Simple Hartley Oscillator Circuit

A Hartley oscillator is like a gadget that makes very high pitched sounds electronically.

These sounds are smooth and continuous, like a perfect whistle tone.

The special thing about this gadget is how it creates these sounds.

It uses a coil with a center tap like two coils taped together or a transformer along with a special capacitor, to control the sound and keep it going.

It also needs an active part, kind of like an engine, to make the sound strong enough.

This is different from other sound making circuits.

Circuit Working:

Parts List:

The above diagram shows a single transistor oscillator circuit using a transformer for tuning and generating a sine wave AF.

This circuit is a hartley oscillator with tuning and feedback achieved through a center tapped winding of the transformer, while the other winding acts as an output coupling coil.

To construct this circuit you will need a small transformer T1 which is a 500 ohm to 30 ohm center tapped transformer meaning the primary winding has 500 ohms and the secondary winding has 30 ohms approx.

In the hartley circuit the upper half of the center tapped primary winding of L1 serves as the base input coil while the bottom half acts as the collector output coil.

Capacitor C3 is responsible for tuning the oscillation on the primary side of the transformer.

The frequency of the circuit is primarily determined by capacitor C3 and the inductance of the total primary winding.

For example, if the value of C3 is 0.02µF the frequency will be approx. 2 kHz.

To increase the frequency decrease the capacitance of C3 to decrease the frequency increase the capacitance of C3.

Capacitor C2 does not play a role in the tuned circuit its purpose is to block the collector DC voltage from reaching the base of the transistor.

The circuit produces an output amplitude of 0.8V RMS when unloaded.

The current consumption is 2 mA when a 6V DC supply is used.

Formulas:

Formula for the frequency of a Hartley oscillator

A Hartley oscillators resonant frequency (f) may be found using the formula:

f = 1 / (2 * π * √(L_eq * C))

where,

• f is the frequency expressed in Hz hertz.
• The two inductors equivalent inductance in henrys H is represented by L_eq.
• The capacitors capacitance in farads F is denoted by C.

Equivalent Inductance (L_eq) Calculation

From the above circuit diagram T1 mentioned as transformer (inductor) is equivalent inductance which may be computed as follows:

L_eq = T1 + 2M

where,

• The mutual inductance of the two coils (here transformer) is represented by M.

Note:

In practical computations, the mutual inductance M is sometimes disregarded for simplicity sake, particularly in cases where the coils are weakly connected.

Like the colpitts oscillator, the hartley oscillators frequency can be affected by parasitic capacitances, component tolerances and the properties of the active device.

How to Build:

To build a Simple Hartley Oscillator circuit you need to follow the below mentioned steps for connections:

• Connect the collector of the Q1 transistor to one end of the primary winding of the transformer.
• Connect the emitter of the Q1 transistor to ground through resistor R2.
• Connect the base of Q1 transistor to Resistor R1 to positive supply of S1.
• Connect capacitor C2 between the transistor Q1 base and resistor R1.
• Connect capacitor C3 in parallel with the primary winding of the transformer to tune the oscillation frequency.
• Connect capacitor C1 from DC input source to ground.

Note:

• Apply a 6V DC power supply to the circuit and it should start oscillating and producing a sine wave AF at the specified frequency.
• Adjust capacitor C3 to fine tune the frequency as needed.

Conclusion:

A Hartley oscillator is an electronic circuit used to generate high-frequency sine waves.

It uses a tapped inductor or transformer for feedback and tuning, making it suitable for RF applications and other devices requiring a stable oscillating signal.

References:

Hartley oscillator

Hartley Oscillator