Temperature Warning Light

When you see your vehicle’s engine temperature warning light turn on, you know what it means! While it doesn't look like a warning light signalling an emergency, the visual notification simply tells you that your engine is overheating which leads to engine damage!

You can obviously build a compact temperature warning light yourself with a handful of discrete electronics components you have in hand. And, you can install it in your lawn mower, leaf blower, genset, etc (or in your new robotic vehicle prototype) to act as an early warning device in case something goes wrong!

This is the schematic of the overtemperature warning light.

Admittedly, this is an odd design revolved around the ubiquitous programmable shunt regulator KA431A (IC1).

KA431A Datasheet https://www.onsemi.com/pdf/datasheet/ka431-d.pdf

The temperature sensor is a common 100K negative temperature coefficient – NTC – thermistor. The thermistor together with its 100K variable pullup resistor (P1) works as a potential divider wired to the reference (R) terminal of IC1 through a 2K2 resistor (R1). Also see the 1uF capacitor (C3) wired in parallel with the NTC thermistor.

The NTC thermistor used in this circuit has a typical ‘zero power resistance’ close to 100KΩ at 25°C (R25<100000Ω). The beta value as indicated in its datasheet is 4400 (B25/50 = 4400K). The beta value is in fact an indication of the shape of the curve that represents the relationship between the resistance and the temperature of a particular thermistor (https://www.ametherm.com/blog/thermistors/thermistor-beta-calculations).

Since the thermistor is a temperature sensitive resistor where its resistance changes with respect to temperature, when the temperature changes, its resistance also changes. The voltage across C3 therefore changes as well when the temperature changes and it follows the standard voltage divider formula. Note that in this circuit the voltage decreases as the temperature increases, and you can set it to a desired threshold (to a certain extent) through the multiturn trimpot P1.

Testing the thermistor alone is easy because you just need a digital multimeter set to measure its resistance. Then use your hot air gun to blow hot air over it to increase the temperature. You will also need to keep a digital thermometer close to it to determine the temperature.

Similarly, you can ‘calibrate’ your prototype with little effort by following these steps:

· Power up the prototype with regulated 5VDC.

· Heat the thermistor to a desired temperature level, 40°C for example.

· And then slowly turn trimpot P1 until voltage across capacitor C3 becomes 2.50V. The indicator (LED1) will go off.

· Ensure that, when the temperature goes above the set point (40°C), the 2.50V drops off, and C1 can then see a voltage well below 2.40V. LED1 wakes up in this state.

Note: Moving the trimpot's wiper completely towards the positive rail (+5V) will damage the fragile thermistor!

In simple terms the KA431A acts a temperature compensated variable/adjustable zener diode and a precise voltage reference. It’s worthy to note that a precision current source can also be created with the KA431A, not directly related to the project being discussed though.

The above pointed idea of current sourcing with KA431A is ridiculously exploited in this project but in a slightly different way. I simply got inspired by that intellectual idea, that’s all!

For that reason, you can always see a somewhat steady current there in between the emitter of S8050 transistor (T1) and the ground rail (GND). In idle state, a little voltage also appears at these points which will raise up sharply in active state, i.e., at the time of an overtemperature detection. I won’t go into much detail here but feel free to ask questions.

My test setup on the breadboard is like this!

My first lab test was by using a 5mm ultrabright red LED as LED1, but later I employed a 1W star red LED (R3=4.7Ω/1W) there. A successful test with one 5V/180Ω signal relay (Panasonic) was also conducted, that is why you can see a freewheeling diode (1N4007) in my breadboard. All thing worked fine!

However, you will need a 5V linear/switched voltage regulator circuitry for the automotive applications of this project. Choose the one that suits you best.

This post offers hobbyists an utterly simple method of building a temperature warning light. The option to use a selection of extra hardware makes this design equally suitable for general or specialized use. Okay, feel free to make your own temperature warning light, and if you spot any issues do let me know!

Thank You!