One Shot (D-Flip/Flop) Circuit  -  Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators.  This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds . . . [Circuit by David A. Johnson P.E.,  06/06/00]

Oneshot And Latch Pushbutton  -  This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v . . . [Hobby Circuit designed by David Johnson P.E.,  03/26/02]

Optical RFID Test Circuit  -  I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section . . . [Circuit by Dave Johnson P.E.,  02/11/02]

Over-Temperature Micro-Power Alarm  -  The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/04/00]

Piezoelectric Device used for Motion Alarm  -  An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Piezoelectric Vibration Sensor Activates Switch  -  An inexpensive piezo device is used as a motion sensing device for this motion alarm . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Piezoelectric Water Flowing Indicator  -  The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezo wafer.  The signal from the wafer is first boosted by a micropower transistor amplifier and then fed to an ultra low power voltage comparator . . . [Hobby Circuit designed by David Johnson P.E.,  07/26/09]

Plastic Pipe Finder  -  I used this circuit once to track down the location of a plastic pipe, buried underground.  It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away . . . [Circuit by David Johnson P.E.,  12/14/04]

Pot Controlled Variable LED Intensity Circuit  -  The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.  A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%.  The beauty of the circuit is that when the . . . [Circuit by David Johnson P.E.,  09/27/09]

Power Line Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac. . . . [Circuit by David A. Johnson P.E.,  07/13/06]

Power Loss Beeper  -  For some medical equipment it is important for an operator to know when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts . . . [Circuit by Dave Johnson P.E.,  03/26/99]

Power Reminder Beeper  -  This circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box . . . [Circuit by Dave Johnson P.E.,  06/03/00]

Precision AC Peak Detector  -  This unique circuit uses a very inexpensive voltage comparator to form a peak detector. The DC voltage produced tracks the positive peak of the input signal.  It works from about ten millivolts to about 10 volts peak to peak.  The maximum frequency is about . . . [Circuit by David A. Johnson P.E.,  02/11/02]

Precision Fullwave Rectifier  -  I have used this handy circuit many times.  It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage.  It works from millivolts to volts.  The circuit shown requires a stable +5v reference if a . . . [Circuit by Dave Johnson P.E.,  02/12/02]

Precision Low Capacitance Meter  -  This circuit was originally designed to measure the volume of fluid inside a medical syringe.  As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . [Hobby Circuit designed by David Johnson P.E.,  12/12/04]

Produces Beep once each Minute  -  This circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/03/00]

Pulsed LED Test Circuit  -  This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test . . . [Circuit by Dave Johnson P.E.,  06/17/00]

Pump Motor Monitor  -  This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on.  It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable.  No direct wire contact is needed . . . [Circuit by David Johnson P.E.,  11/18/04]

Pump Motor Monitor  -  This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on.  It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable.  No direct wire contact is needed . . . [Circuit by David A. Johnson P.E.,  11/18/04]

Pushbutton Oneshot & Latch  -  This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v . . . [Circuit by David A. Johnson P.E.,  03/26/02]

PV Solar Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/01/06]

Radio Tower Lamp Monitor  -  The circuit below uses two 3 amp diodes to produce a fairly constant 2 volts whenever current is routed to the tower lamp.  The voltage is used to power a bright LED.  The LED will only turn on when current is actually flowing to the lamp. With the component values shown, the circuit will work with lamps ranging from 40 to 250 watts. . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/18/06]

Really Loud Output Beeper Circuit  -  I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezolectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries . . . [Hobby Circuit designed by David A. Johnson P.E.,  06/03/00]

Rectifier-Fullwave  -  I have used this handy circuit many times.  It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage.  It works from millivolts to volts.  The circuit shown requires a stable +5v reference if a . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/12/02]

Resistance Tester got UL Ground Requirement  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms. This resistance must be tested with actual line current in excess or 10 amps.  The circuit shown uses a couple of LEDs to indicate . . . [Circuit by David Johnson P.E.,  04/28/01]

RFID Test Circuit  -  I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section . . . [Hobby Circuit designed by David A. Johnson P.E.,  02/11/02]

RS232 Data Bus Status Monitor  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volt . . . [Circuit by David Johnson P.E.,  12/09/07]

RS-232 Serial Interface Status Indicator Circuit  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volts.  Two dual color LEDs are used as indicator lights . . . [Hobby Circuit designed by David A. Johnson P.E.,  12/09/07]

Scare Off Pigeons  -  I have an area on my roof where a large flock of pigeons like to roost.  Their nesting materials and droppings really make a mess. Often there is enough junk from them that it clogs my roof gutters.  Perhaps one way to scare them away . . . [Hobby Circuit designed by David Johnson P.E.,  04/01/05]

Self Powered AC Indicator  -  Often there is a need to monitor when AC current flows into a load.  The circuit below works well for this application.  It uses an off-the-shelf torrid type AC-1020 current transformer from Triad, available from Mouser.  A wire carrying the AC current is . . . [Hobby Circuit designed by David Johnson P.E.,  12/30/07]

Serial Data Bus Status Monitor  -  I designed this circuit many years ago and have used it several times since.  It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link.  The thresholds are set at plus 3 volts and minus 3 volt . . . [Circuit by David A. Johnson P.E.,  12/09/07]

Shadow activated (or hand wave) motion detector  -  The circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both devices produce nearly identical current levels.… [Circuit Solution designed by David A. Johnson P.E., 5/1/06] . . . [Circuit Solution by David A. Johnson P.E.,  08/21/05]

Shadow Activated Motion Detector  -  The hobby circuit below works quite well in typical indoor room lighting.  I would not recommend its use in direct sunlight.  Two small PIN photodiodes positioned about one inch apart form a shadow detector.  With no shadow cast on the devices, both device . . . [Hobby Circuit designed by David Johnson P.E.,  08/21/05]

Slotted Opto-Sensor used in Honeybee Counter  -  I designed a circuit similar to this one a long time ago to help a beekeeper count the number of bees going into or out of a hive. The low power circuit uses a slotted opto-sensor to detect the passing bees. The circuit advances an electronic counting . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/08/00]

Small Battery Capacity Tester  -  I have a client who uses a lot of batteries.  His sells a device which is powered by a 3v lithium coin cell and he sells thousands of them. It seems that many of the batteries he has been importing from China have a milliamp-hour capacity which is much . . . [Hobby Circuit designed by Dave Johnson P.E.,  09/07/08]

Small Battery Capacity Tester  -  I have a client who uses a lot of batteries.  His sells a device which is powered by a 3v lithium coin cell and he sells thousands of them. It seems that many of the batteries he has been importing from China have a milliamp-hour capacity which is much . . . [Hobby Circuit designed by David A. Johnson P.E.,  09/07/08]

Snow Depth Alarm  -  The design of this system alerts a snowplow driver when the snow depth has exceeded four inches. A visible red LED packaged in a 10mm package is pulsed at a rate of about 50Hz with 10uS pulses.  Twenty four inches away a phototransistor detects the light from the LED.  The LED and the phototransistor are housed in a section of ˝ inch PVC pipe, forming a waterproof seal.  . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/18/06]

Solar Cell Test Fixture  -  If you are considering building your own solar panel, you might try building the solar cell tester shown below. When constructing a solar panel it is important to make sure that all the solar cells used in the panel be closely matched for their short circuit . . . [Hobby Circuit designed by Dave Johnson P.E.,  11/25/07]

Solar Insulation Monitor  -  Owners of photovoltaic solar panels often like to monitor how bright the sunshine is. They can use this information to gage how well their solar panels are doing. On a typical clear day, the sun energy at sea level is about 1000 watts per square meter . . . [Hobby Circuit designed by David A. Johnson P.E.,  05/01/12]

Solar Path Light Improvement  -  Use the circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area. Buy a panel with solar panels that . . . [Circuit by David A. Johnson P.E.,  03/13/11]

Solar PV Cell Test Fixture  -  This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions . . . [Circuit by David A. Johnson P.E.,  10/01/06]

Solar-Powered Pump Motor Controller  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . [Circuit by Dave Johnson P.E.,  10/03/10]

Switch activated by Piezo Vibration  -  An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Switch detects Audio Signal  -  This circuit will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.  It could be used to turn on a relay, routing the signal to were it is needed. . . . [Circuit by Dave Johnson P.E.,  07/09/06]

Temperature Sensor MOSFET  -  Recently I was asked by a company to discover why one of their products failed to operate properly in cold temperatures.  Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in that state until another activation of the switch.  . . . [Circuit by David A. Johnson P.E.,  12/01/10]

Temperature Sensor With 4 to 20ma Current Loop  -  I designed a circuit similar to this one years ago to accurately measure the air temperature inside a building 1000s of feet from a control room.  The circuit uses a very robust current loop method.  It uses a highly accurate semiconductor temperature sensor . . . [Circuit by Dave Johnson P.E.,  07/04/00]

Temperature Sensor With 4 to 20ma Current Loop  -  I designed a circuit similar to this one years ago to accurately measure the air temperature inside a building 1000s of feet from a control room.  The circuit uses a very robust current loop method.  It uses a highly accurate semiconductor temperature sensor . . . [Circuit by David A. Johnson P.E.,  07/04/00]

Test UL Ground Resistance  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms. This resistance must be tested with actual line current in excess or 10 amps.  The circuit shown uses a couple of LEDs to indicate . . . [Circuit by David Johnson P.E.,  04/28/01]

Tester for Brown Out Voltage  -  I used this circuit years ago to test AC line powered devices under 95vac line voltage conditions.  It has a rating of 250 watts.  . . . [Circuit by Dave Johnson P.E.,  07/12/06]

Tester for Modulated Laser Diode  -  This circuit can either be used to test laser diodes or as a general purpose modulated light source. The circuit has an adjustable diode threshold current and an adjustable modulation frequency. The frequency can be adjusted from below 30KHz to above 50MH . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/20/99]

Tester for Modulated Light  -  This battery powered circuit will work from DC to 500KHz.  It is designed to connect to an oscilloscope to monitor the light level changes and modulated signals . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]

Tester for Pulsed LEDs  -  This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/17/00]

Tests UL Ground Resistance  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms.  This resistance must be tested with actual line current in excess or 10 amps.  The circuit shown uses a couple of LEDs to indicate if in fact the resistance is less than 0.1 ohms - modified on july 10, 2006 . . . [Hobby Circuit designed by David Johnson P.E.,  04/28/01]

Thermostat for 12v Cooling Fan  -  This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level.  Both the high and low temperatures are both adjustable . . . [Hobby Circuit designed by David Johnson P.E.,  02/06/11]

Time to Dust Indicator  -  I thought about this circuit when I heard that a lot cleaning personal in hotels were either dusting rooms more often than necessary or not enough. I have not yet built and tested this circuit completely but in concept it should work . . . [Circuit by David Johnson P.E.,  06/06/00]

Time to Dust  -  I have used the circuit below for many different applications, which require ultra low power drain.  It uses a single CA4013 flip/flop.  One section forms a pulse generator.  The generator produces 100uS pulses at a rate of about 1 per second . . . [Hobby Circuit designed by Dave Johnson P.E.,  05/07/06]

Timer for Darkroom Camera Shutter  -  This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply . . . [Hobby Circuit designed by David Johnson P.E.,  06/11/00]

Touch Activated Switch  -  This circuit provides about one watt of non-isolated DC power for an automotive type 12v LED array lamp in addition to a standard incandescent table lamp. . . . [Circuit by David A. Johnson P.E.,  01/18/02]

Transistor-Switch detects Audio Signal  -  For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line.  The hobby circuit below will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal.   The switch could be used to turn on a relay, routing the signal to where it is needed.  A single 3v lithium cell will power the electronic circuit for about 5 years . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Turn D-Flip/Flop into a One Shot Circuit  -  Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators. This schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging from nanoseconds to seconds . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/06/00]

UL Ground Resistance Tester  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms.  This resistance must be tested with actual line current in excess or 10 amps.  The circuit shown uses a couple of LEDs to indicate if in fact the resistance is less than 0.1 ohms - modified on July 10, 2006 . . . [Circuit by Dave Johnson P.E.,  04/28/01]

Ultra Low Power detects Clear Fluid  -  Using an ultra low power technique, this circuit detects when a clear fluid is inside a piece of clear plastic tubing. It can be used for fluid level control and monitoring . . . [Hobby Circuit designed by David Johnson P.E.,  10/22/05]

Ultra Low Power Human Finger Detector  -  The circuit below seems to work well and averages a current drain of only 10 microamps from a lithium cell.  It uses a single CA4013 dual D flip/flop.  One section is wired as a pulse generator.  It applies 10us 10ma pulses to a LED inside a slotted optical sensor at a rate of about 50Hz.  The phototransistor in the second half of the sensor detects the pulses and sends a pulse to the data input of the second flip/flop in the 4013 package.  If there is nothing between the LED and the phototransistor, the inverting output of the 4013 remains low. . . . [Circuit Solution by Dave Johnson P.E.,  05/02/06]

Ultra Low Power Human Finger Detector  -  The circuit below seems to work well and averages a current drain of only 10 microamps from a lithium cell.  It uses a single CA4013 dual D flip/flop.  One section is wired as a pulse generator.  It applies 10us 10ma pulses to a LED inside a slotted optical sensor at a rate of about 50Hz.  The phototransistor in the second half of the sensor detects the pulses and sends a pulse to the data input of the second flip/flop in the 4013 package.  If there is nothing between the LED and the phototransistor, the inverting output of the 4013 remains low. . . . [Circuit Solution by David Johnson P.E.,  05/03/06]

Under/Over Voltage Alarm on AC Line  -  Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about +-15% from standard levels . . . [Circuit by Dave Johnson P.E.,  03/29/09]

Unplugged Power Cord Alarm  -  Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries. However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries. So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time . . . [Circuit by David Johnson P.E.,  07/08/00]

Voltage Boost to Series Regulator provided by Capacitor  -  This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-the-wall power adapter power supply . . . [Hobby Circuit designed by Dave Johnson P.E.,  01/01/98]

Voltage Monitor for a 9v Battery  -  Nine voltage batteries are used in a wide variety of electronic devices.  But, you often don’t know when it is time to change the battery until the device stops operating.  The hobby circuit below provides a visual indication when the battery needs to be . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Voltage Monitor for a 9v Battery  -  Nine voltage batteries are used in a wide variety of electronic devices.  But, you often don’t know when it is time to change the battery until the device stops operating.  The hobby circuit below provides a visual indication when the battery needs to be . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]

Watchdog Timer set for 36 Hrs  -  A watchdog timer is often used in computer systems to detect when a computer is hung up and is no longer running properly. If it does become locked up and is not able to supply a reset pulse to the circuit within a defined time frame the timer times out a . . . [Hobby Circuit designed by David A. Johnson P.E.,  04/11/10]

Water Filling Station Controller  -  This circuit was requested by a company needing a simple way to fill a large tank with water with little fuss.  They wanted a low tech method to automatically fill the tank with water without constant monitoring.  They wanted to press a single pushbutton switch and then walk away . . . [Hobby Circuit designed by Dave Johnson P.E.,  04/24/11]

Water Flowing in Pipe Indicator  -  The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezoelectric wafer.  The signal from the wafer is first boosted by a micropower transistor amplifier and then fed to an ultra low power voltage comparator.   When the vibration signal has sufficient amplitude, a FET transistor switch is activated…[Circuit designed by Dave Johnson P.E., 6/06/2009] . . . [Circuit by David Johnson P.E.,  07/26/09]

Water Level Monitor  -  This system operates much like a classic float switch but without any moving parts.  The illustration shows how a system using two copper coils sealed inside a plastic pipe, can detect the level of water outside the pipe.  Whenever the water level is lower . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

Water Pump controlled by Solar Power  -  A Discover Circuits visitor had a problem.  He needed a simple on/off controller for his solar powered water pump.  His system used two 12v 50 watt solar panels wired in series.  The power from the solar panels drove a submerged water pump.  However, during . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/03/10]

Water Seepage Alarm  -  This simple circuit sounds a beeper when its electrodes detect water. It is powered by a single 1.5v N cell. A small 1.5v button battery will also work . . . [Hobby Circuit designed by David Johnson P.E.,  06/11/00]

Water Seepage Monitor  -  A while back I received a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected . . . [Circuit by David Johnson P.E.,  01/16/11]

Water Seepage Monitor  -  A while back I received a request from a company who needed a water seepage detector.  They wanted a device that ran on 24vac and detected water using a fully isolated circuit.  They wanted a circuit which switched off 24vac to a load when water was detected . . . [Circuit by David A. Johnson P.E.,  01/16/11]

Watt-Hour Meter Monitor  -  Have you ever wondered how much power your house or apartment is using?  Most watt-hour meters are like the one shown below.  Some newer ones have a digital display but there millions of these older models still in use . . . [Hobby Circuit designed by David Johnson P.E.,  09/28/08]

White LED is Powered By One Nimh Cell  -  There are a lot of white LED driver circuits floating around.  A popular circuit called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell.  Most of these circuits use one or two transistors to form a voltage boost circuit . . . [Circuit by David Johnson P.E.,  09/26/09]

White LED Night Light  -  This simple circuit is designed to plug into a standard AC electrical outlet.  It uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit. The LEDs are mounted in a box and are angled slightly to bounce the light off of a nearby wall. . . . [Hobby Circuit designed by David A. Johnson P.E.,  05/27/03]

X100 Scope Probe  -  Often a typical X10 oscilloscope probe will produce large measurement errors and will influence the circuit operation.  To reduce the effect such a probe has on the circuit, I reach for my X100 scope probe, with its 100M impedance.  This probe is a commercially made device but if you want to make one, it is not hard to do. . . . [Circuit by David A. Johnson P.E.,  05/01/10]