Simple Crystal Tester Circuit

Have you ever wondered how a quartz watch or radio maintains perfect time?

It uses a special part called a crystal oscillator.

This post 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 difficult.

It is best to do this under adult supervision.

What is a Crystal Tester Circuit:

A Crystal Tester Circuit is a simple electrical circuit designed to test and confirm the workings of quartz crystals which are often found in electronic circuits, especially in oscillators.

Quartz crystals are often used to provide accurate and dependable frequency references in a wide range of electronic devices such as oscillators, clocks and microcontrollers.

Understanding Crystal Oscillators:

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

Oscillator circuits often use quartz crystals because to their durability and adaptability.

The piezoelectric effect also referred to as electrostriction or inverse piezoelectricity causes a crystal to vibrate when it is exposed to an electric field.

In order to create a crystal oscillator the quartz crystal must be tuned to a specific frequency while maintaining stability due to factors like electrode mass, crystal placement and ambient temperature.

With a much higher Q factor that reduces energy loss during each frequency cycle the resultant oscillator operates similarly to an RLC circuit.

Building the Crystal Tester Circuit:

For other electrical components it is not possible to test crystals directly with a meter using standard techniques.

In order to solve this the provided easy circuit for a crystal tester that accurately tells if a connected crystal is faulty or functioning.

Circuit Working:

Parts List:

Using transistor Q1 and the connected RC network the circuit functions as a Colpitts oscillator.

The Q1 circuit begins to oscillate at the crystal frequency when a crystal is inserted into the designated slots.

A two diode rectifier circuit is then reached by applying the oscillating frequency to a 1000pF capacitor.

The oscillating frequency is corrected by the diode network before being sent to the adjacent transistor stage Q2.

The connected LED turns on when the rectified DC from the diodes biases the base of the Q2 transistor.

The included crystals functioning and the circuits proper oscillation are confirmed by an illuminated LED.

Q1 will not oscillate if a faulty crystal is fitted preventing any frequency from reaching the 1000pF capacitor.

As a result the Q2 stage stays off which also keeps the LED off.

Thus the state of the crystal is clearly shown by the crystal tester.

Formulas:

The following formulas show how a quartz crystal behaves in terms of reactance over frequency in an oscillator circuit.

Series Reactance or Xs:

X_s = R²+(X_LS – X_CS⁾²

here,

• R is a small inherent resistance that is the crystals equivalent series resistance or ESR.
• The symbol XLS represents the inductive reactance of the crystals intrinsic inductance Ls.
• XCS is a representation of the capacitive reactance of the crystals inherent capacitance Cs.
• Basically the calculation takes the difference between the inductive and capacitive reactance components XLS-XCS and adds the squared ESR R2.
• The formula which does not take the square root represents the overall series reactance Xs at that frequency.

XCP Capacitive Reactance:

X_CP = -1 / 2πfC_P

here,

• C represents the inherent capacitance of the crystal. Cs.
• The value of the mathematical constant π (pi) is about 3.14159.
• The symbol for the frequency is f in hertz Hz.
• This formula shows how the capacitive reactance XCP and the crystals capacitance and frequency are connected in reverse.
• As frequency increases capacitive reactance XCP decreases.

XP Parallel Reactance:

X_P = X_S x X_CP / X_S + X_CP

here,

• This formula calculates the crystals parallel reactance at a specific frequency (f) using the previously calculated capacitive reactance XCP and series reactance Xs.
• In this formula the total of Xs and XCP is divided by the product of Xs and XCP.

Relevance in Circuits for Oscillators:

These formulas help in the construction of crystal oscillator circuits.

fs and fp the intrinsic resonant frequencies of the crystal decide the circuits steady oscillation frequency.

The circuit design should operate at or near the series resonant frequency (fs) for its best performance.

How it is Build:

To build a Simple Crystal Tester Circuit follow the below mentioned connection steps:

Gather Components:

• Gather all the required components as mentioned in the above circuit diagram.
• Check twice their values and make sure they are in working condition.

Prepare Transistors:

• Identify the BC547 or similar transistors base, collector and emitter pins.
• 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 to the positive power supply of the circuit.
• The negative terminal of the power supply connect 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:

• Verify every connection a second time using the diagram.
• Check all he components with polarization including the LED and capacitors have been placed correctly.

Testing:

• Turn on the circuit.
• Place a crystal on the right slots.
• Check for the LED lit if the crystal is functioning properly.

Adjustments (if necessary):

• Look for weak connections or faulty component values if there are problems.
• Make sure the circuit and the crystal are proper.

Finalization:

• If one wants can think about making the circuit more permanent on a PCB when once the circuit functions as desired.

Conclusion:

The best way of checking a crystals working is to construct a crystal tester circuit by using with common electrical components.

Understanding basic concepts of crystal oscillators and following the instructions will allow engineers and electronic fans to confidently develop an accurate crystal tester for their electrical projects.

References:

Crystal oscillator