Circuit Project Electronic

This is an old discrete component design, now superceded by a PIC.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