Astable operation

An astable timer is one that is constantly pulsing, never staying in one state or the other. It is constantly switching between its two states. Astable operation is characterized by the flashing light or pulsing buzzer associated with starting a car warning that the seat belts are not fastened. In this mode, the output pulse is fed back into the chip to trigger another pulse, and the result is a continuous pulsed signal.

In a stable operation, the 555 timer chip continuously switches between two states: zero volts and 9 volts (if 9 volts is the input voltage). Each 9-volt state is one pulse. How rapidly the pulses occur is called the frequency, which is measured in hertz. One hertz is equal to one pulse per second; 1,000 hertz is equal to 1,000 pulses per second, or 1 kilohertz (1KHz)

Many circuits require the use of two timer chips to achieve the desired results. For instance, you may want to have an LED flash for a specific duration of time, say 10 seconds, each time the circuit is switched on. This is similar to the seat belt warning mentioned earlier. To do this, you would need a monostable circuit designed for one 10-second pulse. In addition, you would need an astable circuit to flash the light. This would require two 555 timer chips. Conveniently, the 556 chip is actually two 555 timer chips in one 14-pin integrated circuit. The pins used for +DC voltage in and ground (-) terminal are common to both internal circuits. In many circuits, two timers are required, and the 556 chip makes a useful package.