As everyone knows, diodes have a stubborn characteristic of "one-way conduction and reverse cutoff," thus playing an important role in circuits. Some people even utilize the reverse voltage drop of diodes as Zener diodes. However, diodes also have a little-known "sensitive" side. This adds to their mystique.
Diodes are highly sensitive components, extremely sensitive to temperature and heat. The 1N4148 diode is so common it's practically ubiquitous. Here, we'll use it as an example to introduce its sensitive characteristics. You can listen while holding one in your hand and observing it.
Let's take a look at the "health check report" of the 1N4148 diode:
Reverse recovery speed: Super fast, less than 0.000000004 seconds (4 nanoseconds)
Forward withstand voltage: Low, approximately 100V
Reverse withstand voltage: lower than approximately
Maximum forward current: approximately 0.2A
There's a whole lot more, but I won't go into detail...
As can be seen from the above, the 1N4148 is a low-current, high-frequency switching diode. However, since we are only using it to measure temperature, the current draw is not a concern.
After researching, I figured it out: the forward voltage drop of a 1N4148 diode decreases by approximately 0.003V (3mV) for every degree Celsius increase in temperature. If you connect five 1N4148 diodes in series, the forward voltage drop will decrease by 0.015V (15mV) for every degree Celsius increase in temperature. The more diodes you connect, the greater the change, and the easier it is to detect. It's truly a case of "the more diodes, the better!"
However, everyone must be careful, as the saying goes, "too much of a good thing can be bad." If you connect too many diodes in series, you're doomed. The forward voltage of a single 1N4148 diode is approximately 0.7V (I think). So, if you connect too many in series, the forward voltage of the diode array will exceed your power supply voltage, and you'll be in trouble. Because the diode array simply won't conduct, and you won't be able to measure the temperature.
Let's analyze a specific feasible solution using a microcontroller: Here we take the STC12C5A60S2 as an example.
The STC12C5A60S2 is currently the most powerful device in STC's 12C series, boasting top-of-the-line specifications in every aspect, including the AD conversion we'll be discussing today.
This microcontroller has an AD conversion capability of up to 10 bits, which means it can output a total of 1024 different values, from 0 to 1023.
Roughly calculating, 5000mV/1024 ≈ 4.88mV. This means the microcontroller's AD conversion can only detect a change of 4.88mV. If the change is smaller than this, it might not be detected at all. Therefore, we need at least two diodes in series; that's the minimum. If you only use one, the thermometer's accuracy won't even reach 1°C. Such accuracy is indeed worrying.
LED
LEDs are pretty, aren't they? But don't assume that LEDs only emit light. They are still diodes, and they can be used to measure temperature. However, I strongly advise against using LEDs for temperature measurement. LEDs have a little-known characteristic: they are also quite "sensitive."
Even a fool knows that an LED will light up when the positive terminal is connected to the positive terminal and the negative terminal to the negative terminal, and it won't light up when reversed, because it is also a diode and has unidirectional conductivity. But you may not know that an LED has another little-known use when reversed—like a photoresistor, it can also measure light.
We will also use the STC12C5A60S2 as an example for introduction.
Use two I/O ports to achieve light measurement: set the I/O port on the positive side of the LED as a high-impedance input, and set the push-pull output on the negative side, and pull the level high.
At this moment, a miracle occurred: the level of the high-impedance input I/O port would change with the ambient light, being high when the light was bright and low when the light was dim.
