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Hobby Circuits designed by Dave Johnson

Last Updated on: Tuesday, June 01, 2021 03:22 PM

List of Hobby Circuit Designs

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

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

Indicate Electrical Current with this Circuit

Indicates 120 VAC or 240 VAC

Indicator for Acceptable Voltage

Indicator for Bad Light Bulb


It's Time to Dust Indicator

Laser Burst Light Detector

LED Test Signal Generator-40Khz

LED/Laser Light Output Intensity Meter

Light Detector

Light Intensity Monitor

Light Monitor Circuit

Lithium Battery Tester

Locate a Plastic Pipe

Low Voltage Indicator for Lead-Acid Battery

Measure Button Battery Cell's Internal Resistance 

Miniature Pushbutton On/Off Switch

Modulated Light Receiver Front-End Circuit

Monitor Water Seepage

Monitors AC Current


NMH/NiCd 6v Battery Tester

NPN/PNP Transistor Tester

One Amp Current Injector

One Nimh Cell powers White LED

Oneshot & Latch Pushbutton

Piezoelectric Water Flowing Indicator

Precision Low Capacitance Meter


Produces Beep once each Minute

PV Solar Cell Test Fixture

Radio Tower Lamp Monitor

Really Loud Output Beeper Circuit

RFID Test Circuit

RS-232 Serial Interface Status Indicator Circuit

Scare Off Pigeons

Self Powered AC Indicator

Slotted Opto-Sensor used in Honeybee Counter

Small Battery Capacity Tester

Snow Depth Alarm

Solar Cell Test Fixture

Solar Insulation Monitor

Switch activated by Piezo Vibration

Ten Turn Pot Forms Frequency Readout

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

Ultra Low Power Detects Clear Fluid

Voltage Boost to Series Regulator provided by Capacitor


Watchdog Timer set for 36 Hrs

Water Filling Station Controller


Water Level Monitor

Water Pump controlled by Solar Power

Water Seepage Alarm

Watt-Hour Meter Monitor


White LED Night Light

10 Turn Pot Forms Frequency Readout

120 VAC / 240 VAC Indicator

12v Battery Voltage Monitor-Expanded Scale 

1Khz to Over 70Mhz Light Receiver

1-Nimh Cell powers White LED

3mw Laser Test-20Mhz VCSEL

4 to 20ma Current Loop Senses Temperature

A Current Transformer used to Harvest Energy

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


Accurate Low Capacitance Meter

Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor

Alarm sounds when Power Cord is Unplugged

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

Bidirectional DC Current Indicator

Brownout Voltage Tester

Capacitance Type Liquid Level Monitor

Characteristics of CMOS Logic Inverter Amplifiers

Cheap Light Power Meter

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

Clip-on AC Current Indicator Circuit

Control Variable LED Intensity

Convert AC to DC signal with is Full Wave Rectifier

Cooling Fan Thermostat (12v)

CT Hour Meter

CW & CCW Alternating Motor Driver

DC Brushless Motor Monitor

DC Current Bidirectional Indicator

Detect Clear Fluid

Detect Motion with a Piezo Device

Detector for Human Footsteps

Detects Low Voltage 120VAC Power Line

Digital noise generator

Divide a TTL Clock Signal by 1.5

DVD Recorder + TV Camera Make Cheap Data Logger

Earthquake Alarm Circuit



Footstep Alarm for Human Traffic

Front End + Power Supply Circuits of the Electric Filed Disturbance Monitor


Generate Short Delayed Pulse

Ground Resistance Tester

High Speed Light Detector from FET Input

High/Low Voltage Tester on AC Power Line 

Home Made Geophone Detects Foot Steps

Improved Solar Path Light


Hobby Circuits by Category

Dave's Circuits

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


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