Question

PIUulem 3 R2 3200 36800 1k0 5600 LED1 LM339N 10kg U1B 2N2222A LM339N 100 1N5222B V =3V The diode D, is a 1N5222B with a Zener voltage of 3V (even if not true). The purpose of the transistor Q1 is to drive an LED so that it shows the status of the circuit. A) What is the purpose of this circuit? (4-words or less B] Find an expression of the lower and upper thresholds between which the LED is on C) Evaluate these thresholds

Answer 1

Answer:

This circuit is a Window comparator. The LED glows when Vcc is between 6V and 12V

Explanation:

1. The LED glows when Q1 is conducting.

2. Q1 conducts when both the voltage comparators' output is high

3. The lower op-amp (U1B) compares 3V with Vcc*10/(10+10+20) = Vcc/4

4. It outputs high voltage when Vcc/4 is less than 3V

5. The upper op-amp (U1A) compares 3V with (10+10)Vcc/(10+10+20) = Vcc/2

6. It outputs high when Vcc/2 is greater than 3V.

7. Therefore LED glows when either (Vcc/4) < 3V OR Vcc/2>3V . These can be re-written as

Vled = high when Vcc<12V OR Vcc>6V

8. Therefore the LED glows when Vcc lies between 6V and 12V. Graphically, these thresholds and the region for which LED is on are shown in the image below.

VIA UIB 12v LED ON

Hope this helps.