Simple Diode Over-Temperature Indicator (LED)

Today, let’s experiment with a simple over-temperature indicator circuit using simple parts. It consists of only four components, all readily available and fun. We believe everyone can try it, and it is useful too. We will use a 1N4148 diode as a simple temperature sensor, an LED to display a blinking light when the temperature is too high, and a transistor to amplify the current.

Meet Our Diode

We have previously used a 1N4148 silicon diode as a temperature sensor. It works quite well, and there are still interesting things or experiments to learn.

The 1N4148 is a small cylindrical glass rod with leads protruding from both sides. The inside is red, and a black band always indicates the K (Cathode) terminal. The opposite side is the A (Anode) terminal. It is also marked with the letters 4148.

When it receives a reverse bias, no current will flow through it. However, if we experimentally increase the temperature to a certain level, it will allow current to flow through it, or its resistance will decrease. Once the temperature returns to normal, the current will stop flowing through it again.

Let’s burn out a diode

We discovered the above principle by accident during a fun experiment.  We only need a kitchen lighter, a 9V battery, and a multimeter (analog is better than digital). Use the lowest current measurement range, or ammeter at 50uA.

You should quickly test the diode by lighting a lighter at it for just 3 seconds. Observe the meter needle; it should rise very rapidly. Then you remove the lighter, the meter needle should return to normal, or current should no longer flow through the 1N4148.

Simple Over-Temperature Indicator (LED)

Since the current flowing through the diode is so small (about 50uA), we cannot connect the LED directly. We need a good and simple helper, which is a transistor. Let’s try connecting the circuit below.

There are very few devices needed. You might wonder why we connect a small LED directly to a 9V battery without a limiting current resistor. Normally, that is always the case, but in this case, it is not necessary.

Minimal Over-Temperature Indicator Using LED

However, this circuit cannot provide a warning at low temperatures, such as warm water or heated transistors. When the temperature drops (below 40 degrees Celsius), the resistance of the 1N4148 in the LED increases, causing IB to become too low, insufficient for a single transistor with a limited gain of only about 50 to 100.

This problem can be solved by adding another transistor and connecting them in a Darlington form, significantly increasing the gain. Additionally, R1 should be increased to a value of 10K to 47K.

The full experimental process and schematics are documented in the extended notes.

Then, we tested the circuit again by immersing the 1N4148 (with insulation) in hot water. We observed that LED1 started to light up at 37 degrees Celsius and became very bright at 47 degrees Celsius. But it should not be allowed to exceed 60 degrees Celsius because the current flowing through the LED, becomes too high and the diode overheats.

And in another test, attaching or clamping the 1N4148 to a high-current ceramic resistor. Then, gradually increase its temperature; it also works well. The LED1 flashing to alert of rising temperature is a beneficial function.

In summary, the 1N4148 diode can be used as a good heat detector because of its small size, high sensitivity, and ease of modification or application. However, it is not designed for direct temperature detection, so its efficiency and accuracy are only moderate. Nevertheless, we think it is a fun, exciting, and challenging experiment. Just be careful not to overheat it and permanently damage it.