User manual

Fig. 36: The flashing light circuit (Schaltung13.jpg)

Remove from the circuit the 1 k resistor that is parallel to the LED: the pause between flashes of light is considerably

prolonged. The left transistor blocks only when the electrolytic capacitor is completely discharged. Only then does its

collector voltage slowly increase so as to enable a new pulse.

Fig. 37: LED flashing light (Aufbau13.jpg)

15. Step: MOSFET touch sensor

This circuit with the MOSFET BS170 (metal oxide semiconductor field effect transistor) controls an LED by means of

two pairs of contacts which you can directly connect or touch with your finger. After a short connection of the contacts,

the respective state is preserved for a longer period of time.

The NPN transistor was presented in the first experiment with a simple basic circuit. A base current has to flow so that

a collector current is possible. A similar experiment with the MOSFET BS170 exhibits completely different behaviour.

The MOSFET has three terminals: gate (G), source (S) and drain (D). This time, the controlled current does not

depend on an input current, but rather on the voltage applied between G and S. If there is a positive voltage of about 2

V or more at the gate, the transistor conducts. The gate terminal is completely insulated and forms a small capacitor

with about 20 pF. Once the gate is charged the gate voltage thus persists for a long time.

Fig. 38: Basic MOSFET circuit (Schaltung14.jpg)

Briefly connect the terminals A and B to charge the gate. The LED goes on and stays on. Connect the contacts C and

D to discharge the gate and turn the LED off. Each of these two possible states persists for a relatively long time. The

experiment thus demonstrates the basic functioning of a dynamic storage unit which also stores an electric charge so

as to illustrate the one and zero states. At the same time the circuit is a simple touch switch, because when the

contacts A and B or C and D touch it has the same effect as a direct contact.

But be careful! A gate voltage of more than 20 V is not allowed and can lead to destruction of the transistor. Therefore

you have to be careful with electrostatic charge. So always first touch a terminal of the operating voltage so as to

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