Analog Electronics AGH Exercices

4 Comparators

Exercise 1 Window blind control

The goal of electronic circuit is to detect the dawn and sun set and automaticly close/open the window blind

As the light detector the photo resistance is used. Its characteristic expressing the resistivity Rp as a function of light (measured in lux) is shown in fig . 1. The intensity of light at the dawn is between 1 and 5 Lux

Figure 1

1. In the first time the dawn detector which controls the window blind motor is realized as follows R2 and R3 = 100 kΩ

Figure 1

1 Illustrate the evolution of Rp, Vs, V+,V- as a function of L for complete cycle of 24 hours

2. Find R1 value for which Vs change its state when L crosses level L=1Lux

3. Propose such a solution that a Led diode lights when L <1 Lux

4 What is the main drawback of the above system if light changes slowly. What will happen when sky is cloudy?

5. In order to avoid the problems described in question 4 , a new solution is proposed. Illustrate the evolution of Rp, Vs, V+,V- as a function of L for complete cycle of 24 hours.

How do the problems are solved?

Figure 2

Find R1 and R4 values for which the window blind closes when L becomes smaller then 1Lux and rests closed until L>5 Lux. R2 and R3 = 100 kΩ

Exercise 2 Voltage Controlled Oscillator

In the circuit shown in Figure 4 , the operational oprifiels are considered as to be perfect. Their supply voltage is symmetric i.e E and –E where E = 15V

Figure 4

1. What function does it fulfill the first amplifier AOP 1.

2. Show that AOP 2 acts as a comparator with hysteresis and find its thresholds; take R1=10 kΩ et R2 = 30 kΩ.. Draw output/input characteristics

3. Find R3 et R4 such as that pic-pic value of V_c is comprised in interval 2 and 20 V. P= 10kΩ..

4. Draw signals V_S1, V_S2 et V_S3.

5. Show that the frequency F of output signal VS2 is proportional to VC.

6. Why does AOP3 is used here, justify it.

7. For R =10 kΩ and C =1 μF, find F=f(P)

8. Circuit is modified as shown in figure 5. The modulating signal m(t) is taken under form:

where f(t) is a non-biased periodic function of period T_m, such as T_m>>1/F. Tracer signal U_o(t) for f(t) of sinusoidal and triangular shape.

Bloc X indicates an analog multiplier of transfer function as:

Figure 5