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Circuits designed by David Johnson, P.E.
Last Updated on: Monday, July 11, 2016 01:59 PM

Master Category List - Dave's Circuits

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Safety / Protection Circuits Circuits

120 VAC Low Voltage Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac. . . . [Circuit by David A. Johnson P.E.,  07/11/06]


12v Battery Load Cutout Circuit  -  Lead-acid batteries should not be discharged below a certain point if they are to last.  Also, you don’t want certain 12v powered devices, which might be plugged into a car’s cigarette lighter outlet, to drain the car battery down to a point where the car . . . [Circuit by David A. Johnson P.E.,  08/22/09]


18v AC to DC Power Supply  -  This is a classic linear power supply which produces a regulated 18v, rated at about 1 Amp . . . [Circuit by David A. Johnson P.E.,  07/09/06]

 

1W White LED Night Light  -  As many of you know, I have a pet peeve with poorly made LED night lights.  Often, the light from the LED quickly fades, so within months, the light is useless.  I have posted several versions of modified night lights using higher quality components.  This circuit is yet another version, which produces much more light than those other designs. . . . [Hobby Circuit designed by Dave Johnson P.E.,  11/06/08]


AC Power Loss Alarm  -  I’ve been having a problem with my instant hot water system lately.  This system circulates electrically heated water throughout my house using a small pump.  The pump is wired into a ground fault interrupter (GFI) module.  Every now and then, the GFI trips, leaving me without hot water . . . [Circuit by Dave Johnson P.E.,  03/29/09]


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 Johnson P.E.,  07/09/06]


Alarm Beeps when the “Check Engine” Light Turns On  -  When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on . . . [Circuit by David A. Johnson P.E.,  07/09/06]


Battery (12v) Load Cutout Circuit  -  Lead-acid batteries should not be discharged below a certain point if they are to last.  Also, you don’t want certain 12v powered devices, which might be plugged into a car’s cigarette lighter outlet, to drain the car battery down to a point where the car . . . [Hobby Circuit designed by Dave Johnson P.E.,  08/22/09]


Beeper indicats Check Engine  -  When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel.  You could drive for many miles before noticing the light.  To provide an audible indication of trouble, the hobby . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]


Check Engine Beeper  -  When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel.  You could drive for many miles before noticing the light.  To provide an audible indication of trouble, the hobby . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]


Circuit Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . [Circuit by Dave Johnson P.E.,  03/09/02]


Detects Low Voltage 120Vac Power Line   -  This hobby circuit will turn on a beeper when the line voltage drops below 100 volt AC . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]


Infrared Safety Switch  -  Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . [Circuit by Dave Johnson P.E.,  07/11/06]


Low Voltage 120vac Power Line Alarm  -  This circuit will turn on a beeper when the line voltage drops below 100 volt AC. . . . [Circuit by Dave Johnson P.E.,  07/09/06]


Modified LED Night Light  -  I bought some night lights a few months ago from Walmart here in Denver.  The devices were made by Elumina Lighting Technologies Inc. (www.eluminalighting.com) , their model LED-6200A.  I think I paid about $15 for a pack of three lights.  I wanted a light . . . [Hobby Circuit designed by David A. Johnson P.E.,  05/21/07]


Night Light from White LED  -  As many of you know, I have a pet peeve with poorly made LED night lights.  Often, the light from the LED quickly fades, so within months, the light is useless.  I have posted several versions of modified night lights using higher quality components . . . [Circuit by David Johnson P.E.,  11/06/08]


One-Watt 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 . . . [Hobby Circuit designed by David A. Johnson P.E.,  11/06/08]


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


Protects from Battery Polarity Reversal  -  This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity.  The N-channel FET connects the electronic device to the battery only when the polarity is correct.  The circuit shown was . . . [Hobby Circuit designed by Dave Johnson P.E.,  03/09/02]


Replace a Pushbutton Switch with this Infrared Safety Switch  -  Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/11/06]


Simple Circuit for 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 . . . [Circuit by David Johnson P.E.,  07/27/03]


Sound Triggered Night Light  -  A while back, I received a schematic from a guy by the name of Karl.  He described a situation he and his wife found themselves in aboard a cruise ship in the Caribbean.  They were staying in a room toward the inside of the ship, which did not have any port holes, windows or a balcony . . . [Circuit by David A. Johnson P.E.,  08/04/13]


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