Noise generator circuit

Noise Generator Circuit: 

Components used:

  1. Transistors ─  2N4401, 2N4403
  2. Resistors ─  100 ohm, 220 ohm, 1.5k, 22k, 33k, 100k, 330k
  3. Capacitors ─  0.1 uF, 22 uF, 100 uF
  4. Speaker (8 ohm)
  5. Power supply (9V DC)
  6. Pushbutton





This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail. The heart of the circuit is the two transistor flasher with frequency modulation applied to the base of the first transistor. When the pushbutton is depressed, the frequency of oscillation climbs to a peak and when the button is released, the frequency descends due to the rising and falling voltage on the 22 uF capacitor. The rate of change is determined by the capacitor value and the 100k resistor from the pushbutton.  The oscillation eventually stops if the button is not depressed and the current consumption drops to a tiny level so no power switch is needed. The 0.1 uF determines the pitch of the siren: A 0.047uF will give a higher pitch siren and a 0.001 uF will give an ultrasonic (at least for me, anyway) siren from 15 to 30 kHz which might have an interesting effect on the neighborhood dogs.  The 33k resistor from the collector of the PNP to the base of the NPN widens the pulse to the speaker giving greater volume.  The flasher circuit drives a PNP transistor which powers the speaker. This transistor may be a small-signal transistor like the 2N4403 in most applications since it will not dissipate much power thanks to the rapid on-and-off switching. The 100 ohm and 100uF capacitor in series with the speaker limit the current to about 60 mA and they may be replaced with a short circuit for a louder siren as long as the transistor can take the increased current. The prototype drew about 120 mA when shorted which is fine for the 2N4403. Transistor substitutions should be fine - try just about any small-signal transistors but avoid high frequency types so that you do not end up with unwanted RF oscillations.

       Circuit Diagram as per implementation



Circuit Description

This circuit is noise maker circuit. It generates noise in LNA & many other devices. This circuit two parts:  1.Modulator Section,  2. Amplifier Section.

 In modulator  part it produces low frequency  and high frequency simultaneously. Amplifier section amplifies that signal generated.

1.Modulator Section:

In this section it works like a oscillator, it acts as a feedback amplifier. It has two transistors. Modulation apply at 1st transistorís base, capacitor 22uF charged when push button is on   then voltage goes high and it will generates high frequency, it will high pitch , and when push button is  released i.e. turn off,  22uF capacitor charged is being discharged. Thus  the  modulator part will generate high frequency and low frequency.

                                      Fig. Modulator Section  


2.Amplifier Section:

Amplifier section simply amplifies the signal which is coming from transistor  2N4401  through 1.5k resistance. It is a simple amplifier  to increase amplitude of signal , because signal may attenuated due to many reasons. This amplifiers drive the speaker to make it audible the noise or siren.

Fig. Amplifier Section


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