Page 29 - The Basics IA by ETron Circuit Labs

Here’s What You’ll Need...R2 R3 R1RESISTORhole 15jhole 30j10 Ohm ' (Brown, Black, Black, Gold) hole 21dhole 29d47 Ohm ' (Yellow, Violet, Black, Gold) hole 15bhole 30b120K & (Brown, Red, Yellow, Gold) Q1DISC CAPACITORSc bNPN TRANSISTOREmitter hole 14dhole 15e hole 7eNPN 2N3904Collector hole 16dBase hole 15dC1 1040.1uFQ1ee cPNP TRANSISTORB1SPKR 1bPNP 2N3906Emitter hole 9dBase hole 8dCollector hole 7dRED a b c d e 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 a b c d e f g h i j 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 f g h i j SPEAKERhole 15i hole 7aWIRESW1 = 30a & 9b W2 = 21c & 30cW3 = 14e & 30hW4 = 8c & 16cBLACKred = hole 29e black = hole 30fSCAN HERE TO SEE MORE ONLINE27second. This 2 transistor direct-coupled oscillator (meaning that the output of transistor Q1 is connected directly to the input of transistor Q2) 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 R1. 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 C1. This small current flowing through the speaker, while C1 is charging, causes the speaker cone to move slightly. As C1 is charging, Q1 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 cone of the speaker to move even more in the same direction. When Q2 conducts, it will discharge C1 which, in turn, causes Q1 to stop conducting which, in turn, causes Q2 to stop conducting and the speaker returns to its rest position. Then, C1 begins to charge again and the cycle is repeated.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and you will hear an approximate 500Hz (500 cycles per second) tone from the speaker. As an example, this tone can be used as an alarm sound to let you know someone has opened a door. Tone generators like this 2 Transistor Oscillator are very useful devices.Check Your Work HereLAB 14Visit us online at www.ETronCircuit.com for more exciting projects