one LED will always be on.
4. An orange LED is to flash on and off repeatedly, in the case that the ambient light level is
low, and the incandescent bulb is not illuminated, i.e., it has failed.

Design recommendations
1. A single 12 volt power supply may be employed.
2. The circuit may be built in a logical form, that is, one functional block at a time, in order that
each stage may be built and tested before the following block is constructed.
3. An operational amplifier may be used as a comparator to “digitise” the voltage across the LDR
light sensor.
4. A transistor may be used to switch power to the incandescent bulb.
5. The voltage across the LDR may be monitored by an operational amplifier configured as a
comparator.
6. A NAND gate may form the basis of the oscillator required to drive the orange LED.

Relevant Equations

N/A

My progress so far:

https://www.multisim.com/content/NV8Ndn6gdNtxbJFroJ8FFN/atempt2/open/

 

[berkeman]

Welcome to PF.

A few comments/questions:

• You mention an incandescent bulb, but before that it is all LEDs for display indicators. How is the incandescent bulb involved?
• I don't see an LDR component anywhere
• The 100TΩ value for R1 is surely a typo?
• When an opamp is used as a comparator, you connect it using positive feedback and adjust the switching hysteresis voltage via resistor ratios. Do you know how to do that?
• Why do you show 3 LEDs when the original problem seems to only ask for a red and a green LED?
• The transistors Q1 and Q2 will likely need emitter resistors, depending on what the next version of your circuit looks like.