2. Analogue Electronics Higher Physics Unit 2 Electricity And Electronics Monitoring & Control Circuits

3. Introduction Both Monitoring and Control circuits operate by having (at least) one input component whose resistance is affected by a physical property such as: light (LDR), temperature (themistor) strain (strain gauge). A thermistor

4. Monitoring Circuits produce a changing output voltage, which is monitored and used to show the changing conditions affecting the input resistors.

5. Control Circuits are more complicated in that they make use of the changing output voltage to control an external component, such as a lamp or motor (often to rectify the initial change in conditions)

6. The role of the transistor In a control circuit, a transistor is required to switch an external device such as a motor or lamp.In a control circuit, a transistor is required to switch an external device such as a motor or lamp. In a simple monitoring circuit, a transistor may be used to switch on a warning LED.In a simple monitoring circuit, a transistor may be used to switch on a warning LED.

7. The Bi-Polar Transistor npn - requiring a positive switching voltage There are 2 types: pnp - requiring a negative switching voltage

8. Example: Using npn & pnp In this circuit T 1 is an npn, T 2 is a pnpIn this circuit T 1 is an npn, T 2 is a pnp When V 2 >V 1, the op-amp output is positive, T 1 operates, the motor turns in one direction.When V 2 >V 1, the op-amp output is positive, T 1 operates, the motor turns in one direction. When V 2 <V 1, the op-amp output is negative, T 2 operates, motor turns in opposite direction.When V 2 <V 1, the op-amp output is negative, T 2 operates, motor turns in opposite direction.