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

 

16

The initial state after switching on is indeterminate; the transistor could thus be completely blocked or completely 
conductive. In both cases, small differences in the gate voltage have no effect. There is thus a start switch with which 
you briefly connect gate and drain. The gate voltage adapts to the middle range at around ca. 2 V.

 

 

 

Fig. 42: The electrometer (Schaltung16.jpg)

 

 

 

 
Fig. 43: Detection of electric charges (Aufbau16.jpg)

 

 

This experiment shows another way to make a simple light sensor. This time you use a BS170. Two LEDs serve as 
light sensors. In Section 16, an LED was used as a light sensor with two NPN transistors in a Darlington circuit. 
Thanks to the almost infinitely large input resistance, a single MOSFET performs the same task alone. But now you 
need two LEDs as light sensors. The LEDs are used as photovoltaic cells that can deliver a voltage. The BS170 
conducts starting at a gate voltage of 2 V. With sufficient illumination, two LEDs together can produce the required 
voltage. Even just a slight increase in brightness shows the effect. Also experiment with different LEDs. A green LED 
delivers somewhat more voltage than a red one. 

 

 

 

Fig. 44: LEDs as photovoltaic cells (Schaltung17.jpg)