Do you put water on to boil and then completely forget about it because something else grabs your attention? Here is a project to help!

This smart weekend project consists of a temperature sensor — current selection; check compatibility and an active fixed-frequency buzzer and can notify you as soon as the water boils. The table below lists the components needed to get started. We recommend buying the component mega mix box — current selection; check compatibility which actually includes two buzzers: an active fixed-frequency and a PWM buzzer. You will also have lots of extra components for future projects.

Components

Quantity	Component image	Component
1		Temperature sensor with comparator — current selection; check compatibility
1		Active buzzer
1		Component mega mix — current selection; check compatibility
1		Coin cell holder (CR2032)

The table below shows exactly what you need from the component mega mix box — current selection; check compatibility. You need an NPN transistor and a resistor. In many ways, the transistor works like a tap: a signal at one terminal allows current to flow through the other terminals. This arrangement switches the buzzer on and off when the temperature reaches a certain level.

A Brief Look at Transistors and Why We Use One

A transistor is an electronic component used in a circuit to control a large current or voltage with a small voltage or current. This small current enters the base terminal and controls a larger current through the collector and emitter. Check the datasheet pinout for the specific transistor and package; terminal order can vary. It can therefore amplify or switch electrical signals or current, making it useful in a wide range of electronic devices.

This is done by sandwiching one semiconductor between two others. Since current is transferred across a material that normally has high resistance (a resistor), it is a “transfer resistor”, or a transistor. We use it here because the digital output cannot directly supply the current the buzzer needs. As described above, the transistor lets this small current control a much larger current directly from the battery to the buzzer. 

Circuit Setup

The image below gives an overview of the circuit, built on a breadboard for testing. A breadboard is included in the component mega mix box — current selection; check compatibility, along with wires to connect the components.

Breadboard Setup

The image below shows a small test setup using the XY-UDP. XY-UDP is a small, smart, compact USB-powered power supply, also available from ebits.dk. To read more about the XY-UDP we have written this article with a short guide to its functions and operating ranges: XY-UDP | A Small, Smart Power Supply!

If you do not want to use the XY-UDP for testing, you can simply connect the coin cell holder with a battery to the system. The advantage of the XY-UDP is that you can also measure the system’s current consumption during testing: standby consumption without the buzzer sounding is 1mA. Actual battery life depends on cell capacity and load; a continuous 1mA draw is significant for a CR2032.

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Final Implementation

The image below shows the completed project in a 3D-printed enclosure. Download the enclosure STL file from GitHub at this link:Cook’s Box

Setting the Trigger Temperature

To set the temperature on the temperature sensor — current selection; check compatibility, turn the small blue potentiometer. The image below shows that turning the screw left requires a higher temperature to trigger the buzzer. This happens because the board has a built-in comparator (LM393) that compares the sensor voltage with a reference set by the potentiometer. LM393 has an open-collector output; output polarity and pull-up depend on the module design and must be checked. The transistor is connected to this digital output. This useful device has a broad operating range of -55°C-125°C.

Once the trigger is set, the project can serve as an experimental temperature reminder. Keep electronics and the battery away from water and steam, and do not use it as a substitute for supervising a hot hob. Enjoy! 

What Is a Comparator?

The temperature sensor — current selection; check compatibility with comparator is equipped with an NTC thermistor. This is the small black part at the front of the board. An NTC thermistor acts as a resistor whose resistance decreases as temperature rises and increases as temperature falls. 

If you place this NTC thermistor in series with an ordinary resistor, the voltage at the junction between them varies. This is where a comparator is connected to examine the difference.In analogue electronics a comparator is a circuit that compares two analogue voltages and changes its output state according to which input voltage is higher. This signal controls the transistor; check the module’s output polarity and pull-up.