The supply receives -20 V from the rectifier/filter which is fed to the collector of the Darlington pnp pass transistor, a TIP105. The base drive to the TIP105 is supplied through resistor R5. The base of the TIP is driven from Vz terminal at pin 9, which is the anode of a 6.2-V zener diode that connects to the emitter of the uA723 output control transistor. [...]
Read more source:
http://www.circuitsuggest.com/15-v-1-a-regulated-power-supply.html

This is an old discrete component design, now superceded by a PIC.
This thing is a very nifty capacitive sensor. For you europeans, this little gem is used in north america mostly for detecting wooden beams behind drywall or plaster. I’ll take one of these over the new design any day, because these ones don’t error out every time you try to use them in a non-standard way.

Read more source:
http://www.members.shaw.ca/novotill/StudSensorAutomatic/index.htm

This is an actual reverse engineered circuit diagram of the one transistor circuit most commonly used to drive Permanent Magnet DC motors in childrens’ toys. This type of circuit almost always uses the 625mW version of the ubiquitous 8050 or 8550 transistor. It is vital to note that there are two different varieties of each of these two transistors, and although they are often stamped with the same part number, they are not interchangeable because one is a 625mW part with a higher VCEsat, while the other is a 1W part with a lower VCEsat. This is very confusing because you often have to run tests on these transistors to tell them apart! The important feature to note about this circuit, is that the maximum power delivered to the motor is normally designed to be limited by the VCEsat of the transistor, and almost never by the HFE or base current. [...]

Read more source:
http://www.members.shaw.ca/novotill/ToyMotorDrivers/index.htm

Here is a simple audio amplifier using a TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar in complexity to a simple one-transistor radio but the performance is superior (with the exception of the amazing one-transistor reflex ). The TL431 is available in a TO-92 package and it looks like an ordinary transistor so your hobbyist friends will be impressed by the volume you are getting with only one transistor and the amplifier may be used for other projects, too. Higher impedance headphones and speakers may also be used. An earphone from an old telephone will give ear-splitting volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms when using high impedance earphones to save battery power.

Read More Source:
http://www.techlib.com/electronics/audioamps.html

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Friends may meet the disadvantages of the electrical equipment that uses battery be battery like to finished the climax always. This equipment no help to give battery be finished have power upward again. But wait for warn when battery low be hurried change or charger to first be finished. This circuit designs to come to apply to volt since 3-15V the dot will to begin to show can fine with VR1. When voltage decrease down more location dot keeps LED bright immediately. The electronics part a little also just transistor BC547 and other. It usual Current about 3mA but enough LED bright will valuable about 15mA. Other detail can see get from the circuit.

Normally ni cad battery often apply current be stable charger. This circuit was designed go up for use to do battery package aforementioned a kind cell 1.25V x 4 amounts are group model AA that 50mA or cell 1.25V model C that 250mA by build serial. And can fine decorate other value get easy. Follow the circuit resistor R1 and R2 use fine output voltage. While have no load appraise 8V current output railings will flow through resistor R6 and R7 any one. And when there is the size increases transistor Tr1 as a result quietly work, make at the dot Y have tall go up. Be TR2 work and dot Z be add less. It make output voltage decrease lower and trend reduction until equilibrium point value filtration of resistor R6 and R7 the that. Must live the experiment thus then must use amp meter just rightly.

This be Active Antenna circuit for boost frequency all signal give the power goes up. By this circuit has tall rapidity cover the frequency about 10Mhz - 110MHz by have electronics equipment the important be FET and Transistor the small-sized. It make a signal that change antenna the power goes up plentiful.

Antennas that are much shorter than 1/4 wavelength present a very small and highly relative impedance that is dependent on the received frequency. It is difficult to match impedances over a decade of frequency coverage. Instead, input stage Q1 is an FET source-follower. A high-impedance input successfully bridges antenna characteristics at any frequency.

Read Source :
http://www.circuitsuggest.com/active-antenna.html

The above circuit is a versatile audio amplifier employing a low cost LM358 op-amp. The differential inputs give the amplifier excellent immunity to common-mode signals which are a common cause of amplifier instability. The dotted ground connection represents the wiring in a typical project illustrating how the ground sensing input can be connected to the ground at the source of the audio instead of at the amplifier where high currents are present. If the source is a power supply referenced signal then one of the amplifier inputs is connected to the positive supply. [...]
Read source:
http://www.techlib.com/electronics/audioamps.html

A class-A audio amplifier is pretty wasteful of power but when plenty of power is available the simplicity is attractive. Here is a simple darlington transistor example intended for use with a 5 volt power supply:
The 5 volts should be provided by a regulated power supply. The efficiency is below 25% and significant DC current flows in the speaker and that additional power should be figured in to the power rating of the speaker. But look how simple it is! The voltage gain is only about 20 and the input impedance is about 12k. The schematic shows two values of bias resistor to be used with the corresponding speaker impedance.

http://www.techlib.com/electronics/audioamps.html