wind-o-meter

This device utilises the fact that an air current has a cooling effect on an object which is warmer than its surroundings. The object cooled in this case is a transistor (T2) which is connected as a diode. To make it warmer than its surroundings it has been thermally coupled to a tran- sistor (T1) which has a current flowing through it continuously. The wind"s speed is measured by com- paring the voltage across the cooling diode with that across a reference diode (T3). These two voltages are fed to the non-inverting and inverting inputs respectively of an opamp. This amplifier, which is preset for a gain of 1000, passes a current through the heating transistor via resistor R1. When the wind cools the diode, the forward voltage across that diode rises (2 mV/°C) causing 5V 17 wind-o-meter RI the voltage at the non-inverting input of the opamp to increase. As a result, the output voltage of the opamp rises to provide more base drive current for T1 thereby generating more heat in thi...