Conrad Course material 3964 14 years and over 3964 Manual De Usuario

 

17

 

 
Fig. 45: The light sensor (Aufbau17.jpg)

 

 

A ceramic capacitor with 100 nF can be used as a temperature sensor. Such a capacitor has a large temperature 
coefficient. The capacity is diminished with heating. In this experiment, the switch must initially be closed and then 
opened again. The gate voltage automatically adapts to the threshold voltage of approximately 2 V; the LED shines. 
There is a voltage of about 7 V at the 100 nF capacitor.

 

 
Now touch the capacitor very lightly with your finger, which leads to an increase in temperature. The charge stored in 
the capacitor remains constant. But since the capacity diminishes, the capacitor voltage increases. This leads to a 
smaller gate voltage and thus to a smaller drain current. Even a light touch suffices to cause the LED to shine 
noticeably weaker. The circuit reacts to small changes in temperature more sensitively than the transistor circuit in 
Section 18. As soon as the sensor capacitor has cooled down again, the original LED brightness returns. 

 

 

 

Fig. 46: Analysis of capacitor voltage (Schaltung18.jpg)

 

 

 

 
Fig. 47: The temperature sensor (Aufbau18.jpg)

 

 

The light is switched on by pressing the key switch and then remains on for about a minute. The transition between 
bright and dark is smooth but relatively fast. The electrolytic capacitor is charged to 9 V by pressing the key switch. It 
discharges via the 470 kΩ resistor. As long as the gate voltage is more than ca. 2.6 V, the FET conducts and delivers 
the base current for the NPN transistor and the LED switches on. If the input voltage drops, the FET conducts more 
weakly. Once the base voltage of the NPN transistor has dropped below ca. 0.6 V, no more noticeable collector 
current flows and therefore the LED goes out.   

 

 

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