Various diagram

A standard tuning fork produces a tone of 440 Hz, that is, the inter- national A (orchestral pitch). lt is not very difficult to make an electronic alternative. An oscillator, a divider, a loudspeaker and a battery are all that is required. To be useful, an electronic tuning fork must, of course, be a compact unit. As the use of special, and therefore mostly, crystals was precluded, a little research showed that it would be possible to use relatively simple and standard components.

 It appeared that the required frequency can be derived from a readily avail- able 1 MHZ crystal which, by means of a trimmer, can be pulled to 1,000,120 Hz which is the nearest frequency containing a whole number times 440 Hz. The oscillator is constructed a- round gates N1, N2 and tuned to 1,000,120 Hz (with a frequency counter if possible), by means of trimmer C2. The oscillator output is fed to lC2 which divides bv 2273 r2° + 25 + 26 + 27 + 2*1).A practcally symmetrical signal of 440 Hz is then available at output O11 of lC2. This signal is tjgien buffered by gates N3 . . . N6 and the balanced output stage gives a level sufficient to drive a small loudspeaker. ln spite of the current consumption of 65 mA, a standard 9 V battery (preferably alkaline-manganese) will suffice, because tuning forks are by their nature used for short periods only. lf the fork is used for longer periods, it might be advisable to consider a rechargeable battery.