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DiscoverCircuits.com -- Hobby Corner
Last Updated on: Monday, August 17, 2015 04:25 AM

Hobby Circuits' Category List

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Hobby Circuits in Progress
designed by David Johnson, P.E.

175KHz Earth Ground Communications:  Two copper rods are driven into the ground, separated by up to 50 feet.  A third shorter rod is pushed into the ground between the two main rods as a reference.  The two main rods are connected to a frequency modulated 175KHz signal source.  Miles away a similar system is used to detect the signal.  The two locations can communicate through the earth.

2.5GHz RF Detector:  Many cordless telephones, computer networks and cell phones use a RF transmitter lefted on the 2.45GHz band.  Using a microwave diode detector and 100uA meter, this passive circuit will display the strength this signal.  The drawing shows how a bent piece of wire can be used as the antenna. Posted

4-20ma Current Loop Tester:  Using just a few components, this circuit can simulate signals from various instruments, which source a constant current.  Posted
AC Current Indicator Light:  This battery powered circuit is designed to flash a LED light, whenever it detects AC current flowing through a power cord.  It uses an inexpensive ferrite inductor to detect the small magnetic field produced by the power cable.  Posted

Analog Circuit Converts Wind Speed into Wind Watts per Square Meter Readout:  This circuit is very handy when conducting wind power surveys.  Wind speed information from a four cup anemometer is feed to a cube function circuit. The output of that circuit can be scaled for readouts in either watts per square foot or watts per square meter.

Automotive 12v Electrical System Voltage Indicator This battery powered unit is designed to quickly aid troubleshooting of 12v electrical systems in a car.  Three LEDs proved a visual indication of a +12v, ground or an open circuit voltage level. Posted

Baseball Pitcher Training Device:  This square shaped device has an opening equal to the typical “strike zone.”   An electronic circuit measures the speed of the balls which pass through the opening.

Battery Amp-hour Monitor:  A new digital pot IC in conjunction with a voltage to frequency converter is used to measure and display the Amp-hours stored in a battery.  The voltage output of this circuit can be used by a commercial digital or analog meter to display between 0 to 100% battery capacity.  Charging current into the battery increases the capacity while current from the battery decreases the displayed capacity.

Battery Isolator:  This circuit uses a few p-channel power FETs and a new IC from Linear Technology.  It allows an alternator to charge two different lead-acid batteries but isolates the individual battery loads, so that loads on one battery do not draw power from the other battery.  This configuration is often used on recreational vehicles.  It is rated at 28v and up to 50 Amps. Posted

Blimp Supported Wind Speed Monitor:  This illustrates how a blimp holding a wireless four cup anemometer high in the air can be used to conduct wind power surveys.

Candle Flicker LED Light:  A simple digital noise generator is used to control the light emitted by a yellow LED.  The result is a light which simulates the random flicker from a candle.

Candle Power:  A single lit candle in conjunction with an inexpensive thermopile voltage generator provides enough power to charge a small battery when used with a small low voltage DC to DC converter. Posted

Capacitance Sensor Measures Fluid Level:  This circuit has been very popular.  It can be used to monitor the level of a water based fluid inside a tank using a simple sensor made from PVC pipe or other fluids using two brass tubes.  Several capacitance sensor designs are illustrated.

Crank Type Flashlight Battery Charger:  This project shows how a common crank type flashlight can be modified so it can charge two AA NiHM batteries.  The charger is capable of producing about 2 watts of power.

DC Current Probe:  This project shows you step by step how to fashion a ferrite toroid core and a Hall sensor into a DC current probe.  DC current flowing through a single wire can be measured by this probe circuit.  The technique provides complete isolation between the wire and the rest of the circuit.  It is perfect for monitoring current flow in and out of a battery.

Digital Delay Fixes Clipped Syllable in Radio Transmitter VOX Circuit:  Radio transmitters will often use a voice operated relay (VOX) which automatically switches on the transmitter when a person speaks into a microphone. However, VOX systems will often clip the first syllable spoken.  This circuit inserts a digital delay into the system, so the transmitter is turned on before the actual audio arrives.

Electrodeless Water Level Indicator:  An inductive method can be used on the outside of a plastic tank or on the inside of a steel water tank, to indicate when the fluid level reaches a specific height. Several sensor designs are illustrated.

Gas Flowing in Pipe Indicator:  This circuit uses the same technique as the flowing water indicator. An inexpensive piezoelectric wafer is used to monitor the acoustical noise associated with gas flowing through a metal pipe.  A transistor switch is activated, whenever the circuit detects flowing gas.  This ultra low power circuit can be powered by a lithium 3v battery for many years

Gauss Meter:  This circuit uses an inexpensive Hall sensor to measure the strength of permanent magnets and magnetic fields from low frequency coils.

Hidden Behind Wall Capacitance Switch:  A super low power capacitance proximity switch is located behind a standard sheetrock wall.  It is connected to an aluminum foil plate about 6 inches by 6 inches.  The switch is triggered when a human hand is placed over the metal plate, on the outside wall.  The switch can be used to reset or even activate a security alarm system.

High Current Low Voltage 3 Phase Rectifier:  By using some power FETs, this circuit can rectify three phase AC power into DC, with better efficiency than diodes.

Home Made AC Current Monitor:  This circuit uses an off-the-shelf split magnetic core transformer, which can easily be slipped over an existing wire carrying AC current.  The AC current flowing through the wire is converted to a small AC voltage in the transformer.  The voltage is converted to a DC voltage proportional to the current.

Incredibly Low Current Oscillator :  If you want to play with some really low current oscillators here is a design for you.  This oscillator draws only 3 nanoamps, yes 0.003 microamps, from a 3v source.  Maybe you can change some components to get the current down to less than a nanoamp.  Details are given for ways to measure such low currents.

Jeopardy Game Control Boxes:  This wireless system uses multiple identical control boxes to play educational games in the Jeopardy TV show format. Contestants are each handed a box.  The box has a single large pushbutton switch.  When contestants think they know the answer to a question, they press the button.  Within a fraction of a second a powerful xenon flash is produced by the box.  The box also emits a beeping sound and flashes a bright LED.  This action directs the moderator toward the person who thinks they have the answer. The xenon light flash from the box reflects off the classroom walls and ceiling.  A light detector in each of the boxes detects the light flash and temporarily disables the box pushbutton switches for a few seconds.  This insures that only the first contestant to press a button will win the race.  The boxes are powered by a 9v battery and draw no power during standby operation.  There is no on/off switch.

Laptop Theft Alarm:  This alarm system is in the shape of a flat flexible pad, placed under the laptop.  Whenever, the pad and the laptop are separated, a loud alarm is activated.  The alarm will continue to sound, until a hidden switch is activated.

Laser Pointer Used to Detect Speeding Cars:  This describes how a laser pointer can be used with a simple light detector circuit to measure the speed of a passing car.

Line Powered Cold Cathode Fluorescent Lamp:  Using some inexpensive parts and a piezoelectric transformer, this circuit can power up to 5 watt lamps.  A lower power version makes a dandy night light. 

Modulated Cold Cathode Fluorescent Lamp Sends Data:  With a peak power of about 7 watts, this circuit can launch modulated light in all directions. 

Motion Shadow Alarm:  This simple circuit uses two inexpensive PIN photo diode light detectors to detect nearby motion.  Any shadow which is cast over the two diodes from a moving object produces a slight voltage change between the two diodes.  When this change is detected, an alarm switch is activated. Posted

Office Cubical Entry Beeper:  This simple battery powered device alerts you when someone has entered your office cubical.

Optical Fiber Pressure Sensor:  This circuit uses a section of plastic optical fiber to monitor motion or any pressure applied to the fiber.

Plastic Sprinkler Pipe Tracer:  This describes a method to determine the location of some plastic sprinkler pipe buried under ground. The circuit uses a low frequency RF current injection technique and a matching short range receiver circuit.

Raindrop and Snowflake Detector:  An unusual ring shaped optical sensor detects raindrops and snowflakes, which pass through the ring.

RFID Smart Key Detector:  Many cars now use a key with an imbedded RFID microchip.  The microchip is read when the key is inserted into the car’s ignition.  The keys containing a microchip often look the same as most other keys. People duplicating car keys will often not know of the microchip.  This circuit will light a LED if it detects a key containing a 125KHz RFID chip. Posted

RFID Tag Zapper:  A powerful magnetic pulser zaps 13.56MHz RFID tags, deactivating them.

Security Loop Alarm:  A wire is looped through various objects which are to be monitored.  The loop wire is typically an aircraft cable but could also be any insulated stranded wire.  The loop is attached to a table or desk.  Should the loop be cut, the alarm is sounded.  Posted

Service Beeper:  This device functions like a lobby desk service request bell.  A short beep is sounded whenever the large pushbutton on top of the device is pressed.  Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed.  This prevents impatient customers from ringing the bell over and over. Posted

Single Solar Cell Battery Charger:  This circuit takes the 0.5 volts from a single solar cell and boosts the voltage so it can charge a battery consisting of four AA NiMH cells.  Multiple broken solar cells of various sizes can be wired in parallel to increase the available power.  It is suggested that the exposed solar cell area not be less than 4 square inches for this battery charger to be practical.  The maximum area can be in excess of 36 square inches or about 2.5 watts.

Solar Insulation Monitor:  An inexpensive PIN photo diode and a 1ma meter movement form a simple sunlight intensity monitor.  The sensor should be placed inside a lightly frosted glass hemisphere to minimize the influence of the sun’s angle on the sensor.  An adjustment resistor calibrates the meter for a full scale reading under a one sun condition.  An unshielded two conductor cable links the sensor to the meter movement.  The sensor and meter can be separated by up to 200 feet. Posted

Super Large Electrets:  An electret is to capacitor as a permanent magnet is to an electromagnet.  Small versions are used in most microphones.  Like a permanent magnet retains a magnetic field, an electret maintains an electric field.  This project describes how to make gigantic electrets.  A mixture of waxes are mixed then melted and poured into cupcake size molds. A metal plate is placed on the top and bottom.  A 10KV power supply is connected to the metal plates, while the mixture is baked in an oven.  The mixture of waxes is allowed to slowly cool.  After the power supply is removed, a high voltage charge remains. 

Tiny Wind Generator:  Using off-the-shelf components, this generator can produce up to 50 watts of power. 

UFO TV Camera:  An electronic switch, suitable for activating a TV camera, is triggered at night by the detection of flashing lights.  The video might capture unusual flying aircraft in the middle of the night.

Ultrasonic Microphone Frequency Shifter:  This circuit uses a heterodyne technique to shift the frequency of signals picked up by an ultrasonic microphone, so humans can hear the sounds.  This is useful in detecting gas leaks, bats and some insects.

Universal 50mv DC Current Shunt Amplifier:  Using some new “high side” current sensing Amps, this circuit will operate from 6v to 200v DC and produce a DC voltage proportional to the DC current.

Universal AC Line Power Controller:  This fully isolated circuit can be used as a light dimmer or an AC motor controller.  A 0v to 5v control signal defines 0% to 100% AC power to any load.

Universal Flasher:  This circuit is powered by a 9v battery and controls any AC to DC load through a set of relay contacts, rated at 10 Amps.  To conserve power, the circuit uses a latching relay.  A variable frequency oscillator controls the flashing speed from 0.2Hz to 2Hz.  A pair of pulse generators first latch the contacts closed then unlatches them open.  The power consumption is so low that a 9v battery will last for several months. Posted

Very Long Range Optical Security Fence Alarm:  Using some inexpensive parts and a visible red laser pointer, this circuit can detect any object which breaks a light beam several miles long.

Water Flowing in Pipe Indicator:  An inexpensive piezoelectric wafer is used to monitor the acoustical noise associated with water flowing through a metal or plastic pipe.  A transistor switch is activated, whenever the circuit detects flowing water.  This ultra low power circuit can be powered by a 3v lithium battery for many years.

Watt-Hour Meter Monitor:  This is a simple and safe way to monitor the power and energy use of a typical home.  A battery powered device is attached to the top of a typical spinning wheel type watt-hour meter.  The device contains a visible red LED and a PIN photo diode.  Light pulses are bounced off of the watt-hour meter’s spinning wheel.  The circuit detects the black mark on the wheel and produces a clean TTL level logic signal, equal to the frequency of the wheel spin.  This information can be connected to a low power RF transmitter to send to a matching receiver, where the information can be recorded and displayed. Posted

Xenon Flash Lamp Transmitter:  This circuit can send encoded optical data during the brief flash of light from a xenon lamp. The peak light is powerful enough to provide a range of a mile from a matching light receiver.  The circuit can be used to send simple commands.:   :  Xenon Flash Lamp Receiver:  This is the receiver circuit for the xenon flash lamp transmitter above.  It is immune to direct sunlight yet has high sensitivity to the signals from the transmitter.

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