Active Short-Wave Antenna

The circuit presented here illustrates the fact that in spite of all kinds of new component and technology, it is still possible to design useful, and interesting, circuits. The circuit is based on two well-established transistors, a Type BF256C and a BF494. In conjunction with the requisite resistors and capacitors, these form a well-working antenna amplifier. Note that they are direct coupled. Transistor T1 is the input amplifier cum buffer, while the BF494, in a common-ground configuration, provides the necessary amplification. The amplifier is designed for operation at frequencies between 10 MHz and 30 MHz, which is the larger part of the short-wave range, and has a gain of 20 dB.

Inductor L1 is wound on an Amidon core Type T-37-6. The primary consists of 2 turns, and the secondary of 12 turns 0.3 mm dia. enameled copper wire. The number of turns may be experimented with for other frequency ranges. The input circuit is tuned to the wanted station with capacitor C1. The response of the tuned circuit is fairly broad, so that correct tuning is easy. The circuit is powered by a well-decoupled mains supply converter that has an output of 9–12 V. The circuit draws a current of about 5mA.

Author: G. Pradeep
Copyright: Elektor Electronics

Rear Light After Glow (For Bicycles)

This article is of interest only to readers whose bicycle lights are powered by a dynamo. The laws on bicycle lights in the United Kingdom are stricter than in other countries and a dynamo is, therefore, a rarity in this country. From the point of view of traffic safety it is advisable (in UK obligatory) for cyclists to have the rear lamp of their bicycle to light even when they are at standstill. In principle, it is not very difficult to modify the existing rear light with afterglow: all this needs is a large enough energy reservoir. Since the after-glow is required for short periods of time only, a battery is not required: a large value capacitor, say, 1 F, is quite sufficient.

As the diagram shows, in the present circuit, the normal rear light bulb is replaced by two series-connected bright LEDs, D2 and D3. These are clearly visible with a current of only 6 mA (compared with 50 mA of the bulb). The current is set with series resistor R1. The LEDs are shunted by the 1 F capacitor, C1. Since the working voltage of this component is only 5.5 V, it is, in spite of its high value, physically small. An effective regulator is needed to limit the dynamo voltage adequately. Normal regulators cannot be used here, since they do not work at low voltages. Moreover, such a device would discharge the capacitor when the cycle is at standstill.

Fortunately, there is a low-drop type that meets the present requirements nicely: the Type LP2950CZ5.0. Of course, the dynamo output voltage needs to be rectified before it can be applied to the regulator. In the present circuit, this is effected by half-wave rectifier D1 and buffer capacitor C2. Diode D1 is a Schottky type to keep any losses low – important for this application, because the ground connection via the bicycle frame usually causes some losses as well. The value of buffer capacitor has been chosen well above requirements to ensure that C1 is charged during the negative half cycles of the dynamo voltage.

Digital Main Voltage Indicator

Continuous monitoring of the mains voltage is required in many applications such as manual voltage stabilisers and motor pumps. An analogue voltmeter, though cheap, has many disadvantages as it has moving parts and is sensitive to vibrations. The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. Presets VR1 through VR16 are used to set the DC voltages corresponding to the 16 voltage levels over the 50-250V range as marked on LED1 through LED16, respectively, in the figure. The LED bar graph is multiplexed from the bottom to the top with the help of ICs CD4067B (16-channel multiplexer) and CD4029B (counter).

The counter clocked by NE555 timer-based astable multivibrator generates 4-bit binary address for multiplexer-demultiplexer pair of CD4067B and CD4514B. The voltage from the wipers of presets are multiplexed by CD4067B and the output from pin 1 of CD4067B is fed to the non-inverting input of comparator A2 (half of op-amp LM358) after being buffered by A1 (the other half of IC2). The unregulated voltage sensed from rectifier output is fed to the inverting input of comparator A2. The output of comparator A2 is low until the sensed voltage is greater than the reference input applied at the non-inverting pins of comparator A2 via buffer A1.

When the sensed voltage goes below the reference voltage, the output of comparator A2 goes high. The high output from comparator A2 inhibits the decoder (CD4514) that is used to decode the output of IC4029 and drive the LEDs. This ensures that the LEDs of the bar graph are ‘on’ up to the sensed voltage-level proportional to the mains voltage.The initial adjustment of each of the presets can be done by feeding a known AC voltage through an auto-transformer and then adjusting the corresponding preset to ensure that only those LEDs that are up to the applied voltage glow.

Note.
It is advisable to use additional transformer, rectifier, filter, and regulator arrangements for obtaining a regulated supply for the functioning of the circuit so that performance of the circuit is not affected even when the mains voltage falls as low as 50V or goes as high as 280V. During Lab testing regulated 12-volt supply for circuit operation was used.)

Turtle Beach Anounces iSeries Media Headsets.

When it comes to audio brands, there is one particular name that stands out above the rest for some people, and Turtle Beach would be one of them. The name Turtle Beach has long been synonymous with high quality audio devices, and this time around the company has announced their spanking new iSeries range of high end media headsets.

The iSeries will arrive in the form of the wireless surround sound i60 and the wireless amplified stereo i30, where these would no doubt be the first headsets that have been specially constructed for use with both Mac and iOS products. The iSeries headsets themselves will arrive with a bunch of innovations that Turtle Beach originally introduced to gaming headsets but so happen to be new for mobile users. Hopefully, with a little bit of something for everyone, folks will be satisfied.

Some of the new features introduced to gaming headsets that mobile users would fall in love with include improved chat and voice call quality as well as an unmatched level of control over audio. Regardless of whether you settle for the i60 or the i30, they will both share the same unique design which blends style, quality, comfort and durability, while boasting premium finishes, where among them include a leather headband and leather-covered memory foam, noise-isolating ear cups.

The two headsets will also be accompanied by a built-in remote which plays nice with the iPhone, iPad and the Mac. They will come with innovative dual boom-less microphones which will hopefully, deliver clear, high-quality audio during chat, voice calling and use with Siri.

Of course, the i60 will be the flagship model, where it features a control unit for use with Mac desktops and laptops that deliver fingertip audio and preset control. Turtle Beach does have some pretty high hopes that the i60 will also be the first wireless headset for the Mac platform that boasts of 7.1 DTS surround sound with adjustable speaker angles and EQ presets, a fully programmable digital signal processor (DSP), dual-band Wi-Fi for uncompressed, interference free wireless audio and dual-pairing Bluetooth 4.0 connectivity. [Via]

Bike Battery Charger

This simple circuit allows a 12V battery pack to be charged via a bike generator. The generator is rated at 3W and with this voltage multiplier circuit provides about 200mA at about 15km/h. A 12V system was chosen because it allows the use of a car horn (get noticed)! Two 6V 3W globes in series provides adequate lighting and they last more than six months

Circuit diagram:

Bike Battery Charger Circuit Diagram

Author: Paul Breuker
Copyright: Siicon Chip Electronics

Google Nexus 5 Android Phone Now Available

Google Nexus 5 has been available at Google Play, including two memory models and two colors. If you’re curious, let’s go on checking the official Android phone. Google Nexus 5 is a high performance Android smartphone that measures 69.17 x 137.84 x 8.59mm and weights 130g.

The smartphone runs Google’s latest Android 4.4 KitKat mobile operating system, and as we can see from the images, Nexus 5 also features sleek ultra-slim design, and comes with a Qualcomm Snapdragon 800 2.26GHz quad-core processor with Adreno 330 450MHz GPU, 2GB RAM, 16GB/32GB internal storage, 4.95-inch full HD IPS display with 1920 x 1080 pixels resolution and Corning Gorilla Glass 3, 1.3-megapixel front-facing camera, and 8-megapixel rear camera with optical image stabilization. Moreover, Nexus 5 also has Bluetooth 4.0, NFC, dual-band WiFi, GPS, built-in wireless charging as well 2300mAh rechargeable battery that offers 17 hours of talk time.

Google Nexus 5 Android smartphone comes in two colors and two memory version, and starting at $349 USD. If you’re interested, jump to Google official site for more details.

Low-Voltage Cutout For 12V SLA Batteries

This simple circuit protects an SLA battery from over-discharge by disconnecting the load when the terminal voltage drops below a preset level. In operation, a sample of the battery voltage is derived from the 22kΩ resistor and 20kΩ trimpot divider. This is applied to the non-inverting input (pin 3) of IC1, where it is compared with a reference voltage on the inverting input (pin 2). When the sampled battery voltage falls below the reference voltage, IC1’s output (pin 1) swings towards ground, switching Mosfet Q2 off and disconnecting the load from the battery. The reference voltage is derived from a 4.7V zener diode (ZD1), which is connected to ground via the collector-emitter circuit of Q1 (ie, when Q1 is on).

Circuit diagram:

However, when the op amp’s output is driven low, Q1 is switched off, causing the non-inverting input to rise towards the full battery voltage. This greatly reinforces the switching action, latching the circuit in the "off" state until the battery is recharged and the reset switch (S1) pressed. The Mosfet used for Q2 should be selected to suit the intended application. The circuit could also drive a relay simply by connecting the coil across the "load" terminals. As is usual practice, a diode should be connected across the relay coil to limit back-EMF spikes.

Author: Tim Nuske - Copyright: Silicon Chip Electronics