Security system or alarms such as doorbell and Door Annunciator are most wanted in industries and institutional use. This is a Simple 2 Door Annunciator Circuit Diagram. When the push buttons at either door are depressed, this circuit generates a different tone for each door. Tones are generated 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 push buttons.
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