Simple Crystal Tester Circuit

Ever wondered how something keeps perfect time, like a quartz watch or a radio?

It uses a special part called a crystal oscillator.

This guide shows you how to build a simple tester to see if crystals are working properly.

The tester itself is built with common parts like transistors, resistors and capacitors.

WARNING: Building circuits with electronics components can be tricky.

It is best to do this with adult supervision.

What is a Crystal Tester Circuit:

A Crystal Tester Circuit is a simple electronic circuit designed to test and verify the functionality of quartz crystals commonly used in electronic circuits, especially in oscillators.

Quartz crystals are often used to provide accurate and stable frequency references in various electronic devices such as oscillators, clocks, and microcontrollers.

Understanding Crystal Oscillators:

A crystal oscillator utilizes the mechanical resonance of a piezoelectric vibrating crystal to produce a fixed and constant electrical frequency.

Typically, quartz crystals are employed due to their reliability and widespread usage in oscillator circuits.

The piezoelectric effect specifically electrostriction or inverse piezoelectricity causes a crystal to oscillate when subjected to an electric field.

The construction of a crystal oscillator involves tuning the quartz crystal to a specific frequency ensuring stability through factors like electrode mass, crystal positioning, and ambient temperature.

The resulting oscillator functions similarly to an RLC circuit but with a significantly increased Q factor minimizing energy loss during each frequency cycle.

Building the Crystal Tester Circuit:

Testing crystals directly with a meter is not feasible using conventional methods for other electronic components.

To address this we present a simple crystal tester circuit that effectively identifies whether a connected crystal is functional or faulty.

Circuit Operation:

Parts List:

The circuit operates as a Colpitts oscillator, with transistor Q1 and its associated RC network.

When a crystal is connected to the indicated slots the Q1 circuit starts oscillating at the crystal frequency.

The oscillating frequency is then applied to a 1000pF capacitor, reaching a two diode rectifier circuit.

The diode network rectifies the oscillating frequency feeding it to the next transistor stage Q2.

The rectified DC from the diodes biases the Q2 transistor base causing it to turn on and illuminate the attached LED.

A lit LED confirms that the connected crystal is operational, and the circuit is oscillating correctly.

If a faulty crystal is inserted Q1 fails to oscillate, preventing any frequency from entering the 1000pF capacitor.

This results in the Q2 stage remaining switched off, keeping the LED off as well.

Thus, the crystal tester provides a clear indication of the crystals condition.

Formulas:

Below formulas represent the reactance behavior of a quartz crystal in an oscillator circuit across frequency.

1. Series Reactance or Xs:

X_s = R²+(X_LS – X_CS⁾²

here,

• below formula determines the crystals series reactance at a given frequency (f).
• R is the crystals equivalent series resistance ESR, which is a little intrinsic resistance.
• The inductive reactance of the intrinsic inductance Ls of the crystal is represented by XLS.
• The capacitive reactance of the intrinsic capacitance Cs of the crystal is represented by XCS.
• In essence, the formula adds the squared ESR R² to the difference between the inductive and capacitive reactance components XLS – XCS.
• The total series reactance Xs at that frequency is represented by the formula, which does not take the square root.

2. XCP Capacitive Reactance:

X_CP = -1 / 2πfC_P

here,

• This formula determines the crystals capacitive reactance at a given frequency (f).
• C stands for the crystals intrinsic capacitance Cs.
• The mathematical constant π (pi) has a value of roughly 3.14159.
• The frequency is denoted as f in hertz Hz.
• This formula demonstrates the inverse relationship between the crystals capacitance and frequency and the capacitive reactance XCP.
• Capacitive reactance XCP diminishes as frequency rises.

3. XP Parallel Reactance:

X_P = X_S x X_CP / X_S + X_CP

here,

• Using the previously determined capacitive reactance XCP and series reactance Xs, this formula determines the parallel reactance of the crystal at a given frequency (f).
• The product of Xs and XCP is divided by the sum of Xs and XCP in this formula.

Relevance in Circuits for Oscillators:

Circuit designs for crystal oscillators are aided by these formulas.

The stable oscillation frequency of the circuit is set by the intrinsic resonant frequencies (fs and fp) of the crystal.

For best results, the circuit design should function at or close to the series resonant frequency (fs).

How it is Build:

Gather Components:

• Collect all the required components mentioned in the list.
• Double check their values and make sure they are in working condition.

Prepare Transistors:

• Identify the transistor pins base, collector, emitter on BC547 or similar transistors.
• Place Q1 and Q2 on the PCB.

Resistor Placement:

• Place resistors on the PCB according to the schematic.
• Connect one end of 100Ω to the collector of Q1 and the other end to the positive power supply.

Diode and Capacitor Placement:

• Connect diodes OA91 with Q1s base.
• Connect the junction of the two diodes to the collector of other BC547 through capacitor 1000pF
• Connect 680pF and 150pF in series and parallel to the crystal under test.

Connect Crystal:

• Connect the crystal to the indicated slots on the PCB.

Connect Power Supply:

• Connect the positive terminal of the power supply to the positive power supply.
• Connect the negative terminal of the power supply to the ground supply.

LED Connection:

• Connect the LED with a current limiting resistor 100Ω from the collector of Q1 to the positive power supply.

Verify Connections:

• Double check all connections against the schematic.
• Ensure that polarized components like capacitors and the LED are correctly oriented.

Testing:

• Apply power to the circuit.
• Insert a crystal into the indicated slots.
• Observe the LED, it should light up if the crystal is good.

Adjustments (if necessary):

• If there are issues check for loose connections or incorrect component values.
• Ensure the crystal is compatible with the circuit.

Finalization:

• Once the circuit is working as expected, you can consider making it more permanent on a PCB if desired.

Conclusion:

Constructing a crystal tester circuit using common electronic components offers a practical solution for assessing the functionality of crystals.

By understanding the principles of crystal oscillators and following the provided guide, enthusiasts and engineers can confidently build a reliable crystal tester for their electronic projects.

References

Crystal oscillator