A basic square wave oscillator is shown in Figure 7-24. The NE555 is connected in the astable mode and uses only three timing components (RA, RB, and Ct). A 0.01µF hypass capacitor is used on pin 5 for noise immunity.
Operating restrictions of the astable mode are few. The upper frequency limit is about 100 kHz for reliable operation, due to internal storage times. Theoretically there is no lower frequency limit, only that imposed by Rt, and C, limitations. There are many vunanons of this astable circuit, hut it is shown here in its simplest form. Oscillators such as this are useful in test equipment or as a signal generator for testing other circuits. The frequency for the circuit in Figure 7-24 may be calculated as follows.