FET Audio Mixer

This simple circuit mixes two or more channels into one channel (eg. stereo into mono). The circuit can mix as many or as few channels as you like and consumes very little power. The mixer is shown with two inputs, but you can add as many as you want by just duplicating the "sections" which are clearly visible on the schematic.

Schematic

 

Parts

Part	Total Qty.	Description
R1, R3	2	10K Pot
R2, R4	2	100K 1/4 W Resistor
R5	1	6.8K 1/4 W Resistor
C1, C2, C3	3	0.1uF Capacitor
Q1	1	2N3819 Junction FET
MISC	1	Wire, Shielded (Metal) Case, Phono Or Other Plug For Output

Notes

1. As many or as few channels as are required can be added to the mixer. Do this by just duplicating the input "sections" which are clearly shown on the schematic. One version of this mixer I saw had 25 inputs!

2. A shielded case is probably needed to reduce hum and help stop oscillations.

3. The circuit can be powered by a single 9 volt battery.

Digital Volume Control

This digital volume control has no pot to wear out and introduces almost no noise in the circuit. Instead, the volume is controlled by pressing UP and DOWN buttons. This simple circuit would be a great touch to any home audio project.

Schematic

Parts:

Part	Total Qty.	Description
C1	1	0.1uf Ceramic Disc Capacitor
U1	1	DS1669 Digital Pot IC (See Notes)
S1, S2	2	Momentary Push Button Switch
MISC	1	Board, Wire, Socket For U1

Notes:

1. U1 is available from Dallas Semiconductor.

2. S1 turns the volume up, S2 turns it down.

3. The input signal should not fall below -0.2 volts.

4. Using a dual polariity power supply (+-5V works fine) will cure most clipping problems. You will have to check the data sheet for the correct pins to connect your voltages.

 

Speech Recorder

PARTS LIST

R1 = 1k
R2 = 470k
R3 = 10k
R4 = 5k1
R5 = 4k7
R6,7 = 100k
R8,9 = 1M
R10 = 10R
C1-10 = 100nF/63V
C11 = 47nF/63V
E1,4 = 220uF/16V
E2 = 4u7F/16V
E3 = 22uF/16V
IC1 = ISD2560 + socket
IC2 =LM78L05
IC3 = LM386 + socket
MIC = Condensator microphone
S1,2 = Pushbutton (S1 = Start and Pause. S2 = Stop and Reset)
S3 = Change-over switch
Hψjttaler = 8R speaker 

The 2 pushbuttons = S1: Start/Pause. S2: Stop/Reset.

If you want to play your message, put S3 at Play. Then push S1 to start playing and again to pause.

If you want to delete your message press S2 twice.

If you want to record a message put S3 at Rec. Then push S1 to start and S2 to stop.

 

10W Mini Audio Amplifier

You can use this powerfull amplifier in any small audio project. It is very small (6.5 x 4.5 cm).It outputs 10W and uses a 9V battery.

                         Componets Layout                                                            PCB

  

Componets List

R1 : 6 Ohm

R2 : 220 Ohm

R3 : nothing

R4 : 10 KOhm pontesiometer

C1 : 2200 uF / 25V

C2 : 470 uF / 16V

C3 : 470 nF / 63V

C4 : 100 nF

C5 : nothing

C6 : nothing

IC1 : TDA 2003

Sound Level Meter Circuit

This sound level meter circuit can be used to control the intensity of a sound recording or in a disco. It has 5 measurement domains between 70 and 120 dB; reading accuracy is 0.5 dB. Microphone M1 is used to receive the acoustic signal and is coupled to C1, C2, R1 and R2. This components, together with the microphone’s capacity and with the input impedance of the amplifier form an input filter. The filtered signal goes to operational amplifier IC1 whose sensitivity can be switched with S2 corresponding to the five measuring domains.

D1 … D4 diodes rectifies the alternating voltage at the amplifier output and feeds the indicator tool through R9. D5 is used in order to protect the sound level meter indicator against high voltages; it limits the rectifier’s output voltage when the sound level is too high.
On normal conditions the input current is about 2 mA that is why the circuit can be powered with 2 x 9V batteries. S1 switch is used to disconnect the sound level meter device after measurement. The indicator tool should have a graded scale in dB with the maximum value of +10.

Sound Level Meter Schematic

 

Interfacing LED using push button switch to 8051

This article is all about how to interface push button switches to an 8051 microcontroller. Push button switches are widely used in embedded system projects and the knowledge about  interfacing them to  8051 is very essential in designing such projects. A typical push button switch has two active terminals that are normally open and these two terminals get internally shorted when the push button is depressed. Images of a typical pushbutton switch is shown below.

                                                                  Pushbutton switch

Circuit diagram.

The circuit diagram for interfacing  push button switch to 8051 is shown above. AT89S51 is the microcontroller used here. The circuit is so designed that when push button S1 is depressed the LED D1 goes ON and  remains ON until push button switch S2 is depressed and this cycle can be repeated. Resistor R3, capacitor C3 and push button S3 forms the reset circuitry for the microcontroller. Capacitor C1, C2 and crystal X1 belongs to the clock circuitry. R1 and R2 are  pull up resistors for the push buttons. R4 is the current limiting resistor for LED.

The Logic 

The first instruction – MOV P0 #83H  - is to turn LED off (Hex 83 in binary = 10000011) and to initialize switches 1 and 2. Switch 1 is connected to port 0.0 and switch 2 is connected to port 0.1. Also note that LED is connected to port 0.7. 

Note:- Po.0 = 1 means switch 1 is OFF and Po.1 = 1 means switch 2 is OFF.  P0.0 = o means switch 1 is ON and p0.1 = o means switch 2 is ON. LED turns ON when P0.7 = 0 and turns OFF when P0.7 = 1

The program has two labels – READSW and NXT. It’s all about reading switch values – that is P0.0 and P0.1. We are using RRC instruction to read switch values. The values of port 0 is moved to accumulator. Since port 0 and 1 are used to interface switches 1 and 2, we can get the values of both port bits in LSB”s 0 and 1 of accumulator by using MOV A,P0 instruction. RRC – means – rotate right through carry. You can learn more about this instruction here –  8051 programming tutorial 1 . What RRC do is simple – it will move value of port 0.0 to the carry bit. Now we can check the carry bit using instruction JC – which means “jump if carry is set” . If carry is SET – then it means port0.0 =1 and this means switch 1 is OFF. If switch 1 is OFF then we have to check status of switch 2 and that is why we jump to label NXT. 

In the mean time if switch 1 is pressed – then value of port 0.0 will be equal to zero. This will get moved to accumulator and hence an RRC will result in carry bit = o. If carry bit = 0 then result of executing JC instruction is negative and it will not jump. The next instruction will get executed – that is CLR P0.7. This clears port 0.7 to zero and hence LED will turn ON. Once turned On- LED will be kept On until switch 2 is pressed. 

The status of switch 2 is checked in NXT label. When NXT is executed, we are using RRC for the second time consecutively.  This means, the carry bit now holds the value of P0.1 – which is status of switch 2. If carry bit = 1 then switch 2 is OFF. This means LED should not be turned OFF. If carry bit = 0 then LED should be turned OFF (The instruction SETB P0.7 turns LED OFF)