Telephone line Based Audio Muting and Light On

Very often when enjoying music or watching TV at high audio level, we may not be able to hear a telephone ring and thus miss an important incoming phone call. To overcome this situation, the circuit presented here can be used. The circuit would automatically light a bulb on arrival of a telephone ring and simultaneously mute the music system/TV audio for the duration the telephone handset is off-hook. Lighting of the bulb would not only indicate an incoming call but also help in locating the telephone during darkness.

Telephone line Based Audio Muting and Light-On Circuit Diagram

On arrival of a ring, or when the handset is off-hook, the inbuilt transistor of IC1 (opto-coupler) conducts and capacitor C1 gets charged and, in turn, transistor T1 gets forward biased. As a result, transistor T1 conducts, causing energisation of relays RL1, RL2, and RL3. Diode D1 connected in antiparallel to inbuilt diode of IC1, in shunt with resistor R1, provides an easy path for AC current and helps in limiting the voltage across inbuilt diode to a safe value during the ringing. (The RMS value of ring voltage lies between 70 and 90 volts RMS.) Capacitor C1 maintains necessary voltage for continuously forward biasing  transistor T1 so that the relays are not energised during the negative half cycles and off-period of ring signal. Once the handset is picked up, the relays will still remain energised because of low impedance DC path available (via cradle switch and handset) for the in-built diode of IC1.

After completion of call when handset is placed back on its cradle, the low-impedance path through handset is no more available and thus relays RL1 through RL3 are deactivated. As shown in the figure, the energised relay RL1 switches on the light, while energisation of relay RL2 causes the path of TV speaker lead to be opened. (For dual-speaker TV, replace relay RL2 with a DPDT relay of 6V, 200 ohm.) Similarly, energisation of DPDT relay RL3 opens the leads going to the speakers and thus mutes both audio speakers. Use ‘NC’ contacts of relay RL3 in series with speakers of music system and ‘NC’ contacts of RL2 in series with TV speaker. Use  ‘NO’ con-tact of relay RL1 in series with a bulb to get the visual indication.

Source: http://www.ecircuitslab.com/2011/12/telephone-line-based-audio-muting-and.html

Continue Reading..

2 4GHz WiFi ISM Band Scanner Description and Schematic Part 1

Have you ever wondered exactly what is going on in the 2.4GHz WiFi and ISM band around your house. What channel is it best to set your wireless router to? Why are you getting such poor performance across your WiFi network? Is your neighbour on the same frequency?

Just what is out there? This neat little gadget will sniff the airwaves and give you a graph of the signal strength vs frequency across the entire band. It connects to your computer by USB 2.0 and with the companion Windows software you can display the spectrum or save the raw data to an Excel compatible file for some more number crunching.

It uses just two significant components, a radio module from Cypress Semiconductor and a PIC microcontroller from Microchip. Total cost to build it should be less than US$30.

The 2.4GHz Band

The 2.4GHz ISM (Industrial Scientific Medical) band is often called the WiFi band because it is used for WiFi networking (ie, 802.11 b/g/n). This band is unlicensed, meaning that you and anyone can transmit on it. As a result it has been used by a multitude of products including video transmitters, portable telephones, Bluetooth devices, wireless keyboards, toys and so on. Because you cannot see what is going in the band on you can experience strange behaviour from your wireless gadget. All of a sudden your wireless keyboard skips characters, is it because someone is using a portable phone on the same frequency?

The biggest victim is WiFi networking. This needs a lot of bandwidth, is always transmitting and is sensitive to interference. This is why people often cannot get a decent range from their wireless network and give up in disgust.

This scanner will draw a graph on your computer screen showing you the activity across the band and indicate the best frequencies to use. If you use a laptop you can also wander around and identify the culprits that are clogging the airwaves.

How It Works

Internally the scanner is very simple. It just contains a radio receiver and a microcontroller…

The radio receiver is the Cypress CYWUSB6935 Radio SoC (System on a Chip). This is a complete low power radio transmitter/receiver chip for the 2.4GHz band and is controlled by a microcontroller over a synchronous serial (SPI) interface. The microcontroller can write to various registers in the chip to set things like operating frequency and can read other registers to retrieve data from the chip.

This chip is designed to operate over the 2.4GHz band and has the ability to listen on a frequency for any other devices that may be already using the frequency. This is to help the microcontroller select a suitably free frequency before transmitting. The chip reports the signal level as a number typically up to 30, with zero representing no signal. We use this facility in this project - simply put, the microcontroller instructs the module to step to a frequency and measure the signal level at that frequency, when done it steps the chip to the next frequency and instructs it to measure the signal level there. And so on, right across the band.

We actually do not use the transmit/receive function, which is normally the chips main purpose in life.

The microcontroller used in this project is the Microchip PIC18F2550 which integrates the complete USB 2.0 functionality. The microcontroller sets the radio chip to a frequency, reads the signal level from the chip, stores the value in its internal memory and steps on to the next frequency. This continues until the complete 2.4GHz band is covered. The 18F2550 then sends the data off to your computer using USB and your computer, using custom software, displays the resultant spectrum.

Physically the scanner is just a small box hanging on the end of a USB cable.

The Circuit

The circuit is the simplest part of this project. Click on the image or go to the download section at the bottom of this page for a full scale drawing.

2.4GHz WiFi & ISM Band Scanner Circuit Diagram

The PIC 18F2550 microcontroller is a 28 pin part with a built in USB 2.0 interface. As mentioned before, the chip integrates everything connected with the USB including a 3.3V regulator, memory buffers and the USB transceiver. All that you need to do is to connect the USB cable to pins 15 and 16 of the chip and place a capacitor on pin 14 to help smooth the inbuilt 3.3V supply.

The clock for the microcontroller is derived from the 20MHz crystal with the two 15pF capacitors providing the correct loading for the crystal. Internally within the 18F2550 the 20MHz is divided by 5 to give 4MHz and then used to synchronise a phase locked loop (PLL) oscillator running at 48MHz. This is the main clock used within the microcontroller and is used to drive both the USB interface and the CPU. Running at 48MHz this is a speedy little chip so we do not have any issues with performance.

The ISCP connector is there so that I could reprogram the 18F2550 without pulling it out of its socket. It is mostly used for prototyping so you can leave it out if you want. Note that the 10K resistor on pin 1 of the 18F2550 is still needed to pull the reset line high.

Power for the circuit is drawn from the +5V supplied by the host computer on the USB cable. The whole circuit only draws a few tens of milliamps so it is not a significant load. This 5V is dropped to about 3V by three 1N4001 diodes to provide power for the Cypress CYWUSB6935 chip which is mounted on a small PCB (the CYWM6935 module). Each diode will drop about 0.7V resulting in a total voltage drop of about 2V. This is a crude way to derive a 3V supply but it is low cost and does the job without any hassles.

The CYWUSB6935 chip has protective diodes on its inputs, which clamp the signal line to its power supply (3V). This means that we can drive it with 5V signals from the microcontroller with series resistors to limit the current. This is the purpose of the 3.3K resistors, they limit the current in the clamping diodes to less than a milliamp when the PICs output goes to 5V.

CYWM6935 Module

The CYWUSB6935 chip comes in a tiny package designed for machine assembly and is virtually impossible for a mortal wielding a soldering iron to solder. Fortunately Cypress have assembled it into the CYWM6935 module along with two aerials, a crystal and a few capacitors. The connector used in the module is still rather tiny and non standard (or rather it does not use the 0.1" grid that we know and love), but it can be soldered to. For details of the CYMUSB6935 chip and CYWM6935 module go to here.

Parts Listing

•     1 x Microchip PIC18F2550-I/SP microcontroller programmed with the firmware available in the download section at the bottom of this page.
•     1 x Cypress CYWM6935 radio module,
•     1 x 20MHz crystal
•     3 x 1N4001 silicon diodes
•     4 x 3.3K resistors (quarter or half watt)
•     1 x 10K resistor (quarter or half watt)
•     2 x 15pF ceramic capacitors
•     1 x 100nF multilayer ceramic capacitor 1 x 220nF polyester capacitor
•     1 x 100uF electrolytic capacitor (6V or higher)
•     1 x 28 pin IC socket
•     1 x USB cable with a type A connector on one end
•     1 x UB5 jiffy box

Continue Reading..

Transmitter and Receiver Infra Red Headphone

Transmitter and receiver circuit using infra red audio signal is applied to emit an audio signal and will received on a headphones. Making his series is not too difficult, the transmitter and receiver circuit scheme can be seen below.

Transmiter schematics

Receiver schematics

After the series finished, you can try to give the audio inputs on the transmitter and point the LED transmitter to the receiver. Maximum distance is a few meters may not be more than 10 meters, but you can listen to music without wires while using infra red.

image[link]

Continue Reading..

The Differential Input and Differential Output

For getting balanced differential output 
we use this circuit. In this circuit  two source is present, so the
superposition theory is applied to get the output. In this circuit both
the inverting and non-inverting terminal is working.It rejects the
common-mode voltages, so  it is very useful in noisy environments.

A  differential input and differential output amplifier using two identical Op amp. It is     most commonly used as a preamplifier and driving push-pull  arrangement. The differential input and output  are inphase or the same polarity provided  V_in  =V_x – V_y and

V_o  =V_ox – V_oy

When we want to find out the 1^st op-amps output V_OX , we will use the  superposition theory.

When we get  V_X  is active , V_Y  is  inactive then ,

In non inverting terminal 

V₁= (1+ )V_X

When we get  V_y   is  active,  V_x   is  inactive then,

In  inverting terminal,

V₁ = - V_y

 So, V_ox = V₁+V₁

                =(1+ )V_X  -  V_y

 

Fig: The circuit diagram of the differential input and output amplifier. 

When we want to find out the 2^nd  op-amps output V_Oy ,we will use superposition theory.

                      When we get  V_X  is active , V_Y  is  inactive then

                      In  inverting terminal ,

                      V₂= - V_x

        

      Again, when  we get  V_y  is  active ,  V_x  is  inactive then

        In non inverting terminal we get, 

V₂= (1+ )V_y

So, V_oy = V₂+V₂

                                      =(1+ )V_y - V_x

So the output result   

V_o = V_ox  –  V_oy 

= (1+ )V_X  -  V_y  –[(1+ )V_y - V_x ]

                                      = (1+ ) ( V_X - V_y ) +( V_X - V_y )

                                       = ( V_X - V_y ) (1+ )

Design

To design a input and differential output amplifier,  taking a  differential output of at least 3.7V and the  differential  input V_in  =10V.

                        We know,

 V_o = ( V_X - V_y ) (1+ )

            Or, 3.7 = (0.1) (1+ )

 Or, 37 = (1+ )

            Or, 36 = 

            Or, R_f  = 18 R₁

                                    Let, R₁ = 100Ώ, then R_f  = 1.8 KΏ . 

                   Fig: The designing circuit diagram of the differential input and output amplifier. 

Continue Reading..

Cheap And Cheerful Transistor Tester

By using a simple visual indicating system, this small transistor tester allows you to run a quick ‘go/non-go’ check on NPN as well as PNP transistors. If the device under test is a working NPN then the green LED (D1) will flash, while the red counterpart will flash for a functional PNP device. However if the transistor is shorted, both LEDs will flash, and an open-circuit device will cause the LEDs to remain off. The circuit is based on just one CD4011B quad NAND gate IC, four passive parts and two LEDs. The fourth gate in the IC is not used and its inputs should be grounded.

Alternatively, you may want to connect its inputs and output in parallel with IC1.C to increase its drive power to the transistor test circuit. IC1.A and IC1.B together with R2, R3 and C1 form an oscillator circuit that generates a low-frequency square wave at pin 4. This signal is applied to the emitter of the transistor under test as well as to inverter IC1.C. The inverted signal from IC1.C and the oscillator output then drive the test circuit (LEDs, device under test, R1) in such a away that the voltage across that part of the circuit is effectively reversed all the time.

For example, with an NPN transistor under test, when pin 10 is High and pin 4, Low, current flows through LED D1 and the forward biased transistor. However, no current will flow when pins 10 and 4 change states, since the transistor is then reverse-biased. The green LED, D1, will therefore flash at the rate determined by the oscillator. As you would expect to happen, a PNP transistor will be forward biased when pin 10 is Low and 4, High, enabling current to flow through the red LED in that case.

A supply rail of around 3 V (two series connected 1.5-V batteries) should be adequate. To prevent damage to the transistor under test, supply voltages higher than 4.5 V should not be used. Because the LED currents are effectively limited to a few mA by the output of IC1.C (also slightly dependent on the supply voltage), it is recommended to use high-efficiency devices for D1 and D2.

Continue Reading..

PWM Speed Control Circuit Using Forward Reverse and Regenerative Braking

This is a circuit design for PWM Speed DC Motor Rotation circuit. This circuit has two functions, Forward-Reverse and Regenerative Braking function. This circuit is control by MOSFET. This is the figure of the circuit;

On this circuit, we can control the speed of DC 12V motor with Power MOSFET IRF150 using a signal PWM. The Relay RY1 work as control Reverse with the digital alarm, change Q10. The Relay RY2 work as function brake resistor. The F1 use to protect through the circuit by control Run or stop with the digital alarm. D1 is used to protect the current turn back from DC motor.

Continue Reading..