Page 88 - The Basics IA by ETron Circuit Labs

ETron Circuit LabsAdvanced Builds 4British SirenTheory:This project uses the 555 Timer IC as an Astable or free- running Multivibrator. As you change the voltage across Pins 6 and 7, you change the frequency of the output pulses. The time it takes to charge and discharge capacitor C1 determines the output frequency from Pin 3. Pressing push button switch S1 causes R5, 470 Ohm, to be put in parallel with resistor R2. This, in turn, changes the voltage across these two pins, and that changes the frequency of the output pulses coming from Pin 3.These output pulses are then sent to transistor Q1 and amplified. This amplified signal is then sent to the speaker.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and you should hear a tone. If you press the push button switch you will hear another tone. As you press and release the push button switch, you will be able to create the sounds of the 2 tone British Siren.86Electronic Motorcyclefrequency of oscillation is low so it sounds like a motorcycle speeding up and slowing down. If you have an oscilloscope you will enjoy observing the interesting wave from passing through the speaker.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and rotate the shaft of the potentiometer to hear different sounds in the speaker. The resistors and capacitor have been chosen to give the approximate sound of a two-cycle engine like the ones used on some motorcycles.Theory:Here you have a typical 2 transistor oscillator using a PNP(2N3906) transistor and a NPN(2N3904) transistor with a potentiometer to control the speed of oscillation. The