ENGR4035 Simulate the system shown in Fig 9.1 using Multisim. Initially set the Variable voltage source VB=1.5V: Basic Electrical Engineering Assignment, OBU, UK

Part A: (30%)
Simulate the system shown in Fig 9.1 using Multisim. Initially set the Variable voltage source VB=1.5V, and ‘open’ the switch and leave it ‘open’. Not an ‘open switch’ is not conducting and a ‘closed switch’ is conducting.

When you open the switch the LED should remain ON, then after a short interval go off. Repeat this sequence by momentarily closing the switch then opening it again. Note that almost instantly the switch is closed the LED comes ON.

Q1. Monitor and record the voltages at pins 2 and 3 of U1, keeping an eye on the LED status. At what voltage on pin 2 does the LED go out? Can you explain why it is no longer ON? Check and record the voltage changes at pin 6 of 741 op-amp (U1) when the LED is ON and OFF.

Q2. Set VB=2V, 4V, 6V and 8V . What happens to the LED for each value and why? Explain what U1 (741 op-amp) is doing?

Q3. The next step is to investigate the current flowing in the transistor (bipolar junction transistor or BJT) Q3. Set the switch to keep the LED OFF and note the values of voltages on Q3 terminals (collector, base and emitter). Now get the LED ON and keep it ON. Measure and record, the voltages across R2 and R4 and calculate the values of IB and IC, these being the base and collector currents respectively. Can you deduce from these measurements the function of Transistor Q3 in the system?

Q4. Go back to the operating conditions needed to get the timer to work for a relatively short time interval and generally probe and record voltages throughout the circuit to see exactly what is happening and try to come up with a brief explanation of how the timer operates.
Explain the operation of the Delay Timer System of Fig. 9.1. Discuss the and explain the performance and limitations of the system. Suggest how could you improve on the limitations?