Two multivibrators with different frequencies can are built using the NAND gates of a 4011 IC. If the output of IC1.B is positive with respect to IC1.C, LED D1 is on. As the levels of IC1.A and IC1.D are exactly opposite, D2 is always on when D1 is off, and the other way around.
The two oscillators have different frequencies, which are determined by the values of R2/C2 and R5/C5 respectively according to the formula f0 = 1 ÷ (1.4 RC) With the given component values, the frequencies are 2.2 Hz and 7.2 Hz. Low-current LEDs should be used since the MOS IC cannot sink or source sufficient current for ‘normal’ LEDs.
The values of series resistors R3 and R6 are suitable for a supply voltage of 12 V, in which case the current consumption of the circuit is around 5 mA. However, in principle the 4011 can be operated over a supply voltage range of 5–15 V. Higher currents can be provided by the HC family (supply voltage 3–6 V) or the HCT family (5 V). Incidentally, the part number of the quad gate IC in the HC family is HC7400.
A multivibrator circuit is nothing but a switching circuit. It generates non-sinusoidal waves such as Square waves, Rectangular waves Saw tooth waves, etc. Multivibrators are used as frequency generators, frequency dividers, and generators of time delays and also as memory elements in computers, etc.
The basic principle of the Astable multivibrator is a slight variation in the electrical properties or characteristics of the transistor. This difference causes one transistor to turn ON fast than the other when power is applied for the first time, thereby triggering oscillations. There are three types of multivibrator circuits depending on the circuit operation: (i) Astable, (ii) Bistable, and (iii) Monostable. An astable multivibrator is internally triggered, whereas the other two types are externally triggered.
Bistable Multivibrator Triggering. To change the stable state of the binary it is necessary to apply an appropriate pulse in the circuit, which will try to bring both the transistors to the active region and the resulting regenerative feedback will result in the change of state.
