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Hobby Circuits designed by Dave Johnson

Last Updated on: Tuesday, October 24, 2017 03:04 PM

List of Dave's Circuit Designs

Text & Graphics Copyright David A. Johnson, PE  --  ALL Rights Reserved.
LINKING to Dave's circuits is permitted but DO NOT COPY CONTENT to your WEB SITE server!

Testers / Monitors Circuits
Circuits Designed by David Johnson, P.E. :

Indicates when it is Time to Dust

Indicator for Acceptable Voltage

Indicator for Bad Light Bulb

Inductor Self-Resonance Tester


Isolated AC Current Monitor

Isolation Transformer Circuits

It's Time to Dust Indicator

Laser Burst Light Detector

Laser Diode Tester

Laser/LED Light Output Intensity Meter

Lead-Acid Battery Low Voltage Indicator

LED Test Signal Generator-40Khz

LED/Laser Light Output Intensity Meter

Light Detector

Light Intensity Monitor

Light Monitor Circuit

Light Receiver Works from 1Khz to Over 70Mhz

Lithium Battery Tester

Locate a Plastic Pipe

Low Power Amp Monitors Light

Low Value Capacitance Meter

Low Voltage 120vAC Power Line Alarm

Low Voltage Indicator for Lead-Acid Battery

Measure Button Battery Cell's Internal Resistance 

Miniature Pushbutton On/Off Switch

Modulated Light Monitor #1

Modulated Light Receiver Front-End Circuit

Monitor Clear Fluids

Monitor Water Seepage

Monitors AC Current

NMH/NiCd 6v Battery Tester

NPN/PNP Transistor Tester

On/Off Flip/Flop Circuit with Automatic Timeout

One Amp Current Injector

One NiMH
Cell powers White LED

One Shot (D-Flip/Flop) Circuit

Oneshot & Latch Pushbutton

Optical RFID Test Circuit

Piezoelectric Vibration Sensor Activates Switch

Piezoelectric Water Flowing Indicator

Pot Controlled Variable LED Intensity Circuit

Power Line Alarm

Power Loss Beeper

Power Reminder Beeper

Precision AC Peak Detector

Precision Low Capacitance Meter

Produces Beep once each Minute

Pulsed LED Test Circuit

Pushbutton Oneshot & Latch

PV Solar Cell Test Fixture

Radio Tower Lamp Monitor

Really Loud Output Beeper Circuit

Resistance Tester got UL Ground Requirement

RFID Test Circuit

RS232 Data Bus Status Monitor

RS-232 Serial Interface Status Indicator Circuit

Scare Off Pigeons

Self Powered AC Indicator

Self-Powered AC Current Indicator

Serial Data Bus Status Monitor

Shadow activated (or hand wave) Motion Detector

Slotted Opto-Sensor used in Honeybee Counter

Small Battery Capacity Tester

Snow Depth Alarm

Solar Cell Test Fixture

Solar Insolution Meter

Solar Insolation Monitor

Solar Path Light Improvement

Solar PV Cell Test Fixture

Solar-Powered Pump Motor Controller

Switch activated by Piezo Vibration

Temperature Sensor MOSFET

Temperature Sensor with 4 to 20ma Current Loop

Ten Turn Pot Forms Frequency Readout

Test UL Ground Resistance

Tester for Brown Out Voltage

Tester for Modulated Laser Diode

Tester for Modulated Light

Tester for Pulsed LEDs

Tests UL Ground Resistance

Thermostat for 12v Cooling Fan

Time to Dust

Timer for Darkroom Camera Shutter

Transistor-Switch Detects Audio Signal

Turn D-Flip/Flop into a One Shot Circuit

UL Ground Resistance Tester

Ultra Low Power Detects Clear Fluid

Ultra Low Power Human Finger Detector

Under/Over Voltage Alarm on AC Line

Unplugged Power Cord Alarm

Voltage Boost to Series Regulator provided by Capacitor

Watchdog Timer set for 36 Hrs

Water Cistern Level Alarm

Water Filling Station Controller

Water Flowing in Pipe Indicator

Water Level Monitor

Water Pump controlled by Solar Power

Water Seepage Alarm

Water Seepage Monitor

Watt-Hour Meter Monitor

Well Pump Flasher & Beeper

White LED is Powered By One NiMH

White LED Night Light

X100 Scope Probe


10 Turn Pot Forms Frequency Readout

120 VAC / 240 VAC Indicator

120VAC Under Voltage Tester

12v Battery Voltage Monitor-Expanded Scale 

12v Cooling Fan Thermostat

12v Lamp Current Indicator

1Khz to Over 70Mhz Light Receiver

Cell powers White LED

20ma Current Loop Tester

20Mhz VCSEL 3mw Laser Test Circuit

36-Hour Watchdog Timer

3mw Laser Test-20Mhz VCSEL

4 to 20ma Current Loop Senses Temperature

40Khz Laser Burst Detector

40Khz LED Test Signal Generator

40KHz Light Receiver Front-End Circuit

6v NMH/NiCd Battery Tester

A Current Transformer used to Harvest Energy

AC Current Controls Hour Meter

AC Current Indicator Light

AC Current Monitor

AC Indicator-Self Powered

AC Isolated Current Monitor

AC Line Under/Over Voltage Alarm

AC Peak Detector that is Precise

AC Power Line High/Low Voltage Tester

Acceptable Voltage Indictor

Accurate Low Capacitance Meter

Alarm Discriminator Battery Monitor of the Electric Field Disturbance Monitor

Alarm sounds when Power Cord is Unplugged

Alarm sounds when Water is Detected

Alternating CW & CCW Motor Driver

Audio Frequency Digital Noise Generator

Automatic Timeout included with this On/Off Flip/flop Circuit

Bad Light Bulb Indicator

Bidirectional DC Current Indicator

Brownout Voltage Tester

Button Battery Cell Internal Resistance Measurement Circuit

Capacitance Type Liquid Level Monitor

Caps Provide Voltage Boost to Series Regulator

Characteristics of CMOS Logic Inverter Amplifiers

Cheap Light Power Meter

Circuit Forms Divide By 1.5 Counter

Circuit Indicates Battery Charge Current

Circuit Monitors 120 Power Line

Circuit Monitors 12v Lamp Current 

Circuit Monitors Power Loss

Circuit tests the self-resonance frequency

Clear Fluid Detector #2

Clip-on AC Current Indicator Circuit

CMOS Logic Inverter Amplifier Characteristics

Control Variable LED Intensity

Convert AC to DC signal with is Full Wave Rectifier

Cooling Fan Thermostat (12v)

Corona Monitor

CT Hour Meter

Current Indicator for Battery Chargers

Current Injector

CW & CCW Alternating Motor Driver

DC Brushless Motor Monitor

DC Current Bidirectional Indicator

Delayed Pulse Generator

Detect Clear Fluid

Detect Motion with a Piezo Device

Detector for Human Footsteps

Detects Low Voltage 120VAC Power Line 

D-Flip/Flop One Shot Circuit

Digital noise generator

Divide a TTL Clock Signal by 1.5

Does Power Line have 120 VAC or 240 VAC?

DVD Recorder TV Camera Make Cheap Data Logger

Earthquake Alarm Circuit

Either indicates 120 VAC or 240 VAC 

Electric Field Disturbance Monitor-Alarm Discriminator Battery Monitor

Electric Field Disturbance Monitor-Front End Power Supply Circuit

Electrical Current Indicator

Electrodeless Water Level Monitor

Energy Harvesting using a Current Transformer

Expanded Scale 12v Battery Voltage Monitor


Fluid Level Monitor

Footstep Alarm for Human Traffic

Freezing Temperature Alarm

Front End Power Supply Circuits of the Electric Filed Disturbance Monitor

Full Wave Rectifier

Generate Short Delayed Pulse

Ground Resistance Tester

High Speed Light Detector from FET Input

High/Low Voltage Tester on AC Power Line 

High-speed 75MHz light receiver

Home Made Geophone Detects Foot Steps

Honeybee Counter

Human Traffic Footstep Detector Preamp

Improved Solar Path Light

Indicate Electrical Current with this Circuit

Indicates 120 VAC or 240 VAC


Hobby Circuits by Category

  David A. Johnson, P.E. about these circuits



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