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LM386 Audio Probe Amplifier
Posted on Thursday, April 28, 2011   •   Category: Test and Measurement


LM386 audio probe amplifier is an essential tool for troubleshooting audio stages in audio related circuits such as amplifiers, oscillators, function generators, phone circuits, radios and lots of our other projects. It is a very handy piece of test equipment that can be built on pre-drilled board and will make a perfect addition to your electronic collection. There are lots of things that can go wrong with an audio stage. It can produce distortion or a “hollow” sound, go weak or simply fail altogether. Likewise tone circuits can present a number of faults and it is very handy to be able to “hear” what is going wrong. It is not sufficient to measure the DC voltages on these stages. This only gives a partial picture of the conditions and does not tell you the quality of the audio being processed. To determine this you need a piece of test equipment that will let you see or hear what is being processed. Some of the projects you can test with the Mini Bench Amplifier are tone circuits while others are audio circuits. Tone circuits and audio stages are surprisingly difficult to test unless you have an audio probe or a oscilloscope. Oscilloscope is an ideal piece of equipment but if your budget does not extend this far, the next best thing is an audio probe.


High Power FM Wireless Microphone
Posted on Thursday, April 28, 2011   •   Category: FM Transmitters


This FM Wireless Microphone has been a very popular project with beginners and experienced constructors alike. It has been used inside guitars and as the basis of a remote control system. I do however, receive many requests for a higher powered circuit and better microphone sensitivity. This High Power FM Wireless Microphone has a better frequency stability, over 1 Km range and is good on microphone sensitivity. This has been achieved by adding an RF amplifier buffer (with 10dB gain) and an AF preamplifier to boost the modulation a little.


BA1404 HI-FI Stereo FM Transmitter
Posted on Thursday, April 28, 2011   •   Category: FM Transmitters


This FM transmitter design is a result of many hours of testing and tweaking. The goal was simple; to test many existing BA1404 transmitter designs, compare their performance, identify weaknesses and come up with a new BA1404 transmitter design that improves sound quality, has very good frequency stability, maximizes transmitter's range, and is fairly simple for everyone to build. We are happy to announce that this goal and expectations have been met and even exceeded. The transmitter can work from a single 1.5V cell battery and provide excellent crystal clear stereo sound. It can also be supplied from two 1.5V battery cells to provide the maximum range. One of the qualities of BA1404 FM transmitter is excellent frequency stability. This is mainly due to a use of high quality 3.5 turn variable coil. Tunable RF coils are ideal for precise frequency tuning because their magnet wire is halfway embedded within the plastic, which minimizes frequency drifts. Regular air coils are not preferred for professional broadcasting because the coil expands and contracts with temperature changes. That's the very reason why variable coil was chosen as a substitution for an air coil and a variable capacitor.


Telephone Conversation Recorder
Posted on Thursday, April 28, 2011   •   Category: Phone Circuits


This circuit enables automatic switching-on of the tape recorder when the handset is lifted. The tape recorder gets switched off when the handset is replaced. The signals are suitably attenuated to a level at which they can be recorded using the ‘MICIN’ socket of the tape recorder. Points X and Y in the circuit are connected to the telephone lines. Resistors R1 and R2 act as a voltage divider. The voltage appearing across R2 is fed to the ‘MIC-IN’ socket of the tape recorder. The values of R1 and R2 may be changed depending on the input impedance of the tape recorder’s ‘MIC-IN’ terminals. Capacitor C1 is used for blocking the flow of DC. The second part of the circuit controls relay RL1, which is used to switch on/off the tape recorder. A voltage of 48 volts appears across the telephone lines in on-hook condition. This voltage drops to about 9 volts when the handset is lifted. Diodes D1 through D4 constitute a bridge rectifier/polarity guard. This ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines.


Temperature Control using a DS1621 Sensor and a ATtiny 2313 Micrcontroller
Posted on Thursday, April 28, 2011   •   Category: Sensors


This circuit uses a Dallas DS1621 temperature sensor which indicates the temperature of the device. The temperature sensor has an thermal alarm output, which becomes high when the temperature of the device exceeds a user defined value. When the temperature drops below a user defined value, the alarm output becomes low. In this way any amount of hysteresis can be programmed. The values are stored in a special register of the device that is nonvolatile. The signal of the alarm output is amplified by a BC557 PNP transistor, that drives a relay that can switch a heater element or a blower on or off. The temperature settings and readings are communicated to/from the device over a simple 2-wire serial interface. An ATMEL 90S2313 microcontroller controls the serial communication to/from the DS1621.The microcontroller also controls three LED, only one of the LED's is on when the temperature is within a certain range. The range of the temperature in which the LED's are on can be set by the user in the program code. The circuit needs to be powered by a 5V power supply, which can be obtained from a wall-wart.


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