In an Audible crystal tester, 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 CD4060 is a CMOS chip with a binary counter and oscillator included. It can be used to produce selectable time delays or to create signals of different frequencies. This is because it has a built-in oscillator module that only requires a few passive electronic components.
From only two resistors and one capacitor, it can create 10 different frequencies. That makes it a very interesting chip, especially for those interested in audio and synthesizers.
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 crystals, and that is why switch S2 enables the selection of one of three different factors. For crystals <1 Mhz. the scale factor is 128; for crystals 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.