A backup alarm for producing an audible warning signal which warbles or beeps, at selected audible output power (Db) levels includes a computer which drives a speaker by switching drive voltage to the speaker with above audible, high frequency pulses in packets repetitive at audible frequency. The high frequency pulses are pulse width modulated at different ratios selected from tables in memory of the computer in accordance with the voltage from a battery power source which provides the drive voltage so that the audible output power remains constant at the selected level over a wide range of power source voltage. By providing for warning/alarm signal generation by means of an operating system which provides both high, above audible frequency and audible frequency switching, power conservation and warning signal output power control are accomplished without interference.