Page 83 - The Basics IA by ETron Circuit Labs

Astable MultivibratorTheory:This project uses the 555 Timer IC as an astable or free- running Multivibrator. Astable or free-running means that it will continue putting out pulses until you remove the battery.The 555 Timer IC is a two-state device, which means its output (Pin 3 can only be HI or LO). Its state, HI or LO, and how long it remains with one of the two states on its output depends on external components, i.e., resistors and capacitors, connected to the pins of the 555 Timer IC. The square-wave pulses emitted by Pin 3 are called clock pulses. Digital circuits need these kind of clock pulses.This circuit is designed to make the outputs change from one state to another about once a second, 1 pulse per second. By changing the values of R1, R2 and C1 you will see the LED blink at different rates.Procedure:Assemble the circuit and connect a 9-volt battery to the Battery Snap. The LED should blink on and off at a frequency of about one time per second.SCAN HERE TO SEE MORE ONLINE81BUILD 1Theory:Procedure:Dual Burglar AlarmClosing switch S1 will put a positive voltage on the gate of the SCR causing it to conduct and to sound the alarm. Or, by opening the normally-closed switch S2, capacitor C1 will discharge, and put a positive pulse on the gate of the SCR causing it to conduct. The only way to stop the SCR from conducting, once it has started to conduct, is to remove the power from the circuit. Therefore, even if the sensor switches S1 or S2 are opened or closed again, the alarm will not turn off. The only way to reset the alarm is to disconnect the battery.Assemble the circuit and connect a 9-volt battery to the Battery Snap and with switch S1 in the open position (wires not touching) and with switch S2, in the closed position (wires touching), the LED should not be lit.Close switch S1 by touching the two wires. The LED should light up and remain lit until you disconnect the battery. If you open switch S2 (by disconnecting the two wires that are touching), the LED will also light up and remain lit until power is removed from the circuit. This is an excellent burglar alarm circuit for normally- closed and normally-open systems.Visit us online at www.ETronCircuit.com for more exciting projects