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@garethbriggs something like ebay.co.uk/itm/Buzzer-3-12Vdc-85dB-Piezo-3-3Khz-CPM202B2-PCB-x-1-pc-/230841377591?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D200781361205%26ps%3D54 seems suitable? not sure how loud it is...