Page 85 - The Basics IA by ETron Circuit Labs

2 Transistor OscillatorTheory:This circuit is a 2 Transistor Oscillator with a fixed frequency. That means the tone will remain at a constant pitch. To change the pitch, you may experiment with the values of the capacitor and resistors R1 and R3.R2 restricts the amount of current through the speaker to keep it from burning out.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and you will hear an approx. 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.Screaming BoxTheory:This project uses the 555 Timer IC as an Astable Multivibrator. The voltage across the photocell controls the frequency of the pulses coming from Pin 3 of 555 Timer IC. Transistor Q1 amplifies the strength of these pulses and sends them to the speaker. The more light that hits the photocell, the lower the resistance and the higher the frequency of the tone emitted from the speaker.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and you will hear an approximately 500Hz (500 cycles per second) tone from the speaker. If you put this circuit in a small box that can close out all the light, the speaker will be “quiet.” But as you try to open the box, the speaker will let out a high-pitched squeal, hence the name Screaming Box. Try it in the refrigerator to catch late-night snackers. Have fun!SCAN HERE TO SEE MORE ONLINE83Automatic Night LightTheory:The photocell is in parallel with the base/emitter circuit of the transistor. Because the photocelland transistor act like two resistors in parallel, the amount of current through the transistor depends on the resistance of the photocell. The higher the resistance in the photocell, the greater the current through the transistor.The more current passing through the transistor, the more current running through the LEDs. The more current through the LEDs, the brighter they shine.Procedure:Connect a 9-volt battery to the Battery Snap and observe the two LEDs. This circuit causes the LEDs to be brightest during the night and dimmest during the day. This is the type of circuit that will act like an automatic security system. When the room gets dark, the LEDs light up.BUILD 2Visit us online at www.ETronCircuit.com for more exciting projects