Posted on Thursday, May 5, 2011 • Category: Headphone Amplifiers
If you built our 20W Class-A Stereo Amplifier described last year, you will be aware that it lacks a headphone socket. Similarly, many hifi valve amplifiers also lack a headphone socket, the assumption being that a true hifi enthusiast will want to listen via good-quality loudspeakers.
A headphone output was not included in the Class-A Stereo Amplifier because it would degrade its superb audio performance. Both the wiring paths and the general circuit layout are critical factors in the design and any changes, however slight, can cause big changes in the signal-to-noise ratio and harmonic distortion figures of the amplifier.
Click for larger image
Fig.1: the Stereo Headphone Adaptor connects between your stereo amplifier and the loud-speakers and can drive two pairs of headphones.
If you do want to listen via headphones, a far better option is to build the simple Stereo Headphone Adaptor presented here. It connects directly to the amplifier’s speaker terminals and switches the loudspeakers and stereo headphone sockets using two DPDT (double-pole, double-throw) relays, so there’s no chance of it degrading the audio performance.
Posted on Thursday, May 5, 2011 • Category: Sensors
I needed to replace two old, unreliable thermostats for controlling the heating and cooling for a large garden shed.
Commercial basic digital thermostats are available quite cheaply, but some lack the ability to control heavy loads or have the extra features that I require for saving energy when the door is often left open or to indicate temperature being out of range etc.
I like the PIC18F1320 microcontroller used in my previous project - so decided to use it again in a very similar design to drive three multiplexed LED displays and read the temperature from a Dallas/Maxim DS18x20 "1-Wire" digital sensor.
Posted on Wednesday, May 4, 2011 • Category: Timer Circuits
Here is a 60 minute countdown timer that can be used as an exposure timer for UV light boxes, photography, egg timer, and many other projects where counting or delay is necessary. The heart of the countdown timer is PIC16F84A chip and 4 digit character LED display. The relay is energized after the count down timer goes down from specified minute and second until zero, and can both turn devices on or off. See the link for details and schematic.
Posted on Wednesday, May 4, 2011 • Category: Sensors
Nowadays, metal detection has become a hobby of many people. Besides as a funny and interesting hobby for them, they also wished indeed a treasure that is embedded in the soil when excavated. For this one hobby, you have to have a tool known as a metal detector.
To undergo this hobby is quite expensive to buy. But for those of you who want to try to make yourself a metal detector, the following will be presented a simple schematic that relates to metal detection.
The operation of metal detector is based on superheterodying principle, which is generally used in a heterodyne receiver. This circuit uses two RF oscillators. Both oscillator frequency is fixed at 5.5 MHz. The first RF oscillator comprises transistor T1 (BF 494) and 5.5 MHz ceramic filter commonly used in TV sound-IF section.
The second oscillator is an oscillator Colpitt realization with the help of the transistor T3 (BF494) and inductor L1 (follow the details of construction) was driven by trimmer capacitor VC1.
Posted on Tuesday, May 3, 2011 • Category: Headphone Amplifiers
This is a dual/stereo headphone amplifier with high quality audio built around OPA2134. This headphone amplifier can drive high or low impedance phones with low noise and distortion. When used with line level signals from CD/MP3 players, etc., requiring only a power supply and volume potentiometer.
Many high-power amplifier audio designs have already provided an output for headphones. To support simple headphones, additional circuitry is required by adding only two resistors in series with the loudspeaker output to limit the drive current and protects the phone that in terms of reinforcing failure.
Considering its simplicity, this scheme works well resistive limit, although it will cause distortion if the load is non-linear - a prospect that may be most headphones. In addition to eliminating potential sources of distortion, there are a number of other reasons why you might consider to build a separate headphone amplifier.
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