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INCUBATOR HEATER
UPDATED:.21:03 02 January 2019
DESCRIPTION SET UP NOTES

This circuit will control a heat source such as a heating mat or a high wattage bulb for an incubator or seed propagator.

The heat source should be central to the incubator or environment it is heating, and the sensor near the edge.

The temperature can be controlled by VR1 on the Comparator Switch.

There are 2 versions of the circuit shown below.

Circuit Voltage: 6 to 9 volts
Heater Voltage: as required

HEAT SENSOR:
Adjust VR1 with a screwdriver to the centre position.
If the temperature you require is outside the range of VR1 on the Comparator Switch, then adjust VR1 on the Heat Sensor.

COMPARATOR SWITCH
The Schmitt setting should be selected by moving the jumper to the right, creating hysterisis in the switching.
BLOCK DIAGRAM - Circuit 1
CIRCUIT BLOX - Circuit 1

EXPLANATION
As the temperature rises, the voltage from the Heat Sensor rises.

As the voltage rises above Vref on the Comparator Switch, the output turns on. This is inverted by the Inverter and switches off the Relay and Heater.

As the temperature falls, the voltage from the sensor falls below Vref and the output turns off. This is inverted, turning on the Relay and Heater.

There is some hysterisis in the switching meaning that the "on" temperature setting is slightly higher than the off setting. This prevents chatter and rapid switching if the heater is too close to the sensor.
BLOCK DIAGRAM - Circuit 2
CIRCUIT BLOX - Circuit 2

EXPLANATION
In this version the Heat Sensor has be connected upside down, ie: the supply polarity has been reversed.
This means that as the temeprature falls the voltage rises and is thus a Cold Sensor, working in the opposite manner from before. This means the Inverter can be omitted, simplifying the design.

As the temperature falls, the voltage rises above Vref on the Comparator Switch and the output turns on, activating the Relay and Heater. As the temperature rises, the voltage falls below Vref, turning the output off, turning off the Relay and Heater.
TEACHING OPPORTUNITIES: DEVELOPMENT
  • Sensors in potential dividers.
  • Reversing compnents for opposite function.
  • Using a variable for R2 to create adjustment.
  • Operational amplifier as a comparator.

This is a common circuit for sensing heat, light and moisture and can be adapted for similar projects.

Vout = Vin x (R2 / R1+R2) to calculate voltages.

This can be expanded to using graphs showing resistance against temperature and predicting votages based on temperature in a potential divider.

 There are many applications for sensing heat and also by using other sensors instead of the thermistor, eg. LDR and moisture sensor.

Some examples are:

  • Cooling System - more
  • Soil Moisture Tester
  • Baby Bath Temperature Alert

.