Two-Door Annunciator

  
When the pushbuttons at either door are depressed, this circuit generates a different tone for each door. Tones are ge
Two-Door Annunciator - schematic

nerated by phase-shift oscillator Q1/Q2. Q3 provides tone frequency change by changing the phase-shift network. U2 and U3 are timers for the tones and Q4/Q5 interface the timers with the pushbuttons.