This unit is actually a two tone door buzzer of the form that generates a basic tone for around one second, accompanied by a tone of lower pitch.
The result resembles an established two tone doorbell. The buzzer tone is developed through IC2 which is generally an LM380N audio power amplifier.
Door buzzer circuit is designed to oscillate through the use of positive feedback amongst its output and non inverting input; and also the frequency of oscillation is dictated through the values of R6, R8, and C3.
The desired values provide an functioning frequency of around 500Hz. The output from IC2 is given to a loudspeaker through DC stopping capacitor C4, and an output power of almost 1 watt RMS is acquired utilizing an 8 ohm speaker.
This drops to only around 200 mW RMS having a 40 ohm speaker, even though this would continue to supply sufficient volume for many circumstances.
The two tone result is attained through the inclusion of IC1 and its connected elements. This is a 555 IC linked within the monostable mode, and it constitutes a positive output pulse of just below one second in timeframe (fixed by R3 and C1) when a negative trigger pulse is placed on its pin 2.
This type of pulse is developed at turn on, considering C2 will certainly at first be uncharged and may acquire pin 2 of IC1 to the negative supply potential.
C2 quickly charges by means of R1 though, in order that the trigger input is easily considered positive, and may not continue to be negative at the end of the output pulse (this might possess the a result of stretching the output pulse).
When power is taken off the door buzzer circuit, R2 quickly discharges C2 so the monostable is activated once the device is controlled yet again.
Q1 is biased hard into conduction through the output pulse through la, and it as a result successfully links the series resistance of R5 and R7 in parallel with R8.
This raises the working oscillator frequency by an amount which is governed through R5. At the finish of the pulse via IC1, the oscillator functions at its standard, lower frequency, providing the specified two tone result.
R5 is modified to provide two tones providing a pleasing impact. The current usage of the door buzzer circuit differs from approximately 100mA having an 8 ohm speaker down to around 40mA using a 40 ohm type.