Principles of Oscillators and a Morse Code Practice Oscillator With 555 IC

 Oscillator in electronics is a circuit which produces periodic oscillations or alternating signal which could be a sine wave, square wave or another type of wave. The oscillator is powered by a direct current source and can be used in a wide variety of applications like radio, television, computer and mobile phone. In the animation, a resistor-capacitor (RC) network is used to feed a portion of the output of the inverting Schmitt trigger into its input. Schmitt trigger is a circuit with hysteresis implemented by applying positive feedback to the non inverting input of a differential amplifier. It is called a trigger because the output retains its value till the input changes sufficiently to trigger a change. That brings us to the most common form of linear oscillator or harmonic oscillator in which a transistor or operational amplifier is connected in a feedback loop with output fed back into its input through a frequency selective electronic filter to provide positive feedback. In the illustration, the RC network does this job. 


When the amplifier is switched on, electronic noise or unwanted signal in the circuit provides a signal input to get the oscillations started. The noise gets amplified and travels along the feedback loop which filters it and converts it to single frequency wave which could be a sine wave or a square wave as shown in the illustration. RC oscillators are used in low frequency range, typically for audio frequency signals. LC oscillators with inductor and capacitor have a tuned circuit instead of the RC network. They are used in high frequency application like radio transmitter and local oscillator of radio receivers. Still another option is a crystal oscillator in which the electric signals from a piezoelectric crystal provides a highly stable signal for the oscillator. But the disadvantage of the crystal oscillator is that frequency cannot be changed by tuning as in RC and LC network oscillators, which can be tuned easily by varying the capacitance of a variable capacitor.

Two common classical types of oscillators are the Hartley oscillator invented in 1915 and the Colpitts oscillator, invented in 1918. Hartley oscillator has a tank circuit consisting of two inductors connected in series or a single tapped inductor and a single capacitor. Signal needed for feedback is taken from the center connection between the two inductors. In the circuit shown, the transistor acts as an amplifier between the high impedance across the entire LC tank circuit and the low voltage/high current point between the coils. Frequency of the oscillation is the resonant frequency of the tank circuit. If the capacitance of the tank capacitor is C and the total inductance of the coils is L, then frequency f = 1/2π√LC. So it is possible to change the frequency of oscillation simply by using a variable capacitor. Function of the radiofrequency choke is to prevent the RF energy setup in L being short circuited through the battery supply.

While the Hartley oscillator has two inductors in series, the Colpitts oscillator has two capacitors in series, in the tank circuit. Feedback for the transistor amplifier is taken from the voltage divider made of two capacitors in the tank circuit. While it is easy to calculate the total inductance in Hartley oscillator by just adding the values of two inductors, it is more difficult in case of Colpitts oscillator with two capacitors in series in the tank circuit. Product of the two capacitances will have to be divided by the sum of the capacitances to get the total capacitance in the formula for calculating the resonant frequency of the tank circuit.

An important use of oscillators for those preparing for amateur radio examination is the Morse Code Practice Oscillator. Though earlier we used to make it with transistors, the trend now is to make it with 555 timer IC, which is widely available and inexpensive. Here is a circuit diagram which I found posted online by VU2MUE, which I have redrawn in colour with slight modifications. While trying to assemble it, pin number 8 of the only 555 IC which I had broke off, after having used it in multiple other breadboard projects. So I had to abandon the project and hence I will not be able to tell you that this circuit actually worked for me. Looks rather simple and I am planning to try assembling it on a breadboard, just for the fun of it, at a later date, when I get a new 555 IC.

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