1. Passive Transducers
Elliott

2. Changes in Environment
Passive transducers are often referred to as resistive transducers. They detect a change in the environment (e.g. light level) by changing resistance (hence resistive). They do NOT generate an electric current. That is why they are called passive. 2 min task: Name 3 resistive transducers. Extension: Explain what they do.

3. Active Transducers What do you think an active transducer is?
Active transducers generate a current.  A pick-up cartridge on a record deck is one such.

4. Passive Devices
In your group, come up with a situation where you may use these transducers

5. Found on weighing scales

6. Check Your Progress
Explain the difference between a positive temperature coefficient thermistor and a negative temperature coefficient thermistor.

7. Answer
The NTC thermistor has a resistance which falls as the temperature rises.
The PTC thermistor has a resistance which rises as the temperature rises.

8. Check Your Progress
A thermistor has a negative temperature coefficient.  At 10 oC it has a resistance of 170 ohms.  It is connected to a 20 V supply. 
a.       What current passes through the thermistor?
b.      What is the power dissipated by the thermistor?
c.       Explain what could happen to the thermistor if it were to be left connected to the supply.       

9. Answer
a.    I = V/R = 20 V ÷ 170 W  = A
b.    P = V2/R = 400 V2 ÷ 170 W  = 2.35 W
c.    It could start to get hot  and go into thermal runaway
      because the resistance falls  and the current gets bigger.

10. Potential Divider Circuit
Resistive transducers like LDRs or thermistors are often put into a potential divider circuit.
In electronic circuits, the resistors are arranged so that there is a voltage "tipping point" on the voltage balance, to switch the circuit on.

11. Although it is simple, the potential divider is a very useful circuit
Although it is simple, the potential divider is a very useful circuit.  In its simplest form it is two resistors in series with an input voltage Vs across the ends. 
An output voltage Vout is obtained from a junction between the two resistors.
The potential divider acts like a voltage balance.  Make the resistance at the bottom bigger, and the voltage balance changes to make the voltage bigger.

12. Formula
This result can be thought of as the output voltage being the same fraction of the input voltage as R2 is the fraction of the total resistance.

13. Check Your Progress
What is the output voltage of this potential divider? 

14. Answer
= 4.4 V

15. Check Your Progress
At a certain light level, an LDR has a resistance of 200 ohms.  It is connected  to a 1000 ohm resistor in a potential divider circuit, as shown:
The output voltage is 0.6 V.  What is the input voltage?

16. Answer