This is the schematic diagram of a typical rf amplifier that is used in an AM radio receiver. The input circuit is the antenna of the radio (L1-a coil) which forms part of an LC circuit which is tuned to the desired station by variable capacitor C1. L1 is wound on the same core as L2, which couples the input signal through C2 to the transistor (Q1). R1 is used to provide proper bias to Q1 from the base power supply (VBB). R2 provides proper bias to the emitter of Q1, and C3 is used to bypass R2. The primary of T1 and capacitor C4 form a parallel LC circuit which acts as the load for Q1. This LC circuit is tuned by C4, which is ganged to C1 allowing the antenna and the LC circuit to be tuned together. The primary of T1 is center-tapped to provide proper impedance matching with Q1.

Source:  http://www.tpub.com

The 555 as astable produces a ’square wave’, this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between ‘low’ and ‘high’.

The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second. That means:

T = 0.7 × (R1 + 2R2) × C1   and

f = 1.4 / ((R1 + 2R2) × C1)

Where:

T   = time period in seconds (s)
f    = frequency in hertz (Hz)
R1 = resistance in ohms ()
R2 = resistance in ohms ()
C1 = capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts  = 0.7 × R2 × C1

Source: http://www.kpsec.freeuk.com

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Here is a two-transistor Wien bridge oscillator that uses an ordinary night-light bulb for stabilization. The output is about 6 volts peak  to peak and can drive fixed loads as low as 2k or 3k  ohms without additional buffering. A 10 k amplitude potentiometer with the wiper going to a high input impedance output amplifier would make an excellent load.

Excellent distortion is achieved by adjusting the 1 k feedback potentiometer until the output amplitude is about a volt less than the maximum level (with the pot set to the highest resistance). Wait a few seconds between adjustments to give the bulb time to stabilize; the audio signal actually heats the bulb’s filament causing the resistance to go up which controls the loop gain. You will see the signal bounce a little as the bulb gains control. This simple version of the popular Wien bridge oscillator uses feedback to hold the junction of the two RC networks (base of first transistor) near zero volts (100 mV p-p) and the ends of the RC networks move in opposite directions like a see-saw.

Source:  http://www.techlib.com/

This is a simple oscillator with multiple resistors in serie. When you press any switch, the circuit starts oscillating. You can user variable resistors instead the 1k resistors. Using variable resistors you will be able to tune the frecuency for each note. You cannot be able to press two keys at the same time because the frecuency will change. To make an electronic organ capable to press many keys at the same time, you may need to use thirteen 555 ICs.

It’s ok to use 9 to 12V as power but it may be quite loud. I do not recommend to use a AC to DC converter as power supply because it may cause electrical shock. Also a “hum” will be audible while it is in use.

Source: www.josepino.com

The circuit gives a voltage that corresponds the current RPM. The advantage over a analogue meter is an accurate  reading since the “needle” is much quicker to react. In fact, as the output is a voltage you could connect it to a moving needle if you require.

The RPM can be converted to a frequency quite easily. There are two sparks per rotation and we are working in seconds so simply devide the RPM by 30! (thus 8000rpm is actually 266 Hertz).

The frequency input to the circuit can be taken from the -Ve terminal of the coil. You would expect the voltage to swing by about 12 volts since the contact breakers simply switch the battery to the coil. Unfortunately the coil is an inductor and the condensor is a capacitor thus we have a highly resonant circuit which can hit over 40 volts. Protection is shown in the circuit diagram.

Source: http://www.niksula.hut.fi/

This almost trivial circuit may be used to charge a pair of AA or AAA sized rechargeable battery cells using sunlight. The circuit has been used to keep a Palm Pilot and any personal stereo running “perpetually”. This is an unregulated charger, proper charging is achieved by placing the unit in the sun for a known amount of time, this time varies according to the battery type.

Source: http://www.solorb.com/

This 1.5 volt led flasher runs for a long time with a single “D” cell and alternately flashes 2 LEDs at about 1 hertz rate. The circuit uses a CMOS 74HC14 hex inverter that operates at very low voltages. Pins 1 and 2 are used as a square wave oscillator while the others are wired to produce a short 10mS pulse on alternate edges of the square wave so the LEDs will alternate back and forth.

Each output sections use a capacitor charge pump to increase the voltage for the LEDs. A regular LED needs 1.8V to produce light, so that circuit is needed as 1.5V is not enough for the LED. The circuit draws about 800uA from the battery and the LED peak current is about 40mA.

Source: http://ourworld.compuserve.com/homepages/Bill_Bowden

This is a simple but reliable electronic circuit for your a water pump from an aquarium, boat, or whatever, but water only. It cannot be used with flammable liquids. Also being careful when working with 115Volt line voltage is a must. You should Take every precaution to avoid electrical shock. Before making any change to R1 the power should be unplugged.

Source: http://www.uoguelph.ca/~antoon

This simple metal detector requires only a handful of components and an evening’s work. Built around a cmos4011 IC, is very robust and versatile. The 250 kHz reference oscillator is built with two gates (U1/1 and U1/2) C1, R1 and P2. The search oscillator uses only one gate (U1/3) two capacitors and the search coil. The output of the two oscilators are fed to the fourth gate acting as a mixer, filtered with C4 then amplified with U2.

After assembly adjust the volume control (P2) for comfortable volume and turn P1. The pitch will get lower until it disappears. Continuing to rotate P1 in the same direction will cause the pitch to rise again. The point at witch the pitch is the lowest and disappears is called “zero beat”. If you can not get this zero beat frequency for the entire turn of P1 you may have to select different values for C1.

Source: http://www.hobby-hour.com

A circuit for driving high-power unipolar stepper motors. This circuit allows step-level control and can be easily modified for other modes of operation. The L297 has several inputs that can be generated by a PC/104 stack or other controller. This circuit allows you to control each step, in full-step mode. Meaning: You can tell it to move one step in either direction (of course you can make it move fast and it will continuously rotate). The two inputs are a direction and a pulse.
Source:  http://hades.mech.northwestern.edu

This message display was based on the dot matrix display in the project area on this site.

The Letters a-z and numbers 0-9 are stored in an array. There is a second array containing the order the letters are displayed on the dot matrix and uses the binary left shift operator (<<) to move them to the left.

For more information about this project, visit the source website.

Source:  http://www.best-microcontroller-projects.com

This Programmer is powered by the RS-232 and it works with RS-232 levels at only < ±8.6V. It programs PIC12C5XX, 12C67X, 24CXX, 16C55X, 16C61, 16C62X, 16C71, 16C71X, 16C8X, 16F8X and ISO-CARD’s with ASF. Other serial programmable chips by adapter.

The high Vpp is obtained by using negative voltage to drive the chip. The voltage is stabilized with zener diodes. They do not need voltage drop as if a voltage regulator, or has much offset current. This makes it possible to use extra low input voltage. Transistor driver guarantee output level > ±3V.

The Programmer supports ICSP, In-Circuit Serial Programming.

Source: http://www.jdm.homepage.dk

Using only 2 capacitors, 3 resistors, 4 seven-segment Display, 1 xtal, 2 switches n.o. and 1 Microcontroller PIC, you can build this Digital Led Clock. you can use common anode or common cathode display, just select the display type.

Pin 3 defines the display type. If you will use common cathode display, connect to negative. For common anode, connect to positive.  Please note: the pin 4 requieres a 10k resistor ONLY for 16F84. On 16F628 is not connected. The minutes displays are upside down.  Short connections on xtal is a must to keep accuracy

Source: http://www.josepino.com

(+)------------+---------------+--------------------------+
 green         |               |                          |
               /               /                          /
               \  2200         \ 100K              100K   \
               /               /                          /
1N4148         \               \                          \
     +---------+               |                          |
  ___|___   ___|___            |                       ___|___/
    / \     \     /   LED      |              10V     /  / \
  /_____\   __\ /__            |             ZENER     /_____\
     |         |               |                          |
     +---------+               |                          |
               | c             |                          |
                \  |           |                          |
         MPSA42   \|-----------+------------- c           |
                  /|        ___|___           \  |        |
                /  |          / \     MPSA42    \|--------+
               | e          /_____\             /|     ___|___
               |      1N4148   |              /  |       / \
               |               |             | e       /_____\
               |               |             |            |
(-)------------+---------------+-------------+------------+
red                                                       1N4148

The circuit I built gives a visual indication at each extension when any extension is off-hook. It is line-powered, and the maximum number that can be used on our system is three. Since they all draw power at the same time to light the LEDs, any more indicators would cause an off-hook condition. Some changes could be made to reduce the current draw, to allow using more indicators, but the brightness of each led would suffer. The LEDs I used are tiny, but amazingly bright on just a couple milliamps. I picked them up from a surplus catalog, I can’t remember which one. If you were to use battery power for the circuit, you could use almost any number of indicators. I had use only for three, and I did not want to worry about replacing batteries. If I remember correctly, our pbx required a load of about 20 milliamps before the line failed to hang up. This circuit draws about 5 milliamps when off- hook, much less when on-hook. It senses the drop in line voltage from about 46 volts to 6 volts when an extension is picked up. The zener voltage should be well above the off-hook voltage of your system, and well below the on-hook voltage. The transistors are small high-voltage npn types I had on hand. The LED also flashes with the ring voltage. Putting a suitable MOV across the line is a good precaution to prevent lightning damage.

Source:  http://www.ee.washington.edu

A simple two-stage amplifier. Use any general-purpose NPN with high gain. It will drive low impedance headphones directly. A speaker will probably require further boost.

Source: http://www.geocities.com/tomzi.geo

This circuit can be used to power a fluorescent lamp or small strobe. It will generate over 400 VDC using just 12 VDC, 2.5 A power supply, an auto or marine battery will be good rnoug. All components are readily available (even from any broken radio or electronics store) and construction is very easy. No custom coils or transformers are required. If wired correctly, it will work.

Output depends on input voltage. Adjust for your application. With the component values given, it will generate over 400 V from a 12 V supply and charge a 200 uF capacitor to 300 V in under 5 seconds.

Source: http://www.repairfaq.org

Q1 amplifies the input signal via C4 from the electret microphone.
Q2 acts as an oscillator and the signal coming off C2 is fed onto the base of Q2.
L1/C1 is a so called ‘tank’ circuit and operates in the 88-105MHz band on your regular AM/FM radio dial.
L1 is a 1uH variable inductor coil to be able to tune it a little bit, and the range of 1uH is approximate.
The antenna can be as simple as a 8″ (21cm) piece of wire of any kind.

Source: http://www.uoguelph.ca/~antoon

Anyone performing their own automobile tuneups knows how important is to know the engine speed. With this tachometer, you can measure the engine speed without any connection or annoying timing lights.

A quite interesting project for your car.

Source: www.aaroncake.net

The operation of the circuit is based on superheterodyning principle, which is commonly used in superheterodyne receivers. The circuit uses two RF oscillators. The frequencies of both oscillators are about 5.5 MHz. The first RF oscillator comprises transistor T1 ( Transistor BF 494) and a 5.5MHz ceramic filter, which is commonly used in TV sound at the IF section. The second oscillator is a Colpitt’s oscillator with the help of transistor T3 (BF494) and inductor L1 shunted by trimmer capacitor VC1. These two oscillators’ frequencies (say Fx and Fy) are mixed and the difference or the beat frequency (Fx-Fy) outs from collector T2.  It is  connected to the detector stage comprising diodes D1 and D2 (both OA 79). The output is a pulsating DC which is passed through a low-pass filter made with the help of a 10k (resistor R12) and two 15nF capacitors (C6 and C10).

The inductor L1 can be build using 15 turns of 25 or 26 SWG wire on a 10cm (4-inch) diameter air-core former.  For proper operation of the circuit it is critical that frequencies of both the oscillators are the same so as to obtain zero beat in the absence of any metal in the near vicinity of the circuit.

Source: http://www.electronic-circuits-diagrams.com

Another inverter for fluorescent tubes. The transformer can be taken from a broken radio or any electronic equipment that uses 220V.

Two 2N3055 or TIP3055 transistors are connected as flip-flop to drive the transformer. The 22nF capacitors define the frequency of this inverter.

Any 12 -Volts battery can be used to power this circuit and get light on vehicles.

Source: www.pablin.com.ar