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

 

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Fig. 19: The contact sensor (F_ Darlington1.jpg)

 

 

This circuit has a sensor wire at the input of the first transistor. If someone moves in the vicinity of the wire, the LED 
lights up. Through motion on an insulating background, every person charges him-/herself electrically without noticing 
it. If you then move in the vicinity of conductive objects, the electrostatic forces lead to a shifting of electric charges, 
i.e., to a small current, which is highly amplified here. The Darlington circuit activates a PNP transistor so that the 
current gain once again becomes 300 times bigger. Now a few picoamperes already suffice to cause the red LED to 
visibly shine. 

 

 

 

Fig. 20:

 

Current gain with three transistors (Schaltung5.jpg)

 

 

 

 
Fig. 21: Sensor amplifier for electric fields (Aufbau5.jpg)

 

 
For the first test of the circuit, a short 10 cm sensor wire is suitable. After some motion on an insulating floor, you 
normally have gathered sufficient electric charge. Then move your hand in the vicinity of the sensor wire. The 
brightness of the LED changes.

 

 
To increase the sensitivity of the circuit, a longer sensor wire can be connected. It can be a bare wire or an insulated 
cable. The sensor becomes even more effective if you additionally earth the negative pole of the battery. For this it is 
sufficient if a second person touches the circuit. Now it is already detected if someone walks by at a distance of half a 
meter. The flashing of the LED shows the individual steps. If there is direct contact with the bare wire end, you see a 
continuous shining. You can trace this back to the unavoidable 50 Hz alternating fields in the room. The LED actually 
doesn’t shine constantly, but rather flashes at a frequency of 50 Hz. 

 

 

red