BUILD A TWO-TRANSISTOR OSCILLATOR

In this experiment, we will build a simple two-transistor oscillator that will generate an audio tone in a speaker.

1. Build the Circuit.
Using the schematic along with the pictorial diagram, place the components on a solderless breadboard as shown. Verify that your wiring is correct.

2. Do the Experiment.
Theory: A transistor oscillator is a circuit that generates a constantly changing current all by itself. The frequency of this varying current tells you how many times per second a complete cycle of change occurs. This oscillator generates a signal of a few volts (about three volts) and approx. 500 cycles per second. This two-transistor direct coupled oscillator uses one NPN and one PNP type transistor. The oscillation is maintained by the feedback from the output (speaker) to the input (base of Q1) through capacitor C1. The frequency of the oscillation is determined by the values of C1 and resistor Rl. The larger the values of R1 and C1, the lower the frequency of oscillation. As the battery is connected, electrons begin to flow from the negative terminal of the battery through R2 and the speaker to charge CI. This small current flowing through the speaker while C1 is charging causes the speaker cone to move slightly. As C1 is charging, Ql begins to conduct current which allows Q2 to begin to conduct current. When Q2 conducts, the electrons travel from the negative of the battery through R2, the speaker, and Q2, back to the positive of the battery. This current is larger and causes the speaker cone to move even more in the same direction. When Q2 conducts, it will discharge C1 which, in turn, causes Q1 to stop conducting, which causes Q2 to stop conducting. The speaker returns to its rest position.

Procedure.
Connect a ninevolt battery to the battery snap and you will hear an approx. 500 Hz (500 cycles per second) tone from the speaker. This one can be used as an alarm sound to let you know someone has opened a door, for example.