# Linear Temperature Sensor Design

__By Geoff Knagge__

The basic requirement of this project was to design a circuit which would :

- Use the transistor junction characteristics to measure a change in temperature
- Represent the actual temperature on a scale 0 to 10V, corresponding to 0 to 100°C.
- Provide a logic signal which goes low if the temperature exceeds 90°C, and returns to high once the temperature falls back below 80°C.

This assignment scored 99%... I lost the mark because it would be better to have added an additional resistor to the hysteresis output stage.

The final design can be divided into the following three modules :

- The sensor contains the temperature detector and outputs an analog signal which is linearly proportional to the Kelvin value of the measured temperature. The design aim was for 0.5mV = 1K
- The scaler converts this output to one which is linearly proportional to the Centigrade value of the measured temperature. The requirement is that 0.1V = 1 °C.
- The hysteresis unit determines if the temperature is in a safe range. It will output a high logic signal if this is the case. When the temperature rises above 90°C (9V), the signal will go low and will remain that way until the temperature drops below 80°C (8V).

Since we are assuming ideal op-amps, each module can be dealt with individually without having to consider interconnection problems such as loading effects.

### Sensor Unit

+V = 15V R R V |

The above circuit is the logarithmic converter presented in lectures, but with a fixed input voltage and no scaling correction.

The temperature sensor design makes use of the non-ideal property of the scaling factor V_{T} changing linearly with temperature, but corrects the problem of the offset I_{E0}.

Hence, from the lecture notes,

The result is an output which changes linearly with temperature, at a rate of 0.5mV / K.

### Scaling Unit

This is a non-inverting amplifier, where we need

Let R_{4}= 1K

R_{3} = 32.67K

R_{1} = 193.09K

V_{- }= V_{+} = V_{in} for linear operation

So,

For V_{in} = 297*0.0005 (0°C), V_{out} = 29.7 - 29.7 = 0V

For V_{in} = 397*0.0005 (100°C), V_{out} = 39.7 - 29.7 = 10V

This verifies that the amplifier is correctly configured.

### Hysteresis Unit

R_{1} = 10KR _{2} = 200KR _{3} = 10KR _{4} = 15K + 2.7K = 17.7KR _{5} = 18 + 2.7 = 20.7KR _{6} = 100KV _{Z} = 10V |

For positive saturation, . For this to be valid, we need

For negative saturation, . For this to be valid, we need .

This circuit has negative feedback and hence is unstable. Therefore, the op-amp output will always be in a saturated state. It goes into positive saturation (turning on the transistor and pulling the output low) whenever it is already in that state with V_{in} > 8V. If V_{in } drops below 8V, the circuit will revert to the other unstable state, negative saturation.

When in negative saturation, the op-amp switches off the transistor and the logic is pulled high by R_{6}. Similarly, it will remain in this state until V_{in} rises above 9V.

Hence, it can be seen that this module functions as required.