Portable Microphone Preamplifier

High headroom input circuitry, 9V Battery operation

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply. Using a stereo microphone the circuit must be doubled. In this case, two separate level controls are better than a dual-ganged stereo potentiometer. Low current drawing (about 2mA) ensures a long battery life.

Circuit Operation:

The circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Circuit diagram:

Portable Microphone Preamplifier Circuit Diagram

Parts:

P1 = 2.2K
R1 = 100K
R2 = 100K
R3 = 100K
R4 = 8.2K
R5 = 68R
R6 = 6.8K
R7 = 1K
R8 = 1K
R9 = 150R
C1 = 1uF-63V
C2 = 100uF-25V
C3 = 100uF-25V
C4 = 100uF-25V
C5 = 22uF-25V
Q1 = BC560
Q2 = BC550

Notes:

• Harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).
• Maximum input voltage (level control cursor set at maximum) = 25mV RMS
• Maximum input voltage (level control cursor set at center position) = 200mV RMS
• Enclosing the circuit in a metal case is highly recommended.
• Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.

Source : www.redcircuits.com

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Fuse Monitor

The idea for this project may have come to me in a flash of inspiration, and its a very simple way to check if a fuse has blown without removing it from its holder. The simplicity of this circuit uses just two components, but with just one resistor and an LED this circuit gives visual indication of when a fuse has blown. LED1 is normally off, being "short circuited " by the fuse, F1. Should the inevitable "big-bang" happen in your workshop then LED1 will illuminate and led you know all about it! Please note that the LED will only illuminate under fault conditions, i.e. with a short circuit or shunt on the load. In this case the current is reduced to a safe level by R1.

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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.

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Sound Effects Generator 2

This circuit uses the Holtek HT2884 IC to produce 8 different sound effects. All sound effects are generated internally by the HT2884 IC. Power is a 3 Volt battery, but the IC will work with any voltage between 2.5 and 5 Volts. Switch S1 is the on / off switch.

Sound Effects Generator 2 Circuit Diagram:

The output at pin 10 is amplified and drives a small 8 ohm loudspeaker. Pressing S3 once will generate all the sounds, one after another. S2 can be used to produce a single sound effect, next depression gives the next sound effect. There are 2 lazer guns, 1 dual tone horn sound, 2 bomb sounds, 2 machine gun sounds and a rifle shot sound. Standby current is about 1 uA at 3 Volt, so battery life is very economical.

The IC may be obtained from Maplin Electronics order code AZ52G

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LONG RANGE FM TRANSMITTER ELECTRONIC DIAGRAM

LONG RANGE FM TRANSMITTER ELECTRONIC DIAGRAM

This circuit works optimally by adding RF amplifier and antenna. Here is the schematic diagram :

Parts list :

•     Diode D1 : BB109
•     Resistor R1 : 10k ohm
•     Resistor R2 : 100k ohm
•     Resistor R3 : 180k ohm
•     Resistor R4 : 4K7
•     Resistor R5 : 15k ohm
•     Resistor R6 : 68 ohm
•     Resistor R7 : 470 ohm
•     Resistor R8 : 39k ohm
•     Resistor R9 : 10 ohm
•     VR1 : 47k ohm
•     VR2 : 22 ohm
•     Capacitor C1-C3, C8 : 0.1 uF
•     Capacitor C4 : 4.7 pF
•     Capacitor C6 : 0.01 uF
•     Capacitor C7 : 5.6 nF
•     Capacitor C9 : 100 pF
•     Transistor T1: BF494
•     Transistor T2:2N3866
•     Trimmer VC1-VC2 : 50p
•     L1 : 4 round 20 cables SWG in plastic with 8mm diameter
•     L2 : 2 round 24 cables SWG
•     L3 : 7 round 24 cables SWG in plactic with 4mm diameter
•     L4 : 7 round 24 cables SWG in ferrid bead

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Frequency Doubler

If you are working at frequencies of the order of 850MHz to 4GHz and find that a frequency multiplier is required, the HMC 187, HMC 188 and HMC 189 (see table) frequency doubler may be just the solution you are looking for. The isolation performance of these devices ensures that the input frequency (fin) and its harmonics 3fin and 4fin are attenuated by 35dB relative to the wanted output frequency 2fin. This excellent isolation specification reduces the need for additional output filtering and is also an advantage where several doublers are connected in series to produce four or eight times the input frequency.

The tiny outline of the HMC18x- series device occupies a board area of 3mm by 4.8mm and measures just 1.07 mm high. Internally the device contains balanced to unbalanced transformers (baluns) to match the doubler circuit with the output and input. The doubler circuit itself is passive and comprises a full wave Schottky diode bridge rectifier. The monolithic baluns which are integrated on-chip give the device a relatively high low-frequency roll-off at 850MHz.

Lower frequencies can also be multiplied but the conversion loss factor (given as typically 15 dB) will increase. The input and output are matched for 50 Ohm operation and the input signal level should be of the order of +15dBm which will give a output level of approximately 0dBm. The main characteristics of the three versions of this device are summarized in the table above.

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Mains Slave Switcher Circuit

There are many situations where two or more pieces of equipment are used together and to avoid having to switch each item on separately or risk the possibility of leaving one of them on when switching the rest off, a slave switch is often used. Applications which spring to mind are a computer/printer/scanner etc or audio amplifier/record deck/tuner combinations or perhaps closest to every electronics enthusiast’s heart, the work bench where a bench power supply/oscilloscope/soldering iron etc are often required simultaneously. The last is perhaps a particularly good example as the soldering iron, often having no power indicator, is invariably left on after all the other items have been switched off. Obviously the simplest solution is to plug all of the items into one extension socket and switch this on and off at the mains socket but this is not always very convenient as the switch may be difficult to reach often being behind or under the work bench. Slave switches normally sense the current drawn from the mains supply when the master unit is switched on by detecting the resulting voltage across a series resistor and switching on a relay to power the slave unit(s).

This means that the Live or Neutral feed must be broken to allow the resistor to be inserted. This circuit, which is intended for switching power to a work bench when the bench light is switched on, avoids resistors or any modifications to the lamp or slave appliances by sensing the electric field around the lamp cable when this is switched on. The lamp then also functions as a ‘power on’ indicator (albeit a very large one that cannot be ignored) that shows when all of the equipment on the bench is switched on. The field, which appears around the lamp cable when the mains is connected, can be sensed by a short piece of insulated wire simply wrapped around it and this is amplified by the three stage amplifier which can be regarded as a single super-transistor with a very high gain. The extremely small a.c. base current results in an appreciable collector current which after smoothing (by C3) is used to switch on a relay to power the other sockets. Power for the relay is obtained from a capacitor ‘mains dropper’ that generates no heat and provides a d.c. supply of around 15 volts when the relay is off.

Circuit diagram:
 

Mains Slave Switcher Circuit Diagram

The output current of this supply is limited so that the voltage drops substantially when the relay pulls in but since relays require more current to operate them than they do to remain energized, this is not a problem. Since the transistor emitter is referenced to mains Neutral, it is the field around the mains Live which will be detected. Consequently, for correct operation the Live wire to the lamp must be switched and this will no doubt be the case in all lamps where the switch is factory fitted. In case of uncertainty, a double-pole switch to interrupt both the Live and Neutral should be used. The sensitivity of the circuit can be increased or decreased as required by altering the value of the T2 emitter resistor. The sensing wire must of course be wrapped around a section of the lamp lead after the switch otherwise the relay will remain energized even when the lamp has been switched off. The drawing shows the general idea with the circuit built into the extension socket although, depending on the space available an auxiliary plastic box may need to be used.

Warning:

The circuit itself is not isolated from the mains supply so that great care should be taken in its construction and testing. The sensor wire must also be adequately insulated and the circuit enclosed in a box to make it inaccessible to fingers etc. when it is in use.

Source :  www.extremecircuits.net

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Maximum Minimum Voltage Indicator

This circuit indicates which of three voltages in the range from about about -4V to about +4V - at A, B and C - is the highest by lighting one of three indicator LEDs. Alternatively, it can be wired to indicate the lowest of three voltages or to indicate both the highest and lowest voltages. Op amps IC1a, IC1b & IC1c are wired as comparators, while the three indicator LEDs and their series 1kO current limiting resistors are strung across the op amp outputs to implement the appropriate logic functions.

For example, LED A will light only when pin 8 of IC1c is low (ie, A greater B) and pin 7 of IC1b is high (ie, A greater C). Similarly, LED B will light only when pin 8 of IC1c is high (ie, B greater A) and pin 1 of IC1a is low (ie, B greater C). LED C works in similar fashion if the voltage at C is the highest. Note that if all the LEDs and their parallel 1N4148 diodes are reversed, the circuit will indicate the lowest of the three input voltages. And if each 1N4148 diode is replaced by a LED, the circuit will indicate both the highest and lowest inputs.

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USB Power Injector For External Hard Drives

A portable USB hard drive is a great way to back up data but what if your USB ports are unable to supply enough "juice" to power the drive? A modified version of the Silicon Chip Usb Power Injector is the answer. For some time now, the author has used a portable USB hard drive to back up data at work. As with most such drives, it is powered directly from the USB port, so it doesn’t require an external plug pack supply.

Projects Picture:

In fact, the device is powered from two USB ports, since one port is incapable of supplying sufficient current. That’s done using a special USB cable that’s supplied with the drive. It has two connectors fitted to one end, forming what is basically a "Y" configuration (see photo). One connector is wired for both power and data while the other connector has just the power supply connections. In use, the two connectors are plugged into adjacent USB ports, so that power for the drive is simultaneously sourced from both ports.

USB Cable:

An external USB hard drive is usually powered by plugging two connectors at one end of a special USB cable into adjacent USB ports on the computer. This allows power to be sourced from both ports. According to the USB specification, USB ports are rated to supply up to 500mA at 5V DC, so two connected in parallel should be quite capable of powering a portable USB hard drive – at least in theory.

Complete Project:

Unfortunately, in my case, it didn’t quite work out that way. Although the USB drive worked fine with several work computers, it was a "no-go" on my home machine. Instead, when it was plugged into the front-panel USB ports, the drive repeatedly emitted a distinctive chirping sound as it unsuccessfully tried to spin up. During this process, Windows XP did recognise that a device had been plugged in but that’s as far as it went – it couldn’t identify the device and certainly didn’t recognize the drive. Plugging the drive into the rear-panel ports gave exactly the same result. The problem wasn’t just confined to this particular drive either. A newly-acquired Maxtor OneTouch4 Mini drive also failed to power up correctly on my home computer, despite working perfectly on several work computers.

Circuit diagram:

The revised USB Power Injector is essentially a switch and a 5V regulator. The Vbus supply from USB socket CON1 turns on transistor Q1 which then turns on power Mosfet Q2. This then feeds a 6V DC regulated supply from an external plug pack to regulator REG1 which in turn supplies 5V to USB socket CON2.

Source: Silicon Chip 26 June 2008

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Ultra Simple Microphone Preamplifier

This little project came about as a result of a design job for a client. One of the items needed was a mic preamp, and the project didnt warrant a design such as the P66 preamp, since it is intended for basic PA only. Since mic preamps are needed by people for all manner of projects, this little board may be just whats needed for interfacing a balanced microphone with PC sound cards or other gear. Unlike most of my boards, this one is double-sided. I normally avoid double-sided PCBs for projects because rework by those inexperienced in working with them will almost certainly damage the board beyond repair. I consider this not to be an issue with this preamp, because it is so simple. It is extremely difficult to make a mistake because of the simplicity.

Photo of Completed Board

As you can see, the board uses a PCB mounted XLR connector and pot, so is a complete mic preamp, ready to go. Feel free to ignore the terminals marked SW1 (centred between the two electrolytic supply caps), as they are specific to my clients needs and are not useful for most applications. The original use was to use them for a push-button switch that activated an audio switch via a PIC micro-controller. They are not shown on the schematic.

The DC, GND and output terminals may be hard wired to the board, you may use PCB pins or a 10-way IDC (Insulation Displacement Connector) and ribbon cable. Power can be anything between +/-9V and +/-18V with an NE5532 opamp. The mic input is electronically balanced, and noise is quite low if you use the suggested opamp. Gain range is from about 12dB to 37dB as shown. It can be increased by reducing the value of R6, but this should not be necessary. Because anti-log pots are not available, the gain control is not especially linear, but unfortunately in this respect there is almost no alternative and the same problem occurs with all mic preamps using a similar variable gain control system.

Figure 1 - Preamp Schematic

The circuit is quite conventional, and if 1% metal film resistors are used throughout it will have at least 40dB of common mode rejection with worst-case values. The input capacitors give a low frequency rolloff of -3dB at about 104Hz. If better low frequency response is required, these caps may be increased to 4.7uF or 10uF bipolar electrolytics. These will give response to well below 10Hz if you think youll ever need to go that low.

The project PCB measures 77 x 24mm, and the mounting centers for the pot and XLR connector are spaced at 57mm. If preferred, a traditional chassis mounted female XLR can be used, and wired to the board with heavy tinned copper wire. The PCB pads for the connector are in the correct order for a female chassis mount socket mounted with the "Push" tab at the top.

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