Free-Running Astable Multivibrator Clock Oscillator Time Base Generator Circuit

Figure 1. Schematic of Free-Running Astable Multivibrator Circuit

The astable multivibrator is a unique form of the relaxation oscillator in which the in-phase feedback is provided by a separate transistor. The first multivibrator was built with vacuum tubes and the circuit was very popular from the start. Millions were built and used in the earliest computers and employed in TV sets in their sweep generation and deflection circuits. Multivibrators generate rectangular waveforms that can also be used for timing generator applications.

There is often a need to have a clock oscillator when working with digital logic. Most often complete IC (Integrated Circuit) modules are used or some logic gates for that purpose. Making the clock oscillator from available spare transistors and a handful of surplus parts not usually done because of needless worry of possible circuit complexity. As demonstrated here the circuit is quite simple and it is very easy to customize it for the application at hand. Almost any transistor available is useable in this circuit if the selected frequency for oscillation is not very high. With the values as indicated on the schematic diagram the oscillators frequency is 1 kHz and the cycle time is mSec (mili-Second). With the circuit operating from a 5-volt power supply the output voltage is acceptable to drive TTL and CMOS gates directly.

Figure 2. Waveforms of Free-Running Astable Multivibrator Circuit

The actual multivibrator oscillator circuit is made up of transistors Q1 and Q2 only for its operation - Figure 1. To be able to obtain the signal from this oscillator without changing its frequency with the external load, transistor Q3 is added as a buffer output amplifier stage. The Q3 transistor isolates the multivibrator and this stage has no voltage gain as it is used for impedance transformation. Transistor Q3 provides low impedance output for the load while presenting high input impedance for transistor Q2. Both the Q1 and Q2 transistors of this circuit work as saturated devices with reduced power dissipation and heat stress.

The timing elements of this oscillator are the R2 and R3 resistors and the C1 and C2 capacitors. The design can be changed; scaled up or down easily. By using a 0.1 uF (micro-Farad) capacitors in place of the present values in C1 and C2 the oscillator frequency would slow down by about ten. Changing R2 and R3 such that their product stays the same changes the duty cycle of the waveform. The oscillator's duty cycle is set to 50% - Figure 2. If R1 decreases to 22K and R3 increases to 120K the frequency stays the same but the duty cycle changes to make the resultant wave a narrow pulse. If a dual 75K potentiometer (dual 100K with paralleled resistors) is used and wired such that while one resistor increases the other decreases the frequency will stay the same while the duty cycle would adjust smoothly.

                Parts and materials:

                    B1 - Battery, 5 Volt 
                    C1 - Capacitor 0.01 uF, 15V, Film or Disc
                    C2 - Capacitor 0.01 uF, 15V, Film or Disc
                    C3 - Capacitor 0.1 uF, 15V, Disc
                    Q1, Q2, Q3 - Transistor, Transistor, 2N3904, NPN, or 2N2102, HS Switch
                    R1, R4 - Resistor Resistor, 2K, 5%, 1/4W, CC
                    R2, R3 - Resistor 71K, 5%, 1/4W, CC
                    R5 - Resistor 100K, 5%, 1/4W, CC