A crystal cannot be tested audibly unless its output is scaled down to the audio frequency range by a circuit as shown in the diagram.
A divider that is particularly suitable for this purpose is the Type 4060 CMOS IC. This circuit contains not only a 14-stage binary scaler, but also a complete oscillator.
The crystal to be tested is connected across the input terminals and S2 set as indicated in the table. The crystal frequency is scaled down in IC1 and. depending on the setting of S2. one of the outputs of the 4060 drives transistor T1 via R2. The transistor, in turn, drives a small loudspeaker, LSi. The power delivered to the speaker is limited by R5 to prevent damaged eardrums.
It is, of course, not possible to use one scale factor for all sorts of crystal, and that is why switch S2 enables selection of one of three different factors. For crystals <1 Mhz. the scale factor is 128; for cristals in the range 1—10 MHz, the scale factor is 4096; and for crystals >10 Mhz, the scale factor is 8192.
Also, crystals operating above 10 MHz oscillate readily at somewhat higher voltages than low-frequency ones. That is why S2a and D2 lower the supply voltage to 4.7 V when crystals below 10 MHz are tested.