Page 87 - The Basics IA by ETron Circuit Labs

Variable TimerTheory:This project uses the 555 Timer IC set up as a Monostable multivibrator or “timer.” Monostable refers to the idea that the output stays at LO until triggered to go HI. Once triggered, it remains HI for a period of time and then returns to LO. The duration of the HI on Pin 3 depends on the values of R2, R4, and C1. When you press push button S1, you put a LO, a negative, on Pin 2. This “triggers” the timer. It now puts out a HI on Pin 3 for a period of time depending on how you have adjusted the potentiometer. Capacitor C1 can be replaced with larger values in order to get longer periods of time.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap. Press and release push button S1 and observe that the LED is lit for a period of time and then shuts off by itself. This is an adjustable time circuit. The time that the output pulse is HI at Pin 3 of the 555 Timer IC is controlled by the potentiometer setting.Morse Code OscillatorTheory:This project uses the 555 Timer IC as a source of pulses which are sent to a transistor which drives a speaker. R1, R2, and C1 control the frequency and duty-cycle (duty- cycle is the amount of time the pulse is on compared to the time that the pulse is off) of the pulses coming from Pin 3 of the 555 Timer IC. By interrupting the power to the circuit with switch S1, you can turn on and off the sound from the speaker.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and press switch S1. You should hear a tone in the Speaker.If you wish, you can replace the push button switch with a Code Key and send Morse Code audio signals. It is still necessary to learn Morse Code to receive some Amateur Radio Licenses.Electronic MetronomeTheory:Here is a 2 Transistor Oscillator set up to make a tic-tic sound in the speaker. The potentiometer adjusts the bias on transistor Q1 and this in turn controls the speed of the turning on and off of transistors Q1 and Q2.The full musical tempo range should be able to be reached by adjusting the potentiometer from one extreme to the other.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap and you should hear a tic-tic sound in the speaker. If you hear only one click from the speaker, twist the potentiometer shaft from one extreme to the other until you hear the tic-tic sound. The speed of the tics can be adjusted using the 100k potentiometer. You can adjust this to keep time to the music just like you would do with the standard mechanical metronome.SCAN HERE TO SEE MORE ONLINE85BUILD 3Visit us online at www.ETronCircuit.com for more exciting projects