In this post we discuss how to build a simple crystal tester circuit using ordinary parts like transistors, resistors, diodes and capacitors.
What is a Crystal
A crystal can be used for making an electronic oscillator circuit by using the mechanical resonance of a piezoelectric vibrating crystal to generate an electrical frequency having a fixed, and a constant frequency. This frequency can be typically employed to monitor time, such as in quartz watches. Crystals are also popularly used for getting a constant, reliable clock signal for electronic ICs, and to ensure stable frequencies for radio transmitters and receivers.
The commonest form of piezoelectric resonator utilized is the quartz crystal. Therefore, oscillator circuits depending on quartz for stabilizing the frequency, became popular as crystal oscillators. However various other forms of piezoelectric components such as polycrystalline ceramics can be also found in related circuits.
A crystal oscillator begins oscillating due to the small alteration in its shape when it is subjected to an electric field, a characteristics known as electrostriction or inverse piezoelectricity.
When a crystal is subjected to an potential difference, it results in a change in its shape; and as soon as the potential is removed, the crystal produces a tiny voltage since it flexibly restores to its initial condition.
The quartz can oscillate with a constant resonant frequency, working in the same way an RLC circuit would, except with a much increased Q factor (minimal loss of energy during each cycle of the frequency).
After a quartz crystal is fine-tuned to a certain frequency (which can be dependent on the the mass of electrodes mounted on the crystal, the positioning of the crystal, ambient temperature and various other related factors), it successfully sustains this frequency with an enhanced stability.
Making a Crystal Tester Circuit
Crystals cannot be tested directly with a meter. There's no way these components can be verified using ordinary methods that is normally used for measuring parts like resistors, capacitors, or transistors.
However, the following simple crystal tester circuit works extremely well to detect if a connected crystal is faulty or working without any issues.
The above circuit will provide you with a direct indication whether the connected crystal is good or a bad one.
The configuration around the transistor Q1 and its associated RC network work like a Colpitt's oscillator. As soon the the crystal is hooked up into the indicated slots, the Q1 circuit starts oscillating at the crystal frequency.
This oscillating frequency is applied to the 1000 pF capacitor through which it reaches the two diode stage wired like a rectifier circuit. The oscillating frequency is appropriately rectified using the diode network and fed to the next transistor Q2 stage.
The rectified DC from the diodes provide the necessary biasing to the Q2 transistor base, so that it turns ON illuminating the attached LED.
The switched ON LED confirms that the crystal under test is a good crystal, and the circuit is correctly oscillating with the help of the crystal.
If a bad crystal is inserted into the slot, the Q1 fails to oscillate which does not allow any frequency to enter the 1000 pF capacitor causing the Q2 stage to remain switched OFF. The LED consequently also remains switched OFF, indicating that the connected crystal is a faulty one.
The Q1, Q2 can be any general purpose transistors such as the BC547.