RAIN SOUND EFFECT GENERATOR

Circuit diagram:

Parts:

R1,R2,R13______10K   1/4W Resistors
R3,R5__________33K   1/4W Resistors
R4,R6___________1M   1/4W Resistors
R7____________100R   1/4W Resistor
R8____________330K   1/4W Resistor
R9____________100K   1/4W Resistor
R10____________47R   1/4W Resistor
R11_____________1K   1/4W Resistor
R12____________15K   1/4W Resistor
R14____________47K   1/4W Resistor
R15____________10M   1/4W Resistor
R16_____________1M8  1/4W Resistor

C1,C4,C7______100�F   25V Electrolytic Capacitors
C2,C3,C6______100nF   63V Polyester or Ceramic Capacitors
C8,C10________100nF   63V Polyester or Ceramic Capacitors
C5_____________47�F   25V Electrolytic Capacitor
C9_____________10�F   63V Electrolytic Capacitor

D1___________1N4148   75V 150mA Diode

Q1___________2N3819   General-purpose N-Channel FET
Q2____________BC337   45V 800mA NPN Transistor
Q3____________BC327   45V 800mA PNP Transistor
Q4,Q5_________BC547   45V 100mA NPN Transistors

IC1___________TL062   Low current BIFET Dual Op-Amp
IC2____________4060   14 stage ripple counter and oscillator IC (See Notes)

SW1____________1 pole 3 ways Rotary Switch
SW2____________SPST  Slider Switch

SPKR___________8 Ohm Loudspeaker (40 to 85mm. diameter)

B1_____________3V Battery (two AA or AAA cells wired in series etc.)

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Comments:

Sound effects generators trying to imitate rain sound or sea surf are well known to hobbyists from many years: their purpose is to induce relaxation and sleep or to help in concentration and study.
The sound generated is restrained to a background level and these devices are frequently kept on the night table.
Common designs use invariably Zener diodes or reverse-biased transistors base-emitter junctions as white noise generators. The main snag of these circuits is that a supply of at least 12V is required, therefore a big battery pack or (more commonly) mains supply is used as power source.
The aim of this project was to design a small, portable unit, powered by a 3V battery and capable of shutting-down after a preset delay, in order to save power.

Circuit operation:

Two BIFET Op-Amps are used as a good, low voltage supply, very low current, white noise source. A sound resembling to a rain shower is reproduced by the speaker after being amplified by Q2.
The higher part of the white noise spectrum is attenuated by C8, slowly driven into operation by means of Fet Q1 acting as a variable resistor. Therefore, a sort of automatic tone control is obtained.
IC2 provides all the timings: it auto-resets at switch-on, shutting-down the generator after one of three time-delays, chosen by means of SW1. It provides also, through R8, slow charge and discharge of C5, in order to change smoothly high-frequency attenuation. Q3 is used as a dc switch for the generator circuit. Q4 and Q5 are its drivers.

Notes:

• Different operating delays can be chosen by changing R16 and/or C10 value.
• 4060 ICs by some manufacturers are unable to oscillate at 3V supply. Motorola's MC14060 is therefore highly recommended for IC2.
• If a fixed operating delay is desired, SW1 can be omitted and R14 and D1 anode can be hard wired to the proper pin of IC1.
• Output volume can be increased lowering R9 value to about 47K. On the other hand it can be reduced increasing R9 value to about 150K.
• If variable high-frequency attenuation is not needed, C5, C8, Q1 and R8 may be omitted.
• If auto shut-down is not needed, omit R11, R12, R13, R14, Q3, Q4, Q5, C9, D1 and SW1, connecting pin #12 of IC2 to negative ground.
• If only a straightforward white noise generator is required, omit also IC2, R15, R16 and C10 besides the above listed parts.
• Current consumption is about 7mA and less than 600�A when in stand-by mode.
• After shut-down, the circuit can be restarted opening SW2, then closing it again.

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