The 741 is a versatile chip and it can be used in the design of a wide variety of sound-effect generators. This circuit produces a siren that can be used in conjunction with other circuits. You can also use an LM358 dual op-amp chip.
The operation of the op-amp was not discussed correctly in the original article, so a full explanation has been provided:
The principle of an op-amp is to provide a very high gain. This means a small change in either input produces an almost full rail swing on the output.
The circuit starts to work like this.
As soon as you put a slight voltage on the "+" input, the output goes full HIGH.
The two 100k resistors on the "+" makes the output go full HIGH.
Now we connect a resistor from the output to "+" and this makes no difference. The output remains full HIGH.
Now we put a resistor from output to "-."
If the "-" input is slightly higher than "+" the output goes LOW. This is what happens. The output voltage drops until the "-" input is slightly lower than the "+" input and that's why the output falls until its voltage is equal to the "+" input.
Now we connect a capacitor to the "-" input.
It does not matter if we add the capacitor later or turn the circuit on with the capacitor fitted.
The voltage on the "-" input will be lower than the "+" input and this will start the circuit oscillating.
This is how it oscillates:
Because the "-" input is lower than the "+" input, the output rises towards the positive rail and this begins to charge the capacitor.
The voltage on the "-" input can rise higher than the "+" input and when it is about 15mV higher, the output drops towards the 0v rail.
This reduces the voltage on the "+"input and the capacitor has to discharge a considerable amount before it is lower than the "+" rail. (Actually before the "+" input is higher than the "-" input).
The voltage on the "+" input is rising and falling by about 30% of rail voltage and this is the amount the capacitor has to charge and discharge for the circuit to work.
The operation of the op-amp was not discussed correctly in the original article, so a full explanation has been provided:
The principle of an op-amp is to provide a very high gain. This means a small change in either input produces an almost full rail swing on the output.
The circuit starts to work like this.
As soon as you put a slight voltage on the "+" input, the output goes full HIGH.
The two 100k resistors on the "+" makes the output go full HIGH.
Now we connect a resistor from the output to "+" and this makes no difference. The output remains full HIGH.
Now we put a resistor from output to "-."
If the "-" input is slightly higher than "+" the output goes LOW. This is what happens. The output voltage drops until the "-" input is slightly lower than the "+" input and that's why the output falls until its voltage is equal to the "+" input.
Now we connect a capacitor to the "-" input.
It does not matter if we add the capacitor later or turn the circuit on with the capacitor fitted.
The voltage on the "-" input will be lower than the "+" input and this will start the circuit oscillating.
This is how it oscillates:
Because the "-" input is lower than the "+" input, the output rises towards the positive rail and this begins to charge the capacitor.
The voltage on the "-" input can rise higher than the "+" input and when it is about 15mV higher, the output drops towards the 0v rail.
This reduces the voltage on the "+"input and the capacitor has to discharge a considerable amount before it is lower than the "+" rail. (Actually before the "+" input is higher than the "-" input).
The voltage on the "+" input is rising and falling by about 30% of rail voltage and this is the amount the capacitor has to charge and discharge for the circuit to work.
