Stereo Parabolic Microphone

This circuit is a stereo amplifier for a high sensitivity stereo parabolic microphone that able to used for listening to distant sounds. Typical parabolic microphones are monophonic, this unit has a stereo audio path that helps produce more realistic sounding audio. The Big-E can be used with headphones or as an audio source for a stereo tape recorder or a PC sound card.

This circuit also works nicely as a remote stereo audio receiver for accompanying a video surveillance system. It is capable of operating on the end of a four wire shielded cable that is more than 100 feet long. For remote operation, a set of inexpensive amplified PC speakers can be connected to the outputs for monitoring the sound.

Specifications
Operating Voltage: 9-15V (9V Nominal) DC
Operating Current: 7ma at 9V DC

How Does It Work
The circuit consists of two identical audio channels and some basic power supply filtering components. Only the left channel will be described.

The mini condenser microphone converts sounds into an electrical signal. Resistor R1 provides bias for the condensor microphone's internal amplifier transistor. The 2N3906 PNP transistor acts as a low noise microphone input amplifier. The 10K gain potentiometer is used for adjusting the audio signal level. A stereo 10K audio taper pot can be used for adjusting both channels simultaneously, or individual 10K trimmers can be used for fixed gain applications. The preamp output signal is fed into the 1458 op-amp, which boosts the audio to a level that is sufficient for driving an 8-ohm headphone or a tape recorder input. The 1458 amplifier stage is fixed gain (10X) in the inverting configuration, it drives the headphone speakers.

Capacitor C9 provides DC isolation from the 1458 op-amp output, which sits at half of the supply voltage. Resistor R13 provides impedance protection for the op-amp output and reduces audio distortion when driving low impedance headphones.

DC bias for the 1458 op-amps is set at half of the supply voltage by the R16/R17 voltage divider. Capacitors C13 and C14 filter the DC power supply for the op-amp stage. The DC is further filtered for the input preamp transistors through resistor R15 and capacitor C11. Diode D1 and resistor R18 protect the circuit from reverse battery polarity.

Construction
The Big-E circuit can be assembled on a circuit board, or hand wired. The board should be installed in a metal box for shielding from unwanted hum. For surveillance applications, the condenser microphones can be mounted directly on the PC board or on the edge of the metal box. The volume control can be mounted on the edge of the box, two 3.5MM mono jacks were used for the microphone inputs, a 3.5MM stereo jack was used for the headphone output. The 9V battery was mounted inside of the box, power is switched via a switch on the 10K stereo potentiometer.

The parabolic microphone assembly was made from an old Chinese wok cooker lid. The microphones are mounted on a metal standoff that places them at the focal point of the parabolic reflector. Pre-formed computer microphones were used for the model shown. The optimal microphone position can be found by pointing the reflector at a distant audio source, then moving the microphones for the loudest sound. The circuit box was mounted on the back side of the wok lid, it was attached to a piec of 1/2" square aluminum tubing, which forms a handle.

Parabolic Microphone Use
Start with the volume turned down, point the Big-E at a remote sound source, then gradually turn the volume up until the sound is heard. Be careful not to hit the side of the parabolic dish when listening, loud sounds can result. Also, beware that a malicious friend can cause you pain in the ears by talking loudly at the parabolic mic. It is advisable to wear the headphones partially off of your ears while you get used to the operation of the device. The Big-E is great for listening to birds and distant thunderstorms. It is also possible to hear the rustling of leaves on the top of a distant tree during a breezy day. Close-in wind noise may overpower distant sounds.

Stereo Parabolic Microphone PCB
Printed Circuit Image
Component Placement Silkscreen

Source: The Big-E Stereo Parabolic Microphone

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AC Power Supply-Low Voltage

Here's AC power supply circuit with low voltage output (step down transformer converter). Warning! This project involves the use of dangerous voltages. You must make sure all high-voltage (120 volt household power) conductors are safely insulated from accidental contact. No bare wires should be seen anywhere on the "primary" side of the transformer circuit. Be sure to solder all wire connections so that they're secure, and use real electrical tape (not duct tape, scotch tape, packing tape, or any other kind!) to insulate your soldered connections.

If you wish to enclose the transformer inside of a box, you may use an electrical "junction" box, obtained from a hardware store or electrical supply house. If the enclosure used is metal rather than plastic, a three-prong plug should be used, with the "ground" prong (the longest one on the plug) connected directly to the metal case for maximum safety.

Before plugging the plug into a wall socket, do a safety check with an ohmmeter. With the line switch in the "on" position, measure resistance between either plug prong and the transformer case. There should be infinite (maximum) resistance. If the meter registers continuity (some resistance value less than infinity), then you have a "short" between one of the power conductors and the case, which is dangerous!

Next, check the transformer windings themselves for continuity. With the line switch in the "on" position, there should be a small amount of resistance between the two plug prongs. When the switch is turned "off," the resistance indication should increase to infinity (open circuit -- no continuity). Measure resistance between pairs of wires on the secondary side. These secondary windings should register much lower resistances than the primary. Why is this?

Plug the cord into a wall socket and turn the switch on. You should be able to measure AC voltage at the secondary side of the transformer, between pairs of terminals. Between two of these terminals, you should measure about 12 volts. Between either of these two terminals and the third terminal, you should measure half that. This third wire is the "center-tap" wire of the secondary winding.

It would be advisable to keep this project assembled for use in powering other experiments shown in this book. From here on, I will designate this "low-voltage AC power supply" using this illustration:

 

 

Source: Transformer -- power supply

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ATX Power Supply Connector-Pinouts Diagram

Here's a standard for ATX Power supply connectors dan ATX power supply pinouts. Standard power supplies turn the incoming 110V or 220V AC (Alternating Current) into various DC (Direct Current) voltages suitable for powering the computer's components.

Power supplies are quoted as having a certain power output specified in Watts, a standard power supply would typically be able to deliver around 350 Watts.

The more components (hard drives, CD/DVD drives, tape drives, ventilation fans, etc) you have in your PC the greater the power required from the power supply.

By using a PSU that delivers more power than required means it won't be running at full capacity, which can prolong life by reducing heat damage to the PSU's internal components during long periods of use.

Always replace a power supply with an equivalent or superior power output (Wattage).

There are 3 types of power supply in common use:

• AT Power Supply - still in use in older PCs.
• ATX Power Supply - commonly in use today.
• ATX-2 Power Supply - recently new standard.

The voltages produced by AT/ATX/ATX-2 power supplies are:

• +3.3 Volts DC (ATX/ATX-2)
• +5 Volts DC (AT/ATX/ATX-2)
• -5 Volts DC (AT/ATX/ATX-2)
• +5 Volts DC Standby (ATX/ATX-2)
• +12 Volts DC (AT/ATX/ATX-2)
• -12 Volts DC (AT/ATX/ATX-2)

A power supply can be easily changed and are generally not expensive, so if one fails (which is far from uncommon) then replacement is usually the most economic solution.

ATX Power Supply Connectors Diagram

ATX Power Supply Pinouts Diagram

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12V Inverter Circuit

This 12V inverter is very easy to build, cheap components that many electronics hobbyists may even already have. Though it is possible to build a more powerful circuit, the complexity caused by the very heavy currents to be handled on the low-voltage side leads to circuits.

The circuit diagram of 12v inverter is easy to follow. A classic 555 timer chip, identified as IC1, is configured as an astable multivibrator at a frequency close to 100 Hz, which can be adjusted accurately by means of potentiometer P1. It is used to drive a D type flip-flop produced using a CMOS type 4013 IC. This produces perfect complementary squarewave signals (in antiphase) on its Q and Q outputs suitable for driving
the output power transistors.

As the output current available from the CMOS 4013 is very small, Darlington power transistors are used to arrive at the necessary output current. We have chosen MJ3001s from the now defunct Motorola (only as a semi-conductor manufacturer, of course!) which are cheap and readily available, but any equivalent powerDarlington could be used.

These drive a 230 V to 2 × 9 V centre tapped transformer used ‘backwards’ to produce the 230 V output. The presence of the 230 VAC voltage is indicated by a neon light, while a VDR (voltage dependent resistor) type S10K250 or S07K250 clips off the spikes and surges that may appear at the transistor switching points.

12 Inverter Parts List
Resistors
R1 = 18kΩ
R2 = 3kΩ3
R3 = 1kΩ
R4,R5 = 1kΩ5
R6 = VDR S10K250 (or S07K250)
P1 = 100 kΩ potentiometer
Capacitors
C1 = 330nF
C2 = 1000 μF 25V
Semiconductor
T1,T2 = MJ3001
IC1 = 555
IC2 = 4013
Miscellaneous
LA1 = neon light 230 V
F1 = fuse, 5A
TR1 = mains transformer, 2x9V 40VA (see text)
4 solder pins
PCB,

The Darlington transistors should be fitted onto a finned anodized aluminium heat-sink using the standard insulating accessories of mica washers and shouldered washers, as their collectors are connected to the metal cans and would otherwise be short-circuited.

An output power of 30 VA implies a current consumption of the order of 3 A from the 12 V battery at the ‘primary side’. So the wires connecting the collectors of the MJ3001s [1] T1 and T2 to the transformer primary, the emitters of T1 and T2 to the battery negative terminal, and the battery positive terminal to the transformer primary will need to have a minimum crosssectional area of 2 mm2 so as to minimize
voltage drop. The transformer can be any 230 V to 2 × 9 V type, with an E/I iron core or toroidal, rated at around 40 VA.

Properly constructed on the board shown here, the 12 inverter circuit should work at once, the only adjustment being to set the output to a frequency of 50 Hz with P1.

The circuit should not be too difficult to adapt to other mains voltages or frequencies, for example 110 V, 115 V or 127 V, 60 Hz. The AC voltage requires a transformer with a different primary voltage (which here becomes the secondary), and the frequency, some adjusting of P1 and possibly minor changes to the values of timing components R1 and C1 on the 555. Author: B. Broussas

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Audio Power Meter Using Duo LED

Audio Power Meter indicates the amount of power that goes to a loudspeaker. This simple audio power meter circuit uses dual-colour LED that shows green at an applied power level of about 1 watt. At 1.5 watts it glows orange and above 3 watts it is bright red.

The audio power meter circuit is connected in parallel with the loudspeaker connections and is powered from the audio signal. The additional load that this represents is 470 Ohm (R1//R3) will not be a problem for any amplifier.

How The Audio Power Meter Works
During the positive half cycle of the output signal the green LED in the dual-colour LED will be turned on, provided the voltage is sufficiently high. At higher output voltages, T1 (depending on the voltage divider R2/R1) will begin to conduct and the green LED will go out.

During the negative half cycle the red LED is driven via R3 and will turn on when the voltage is high enough. In the transition region (where T1 conducts more and more and ‘throttles’ the green LED as a result) the combination of red/green gives the orange colour of the dual-LED.

By choosing appropriate values for the resistors the power levels can be adjusted to suit. The values selected here are for typical living room use. You will be surprised at how loud you have to turn your amplifier up  before you get the LEDs to go!

The resistors can be 0.25 W types, provided the amplifier does not deliver more than 40 W continuously. Above this power the transistor will not be that happy either, so watch out for that too. Because T1 is used in saturation, the gain (Hfe) is not at all important and any similar type can be used. The power levels mentioned
are valid for 4-Ohm speakers. For 8-Ohm speakers all the resistor values have to be divided by two. Author: Michiel Ter Burg

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Analog Milliamp Meter Used as Voltmeter

A milliamp meter can be used as a volt meter by adding a series resistance. The resistance needed is the full scale voltage reading divided by the full scale current of the meter movement. So, if you have a 1 milliamp meter and you want to read 0-10 volts you will need a total resistance of 10/.001 = 10K ohms.

The meter movement itself will have a small resistance which will be part of the total 10K resistance, but it is usually low enough to ignore. The meter in the example below has a resistance of 86 ohms so the true resistor value needed would be 10K-86 or 9914 ohms. But using a 10K standard value will be within 1% so we can ignore the 86 ohms. For a full scale reading of 1 volt, the meter resistnace would be more significant since it would be about 8% of the total 1K needed, so you would probably want to use a 914 ohm resistor, or 910 standard value.

The milliamp meter can also be used to measure higher currents by adding a parallel resistance. The meter resistance now becomes very significant since to increase the range by a factor of ten, we need to bypass 9/10 of the total current with the parallel resistor. So, to convert the 1 milliamp meter to a 10 milliamp meter, we will need a parallel resistor of 86/9 = 9.56 ohms.


Source: Analog Milliamp Meter Used as Voltmeter

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Car Anti-Theft Wireless Alarm

This alarm circuit is an anti- theft wireless alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF FM radio-controlled, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park.

The receiver unit of the wireless alarm uses an CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside. Receiver is tuned to the transmitter's frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transis- tor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised.

When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link betw- een the car (transmitter) and alarm (receiver) is broken. As a result FM radio module gene-rates hissing noise. Hissing AC signals are coupled to relay switching circ- uit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2. Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level. The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on.

If, by chance, the intruder finds out about the wireless alarm and disconnects the transmitter from battery, still remote alarm remains activated because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable. (Ed: You may have some problem catching the thief, though, if he decides to run away with your vehicle_in spite of the alarm!)

Go to Car Anti-Theft Wireless Alarm Forum

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