Automated Greenhouse Window Vent- Motor Control

Introduction

This session will explore the use of a bimetallic strip as a temperature-sensitive switch to automatically control the opening and closing of the greenhouse. A bimetallic strip bends in response to temperature changes, allowing it to make or break an electrical contact in the motor control circuit.

The strip will be in the neutral position at 20°C. When 25°C is reached, the strip will bend upwards and make contact, driving the motor to open the greenhouse. When 15°C is reached, the strip will bend down and make contact with the opposing contact to close the greenhouse.

Part 1: Simple Design

The strip is made of copper and steel. The length, L, of the bending part of the strip is 3 cm. The required angle of deflection, θ, is +/-10 deg. Coefficients of thermal expansion can be found through this link.

Part 2: Spiral Bimetallic Strip

An alternative design is suggested which uses a spiral bimetallic strip, offering a greater length (L) for thermal expansion within a compact form. This design ensures a more significant and sensitive mechanical response to temperature changes.

This online calculator can be used to calculate the geometry of an Archimedes spiral (one with constant gap between successive spiral turns).

Extension Tasks:

Now that you have a basic design for the bimetallic switch, you may consider looking into the motor control circuit. The circuit should do the following enable the three different states below based on the location of the bimetallic strip.

References:

1. https://www.cooksinfo.com/bimetallic-coil-thermometers
2. https://demonstrations.wolfram.com/BimetallicSpiralThermometer/