This Design Idea describes a method by which you can detect and assess air or liquid fluid flow using an externally heated semiconductor diode. Airflow across the heated diode reduces its temperature, causing a variation in the diode's voltage drop (Figure 1). This principle is similar to that used in hot-wire anemometers.
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| Figure 1. | When cooled by an applied fluid flow, a heated diode increases its forward voltage. |
You can use this effect to make a novelty electronic candle as shown in Figure 2a. By placing the sensor diode close to the LED, one can make it turn off by blowing air at it. (A few of these each powered by a pair of AAA cells stacked in tubing to actually resemble candles would make an amusing addition to a birthday cake.) The comparator’s inverting input is biased using the 100k trimpot, while the non-inverting input is connected to the diode, which is heated to around 50 – 70 °C. Adjust the trimpot so that the LED is just fully on. At this point, the non-inverting input is at a slightly lower potential than the inverting input. When air is blown, the temperature of the diode decreases, causing an increase in the voltage drop (a typical silicon diode has –2.1 mV/°C voltage drop variation). This causes the non-inverting input's voltage to exceed that of the inverting input, causing the comparator output to open and turning off the LED.
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| Figure 2. | These circuits demonstrate applications using a diode as a temperature sensor: a) The electronic candle LED turns off when air is blown onto the 1N4148 diode. By using a resistor-trimpot combination, more precision adjustment is possible. Perhaps –5 V for LM393 is not required; ground would do in this case. b) The wind meter works on the same principle as a hot-wire anemometer. |
The simple wind meter (Figure 2b) operates on the same principle as a Thermal Probe (Reference 1.) It can be calibrated by heating the diode with a certain current and adjusting the right trimpot to get a zero deflection in the meter at room temperature without any air flow. Air flow across the sensor diode then results in a voltage difference between the two ends of the meter causing it to deflect. The marked (*) 1N4148 diode should be exposed to the same air as the heated diode to compensate for air temperature variations.
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| Figure 3. | Close-up of the sensor diode. |
Another possible application would be in a respiration meter where the inhaling air can cool the diode. One can also think of different methods of heating the diode so that the cool air comes in more contact with the diode surface and leads. Thin wire from a metal wire resistor is an easily available material for the diode-heater coil (Figure 3).
