1. Team Activity: Problem 1 on p. 183 – Method 2
Find the equivalent resistances R12 & R34.
Find the total resistance RT.
Find the total current IT.

2. A Second Example: Parallel Circuits Connected in Series

3. Team Activity: Problem 5 on p. 184.
Find the equivalent resistances R12 & R34.
Calculate the total current IT.

4. Team Activity Calculate the following equivalent resistance:
Rtot = [(R7 + R6)||R5||(R4 + R3)] + (R2||R1) where Ri = i kΩ
Draw the circuit in Multisim and verify your resistance with the multimeter. Type your mulitmeter reading, your names and your group number on the circuit.
Turn in your Multisim schematic and your hand calculations.

5. Wheatstone Bridge
Galvanometer – a sensitive ammeter

6. Bridge Operation – based on three possible states:
VA = VB VA > VB
VA < VB

7. Wheatstone Bridge: VA = VB means at “balance”
No current flows through the Galvanometer when the bridge is balanced so you can think of it as:

8. Wheatstone Bridge: A series-parallel arrangement at “balance”
Resistors R1 and R2 are in series
R12 = R1 + R2
Resistors R3 and R4 are in series
R34 = R3 + R4
Equivalent resistance R12 is in parallel with equivalent resistance R34

9. Circuit Analysis at “balance”, galvanometer current =0

10. Application of a Wheatstone Bridge: Smoke Detector
Photoconductive Cells – Light-Controlled Resistors Used as Sensor and Reference
When the amount of light striking the device increases, the resistance decreases (and vice-versa).

11. Detector Diagram

12. How it works When the air is clear, the bridge is balanced.
If smoke enters the lower chamber via the vents, the resistance of the lower photoconductive cell decreases, causing the bridge to become unbalanced, triggering the alarm.
SMOKE

13. Switches
A switch allows you to “make” or “break” the connection between two or more points in a circuit.
Pole – the moving contact(s)
Throw – the non-moving contact(s)
Switches are identified by the number of poles and the number of throws

14. Switch Reference Material
Sections & 2.8.2, pp in your textbook.

15. Switch Definitions One moving contact Two non-moving contacts
SPDT
Single-Pole
Double throw
SPDT – Single-Pole, Double-Throw switch
Already seen in RC Circuits 15

16. Toggle Switch Activate by pushing against a lever arm
These two contacts connected when switch is in position shown

17. SPST – Single-Pole, Single-Throw switch
Switch Definitions
One moving contact
One non-moving contact
Single-Pole
Single throw
SPST – Single-Pole, Single-Throw switch
(On-Off Switch)

18. Switch Application

19. Push-Button Switches Activate the switch by pushing a button
Two types of Push-Button Switches
Normally-closed (NC)
When the button is NOT pushed, the contact is closed
Normally-open (NO)
When the button is NOT pushed, the contact is open

20. Normally-Closed (NC) Normally-Open (NO)