space defined by the bottom and the cover a circular sound washer 20 rests on a sharp edge of a circular rim 21 projecting from the upper side of bottom 15 and is attached to the rim by means of an elastic glue double stick tape or any other suitable adhesive the sound washer includes a circular piezoelectric layer 22 on the upper surface thereof this layer can be made as a circular disc which is attached to the upper surface of the washer e g by means of an adhesive openings 23 are provided in the bottom for the insertion of two batteries 24 and 25 of which one has a positive pole facing upwards and the other a positive pole facing downwards the bottom has upstanding guide formations 26 for localizing the batteries in the correct position see fig4 a lid 27 for covering openings 23 has spring hooks 28 which at closing of the lid engage sloping surfaces formed by the bottom the hooks being deflected by sliding against said surfaces when the lid is pushed against the lower side of the bottom to close the openings the hooks snap into engagement with shoulders 29 in the closed position of the lid as shown in fig5 circuit board 18 has contacts 30 for connection of the batteries to the circuit on the circuit board a spring blade 31 is provided as a contact element connecting the batteries in series see fig3 there are also on the lower side of the circuit board contact blades 32 which connect the sound washer with the circuit board see fig5 according to a first.