Non sinusoidal Oscillator

So far every oscillator produced a sine wave. A non-sinusoidal oscillator produces a repeating output of some other shape, and the standard device for building one is the 555 timer. The 555 was first introduced in the early 1970s by Signetics Corporation as the SE/NE 555 timer. It is a monolithic timing IC that gives accurate and highly stable oscillations, is very reliable, easy to use and low in cost.

555 timer pin configuration

The 555 is available as an 8-pin metal can, an 8-pin mini DIP (dual-in-line package) or a 14-pin DIP. Internally the IC contains 23 transistors, 2 diodes and 16 resistors. Its eight pins (Figure 6.5.1) have the following functions:

• Pin 1 — Ground: the reference pin against which all voltages are measured.
• Pin 2 — Trigger: the trigger pulse is applied here; the output goes low when the trigger voltage is greater than one-third of Vcc.
• Pin 3 — Output: the timer output pin.
• Pin 4 — Reset: the timer is reset by a negative pulse at this pin; it should be tied to Vcc when not used, to avoid false triggering.
• Pin 5 — Control voltage: an external voltage applied here changes the internal threshold reference (normally two-thirds of Vcc).
• Pin 6 — Threshold: when the threshold voltage rises above the control-voltage level (two-thirds of Vcc), the output of comparator 1 goes high and the timer output goes low.
• Pin 7 — Discharge: the collector of the internal discharge transistor is connected here; it saturates or cuts off according to the state of the flip-flop.
• Pin 8 — +Vcc (Supply): the supply pin; the 555 works from +5 V to +18 V.

Block diagram of the 555 timer

The functional block diagram (Figure 6.5.2) shows that a 555 timer contains two comparators, an RS flip-flop, an npn (discharge) transistor and three equal 5 kilo-ohm resistors. The three equal resistors form a voltage divider across the supply, fixing the comparator references at one-third and two-thirds of Vcc.

Working

When the threshold voltage exceeds the control voltage, comparator 1 sets the flip-flop, so its output goes high ($Q = 1$). This high output saturates the discharge transistor T2 (turns the discharge path on), while the complementary output ($\overline{Q} = 0$) sent to pin 3 keeps the output low. These conditions hold even if the threshold input later falls below two-thirds of Vcc — comparator 1 can only force the flip-flop high; it cannot change it back.

To change the output to low, the trigger input must fall below one-third of Vcc. Comparator 2 then triggers the flip-flop to the low state ($Q = 0$), which turns the discharge transistor T2 off, and the output at pin 3 goes high ($\overline{Q} = 1$).

Bistable multivibrator …