Simple Temperature Alarm System (Has No Microcontroller Just Basic Electronics)

This project is what I consider to be my first actual circuitry project. Since I don't have much experience, I wanted it to be simple, and I didn't want to dabble with microcontrollers just yet. Because there are no microcontrollers, there is no coding, which makes it a bit simpler. This circuit only uses resistors and a comparator to detect whenever it is cold. The piece that actually does the sensing is a variable resistor that gains resistance with less temperature (An NTC Thermistor). This simple thermometer can be "coded" to other temperatures by changing the resistance of a certain resistor. I will go over this more in other steps.

This cheap project only costs ~$2.50 per unit.

I got the idea for this because of my rabbits. My rabbits reside in a small shed outside, equipped with a heater and an A/C unit. I only have one thermometer, though, and the shed lacks circulation; because of this, the top can be up to 15 degrees hotter than the bottom. So to fix this problem, I designed and made this makeshift thermometer that is wireless and can run for 48 - 240 hours on batteries.

Circuit Board

5mm LED Diode (any color) ~$0.03

24 AWG Stranded Wire (~8 in.) ~$0.10

10K Ω NTC-MF52AT Thermistor ~$0.09

2 Slot AA Battery Case ~$1.00

LM393 Comparators* ~$0.05

Black PCB Board 3cm x 7cm (Perfboard) ~$0.16

1/4 Watt Resistors (5) ~$0.05

10k Ω (3)

220 Ω (1)

33K Ω (1)

Casing

White PLA (~25 grams) ~$0.40

M2.5 10mm Screws (3) ~$0.14

M2.5 Hex Nuts (3) ~$0.10

M2.5 Washers (6) ~$0.08

Tools / Miscellaneous

3D Printer

Soldering Iron + Solder

2mm Allen Key

Hot Glue Gun + Hot Glue

Needle Nose Pliers

Wire Strippers

Patience

I got all of my hardware from Aliexpress in bulk, and I recommend you do the same so you can have plenty of room for error.

*(recommend getting multiple as they are hard to work with)

At the core of the circuit is an LM393 comparator, which acts as a decision-making component. It continuously compares two voltages:

1. A sensor voltage derived from a thermistor (temperature-dependent resistor)
2. A reference voltage set by fixed resistors

When these two voltages cross, the comparator switches its output, turning an LED on or off.

The device works with 2 main parts: sensing the temperature and comparing it to a fixed voltage, which determines when to turn on the LED. The sensor used is a 10k NTC thermistor, which changes resistance with temperature:

1. Warmer temperatures allow for lower resistance
2. Colder temperatures allow for higher resistance

This thermistor is paired with a fixed resistor to form a voltage divider, converting temperature into a measurable voltage.

A second voltage divider creates a stable reference voltage. This represents the temperature threshold (for example, freezing).

The comparator continuously evaluates:

1. If the sensor voltage is higher than the reference → one output state
2. If it is lower → the opposite state

This allows the circuit to behave like a temperature switch.

The reference voltage, of course, can be changed, and depending on how you change it, determines at what temperature the LED switches on.

If possible, test your circuit on a breadboard first before soldering, to make sure the circuit works and that you understand my diagram.

Here are some basic things I learned from my first time soldering, and from YouTube.

1. Heat both the metal pad and the component lead at the same time
2. Apply a small amount of solder — you don’t need much
3. Remove the solder, then the iron
4. Let it cool without moving the joint

Some parts in this project are small and close together, especially:

1. The comparator pins
2. Resistor leads
3. Thin wires

Because everything is so small, make sure that nothing crosses and if something does, you can remelt the solder and fix it. I did have to bend the comparator pins so I could get them far enough away to be able to solder them.

I assembled everything in the order of, Resistors / LED, connect the Resistors to ground, then add the comparator and connect them to the resistors.

This was very simple: all I did when the circuit was fully assembled was first use a multimeter to see if anything was touching that wasn't supposed to, then I hooked all the wires up and threw it in the freezer.

It took a long time for the entire system to cool down, but when it did, it worked flawlessly. Whenever I touch the sensor, it warms up and turns on. My freezer was also much colder than freezing temperature, so I instead put it outside, which was about 30 degrees, and it still worked well.

This step was also fairly simple, as all I did was measure the circuit now that it worked and was fully assembled. I gave a clearance of .75 mm on all sides, made air vents for the sensor, and added a hole for the LED to stick out of. I also added some text to the top so I wouldn't forget what it was set to.

The hardest part of this step for me was figuring out how to attach the back to the main holder. After trying to make it a friction fit for a while, I eventually just added screw holes, as they are simple and are not very hard to print.

It takes a while for the entire system to cool down, but when it does cool down, it stays cool and doesn't flicker. The sensor is very accurate and from my limited testing it seemed to be within a degree of error.