Hobby Circuits designed by David Johnson
Circuit Name and Description

#'s          A         B - C          D - F        G - K          L          M - O          P - R         S - Z

A Current Transformer used to Harvest Energy   -  Energy harvesting is all the rage these days.  With some modern electronics, information from low power sensors can be sent to a distant data collection point using a low power RF transmitter.  To power these remote sensors, various energy sources can be . . . Hobby Circuit designed by Dave Johnson P.E.-January 17, 2010

AC Current Indicator Light   -  This circuit could be wired into a 120vac power line, which feeds power to any load, ranging from 40 watts to 250 watts.  It will turn on a LED light whenever current is being drawn by a load.  It is especially useful for remote lights, where you may not . . . Hobby Circuit designed by Dave Johnson P.E.-July 11, 2006

AC Current Monitor   -  This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding.  The transformer contains a single turn primary with a low 0.  001-ohm resistance.  It can easily handle 30 . . . Hobby Circuit designed by David A. Johnson P.E.-July 10, 2006

AC Indicator-Self Powered   -  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.-December 30, 2007

AC Isolated Current Monitor   -  The hobby circuit below shows how a small current transformer from Magnetek can be used to monitor AC current to a load.  The AC voltage developed at the secondary of the transformer can be scaled so it can be read by any digital multimeter.  The transformer . . . Hobby Circuit designed by Dave Johnson P.E.-July 10, 2006

AC Line Powered LED Flasher   -  Some of the latest LEDs are amazing.  They can produce a surprising amount of light with very little current.  In the light flasher application shown below I take advantage of a nice surface mounted LED from King Bright.  This flasher circuit draws only about 3ua of average current from the AC power line to flash this green LED.  This flasher might be used as a power indicator light . . . Hobby Circuit designed by Dave Johnson P.E.-December 07, 2008

AC Line Powered LED Pilot Light   -  This simple circuit can be used to light a LED indicator lamp, powered from the 120vac line.  I have included component values for 240vac lines as well . . . Hobby Circuit designed by Dave Johnson P.E.-July 09, 2006

AC Line Under/Over Voltage Alarm   -  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 . . . Hobby Circuit designed by Dave Johnson P.E.-March 29, 2009

AC Peak Detector that is Precise   -  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 . . . Hobby Circuit designed by David Johnson P.E.-February 11, 2002

AC to DC (240v to 5v)Power Supply   -  This is simple way to power some 5v logic from a 240vac source.  If a 120vac power adapter is used, the circuit will also work for 120vac power lines . . . Hobby Circuit designed by David A. Johnson P.E.-February 12, 2002

AC to DC 18V Power Supply   -  This is a classic linear power supply which produces a regulated 18v, rated at about 1 amp . . . Hobby Circuit designed by David A. Johnson P.E.-July 09, 2006

AC to DC Power Supply requires Low Power   -  Last month I showed how to use a small common mode choke as a fully isolated transformer and a 130v sidac to generate about 100mw of DC power from the AC line.  This second circuit still uses the same inexpensive tiny off-the-shelf 5mH common mode filter coil with a split bobbin as a high frequency transformer but uses two SCRs instead of a sidac to discharge a capacitor . . . Hobby Circuit designed by David Johnson P.E.-November 23, 2011

AC to DC Power Supply, Non-Isolated Off-Line    -  This compact efficient circuit can provide up to 100ma of a regulated 5 volts from an AC power source ranging from 20vac to 120vac . . . Hobby Circuit designed by David Johnson P.E.-July 13, 2006

AC/DC relay from Charge Coupled MOSFET   -  This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current.  The differential oscillator supplies voltage to the gates of the two FETs with good isolation will drawing only 1.5ma of current . . . Hobby Circuit designed by Dave Johnson P.E.-July 11, 2006

Accurate Low Capacitance Meter   -  The circuit measures the unknown capacitance by applying a clean 20KHz 5v peak to peak triangle signal to the capacitor.  The current routed through the capacitance is sent to a current to voltage converter.  The voltage that emerges is therefore a 20KHz square wave signal, whose amplitude is proportional to the capacitance.  The signal’s peak amplitude is rectified and filtered to produce a DC voltage.   . . . Hobby Circuit designed by Dave Johnson P.E.-December 12, 2004

Active High Pass Filters   -  This is a collection of inverting and non-inverting active high pass filter circuits.  I included one, two three, four and six pole filter circuits.  You can change the component value ratios shown to achieve any frequency cut-off you may need . . . Hobby Circuit designed by David A. Johnson P.E.-February 11, 2002

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor   -  This schematic is the alarm discriminator plus the battery monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed by Dave Johnson P.E.-March 26, 2002

Alarm Sounds At Freezing Temperatures   -  This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F).  A 9v battery powers the hobby circuit.  The average 9ua current is so low that the battery should last for many years . . . Hobby Circuit designed by David Johnson P.E.-July 11, 2006

Alarm Sounds when Phone Line is Cut   -  Burglars, rapists and other criminals will often cut a phone line before they break into a home.  The cut phone line will prevent many alarm systems from alerting the police.  The battery powered circuit below will sound an alarm when the unit . . . Hobby Circuit designed by David Johnson P.E.-October 16, 2005

Alarm sounds when Power Cord is Unplugged   -  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 . . . Hobby Circuit designed by David A. Johnson P.E.-July 08, 2000

Ambient Light Dectector with Compensation   -  These circuits were taken from a few application notes on infrared remote control devices.  They use a current compensation method to separate the modulated light pulses from ambient light.  They appear to have limited bandwidth and may only work at the 30K . . . Hobby Circuit designed by David A. Johnson P.E.-February 15, 2002

Appliance Unplugged Alarm   -  I received a call from an administrator at a large prison.  He told me that he was having problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens in a commons area.  He wanted a battery-powered device, which could be mounted to the outside of the power cord of an appliance.   . . . Hobby Circuit designed by Dave Johnson P.E.-April 25, 2010

Astable Multivibrator with Very Low Power   -  This classic circuit draws only 200 nanoamps from a 1.5v supply . . . Hobby Circuit designed by David Johnson P.E.-June 10, 2000

Astable Ultra Low Power Multivibrator   -  Taking advantage of some new voltage comparators, this circuit can produce a nice square wave signal while drawing only 1.  6 micro amps.  With the inclusion of a diode, the circuit can also produce short pulses instead of a square wave signal . . . Hobby Circuit designed by David A. Johnson P.E.-December 18, 2004

Automatic DC Motor Brake   -  in many DC motor powered systems you would like the motor to quickly come to a full stop.   This circuit provides an automatic electronic braking action to any DC motor ranging from 6v to 24v up to 1 amp of current.  Some of the component values could be changed . . . Hobby Circuit designed by David Johnson P.E.-July 13, 2006

Automatic Timeout included with this On/Off Flip/flop Circuit   -  This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs to turn itself off after some period of time.  With the components shown, the output will stay on for only about 20 seconds . . . Hobby Circuit designed by David A. Johnson P.E.-December 16, 2004

Auxiliary 12v Automotive Style LED Light Makes Table Lamp   -  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 . . . Hobby Circuit designed by Dave Johnson P.E.-July 09, 2006