Here is a simple circuit that can be used to test a crystal before using it in a circuit. The circuit is built around two BC547 transistors(T1 and T2) and a few discrete components.
The oscillator circuit formed by transistor T1, Resistor R1 and R3, and capacitors C1 and C3 oscillates if a good crystal is connected to the test points. The output from the oscillator is rectified by the diode D1 and filtered by capacitor C2. The positive voltage appearing across the capacitor is fed to the base of transistor T2, causing it to conduct.
Component values:
C1, C3 : 220pf
C2 : 100pf
C4 : 0.01uf
R1 : 100Kohm
R2 : 270 ohm
R3 : 2.7K ohm
D1 : 1N4148
Testing of a crystal is simple. Insert the crystal as points for it and press the switch. If LED glows, your crystal is good and you can use it in your circuit.
The circuit is powered by a standard 9V battery. Push-to-on switch is included to prolong the battery life but it's not needed if you use a socket for the crystal under test.
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