$8 · SONALERT

Both provide an annoying audible single tone and both have a collar to be panel mounted, the Mallory SC110 works on AC or DC volts 30-120 volts and at 30 volts is 68db the Mallory SC628 works on 6-28 VDC and at 6 volts is 68 db, $8 each or pair $15. Lots of tools when you click on toolguy.