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Indicator Circuits
Indicators:  #'s - A       B - C       D - F       G - I       J - L       M - P        R - S       T - Z
Last Updated on: Thursday, November 30, 2017 10:29 AM

Circuits Designed by Dave Johnson, P.E. :
12V battery level indicator for car batteries -  This kit measures battery voltage.  The red LED    (closest to the IC) indicates a low status11.5Volts or less.  The yellow LED indicates 50% charge12.0V.  The green LED indicates 12.6V or full charge.  This kit also measures the charging status.  Turn the charging system onor start the vehicle.  The red LED    (at the extreme end from IC) indicates overcharge15.5V.  The green LED indicates maximum charge voltage14.5V.  The yellow LED indicates normal charging voltage13.2V.  __ CdS Electronic

12v Lamp Current Indicator -  This circuit turns on a LED indictor light whenever DC current flows into a 12v lamp. . . Circuit by David Johnson P.E.-July, 2006

12v Touch Switch Exciter Circuit -  This circuit is designed to generate a 20KHz pseudo sine wave signal that can power about 50 remote touch activated switch circuits.    It can support a cable length of about 2500 feet.    A typical remote switch circuit is also shown as well as a receiver circuit for those switches.. . . Circuit by David Johnson P.E.-February, 2002

175Khz Inductive Pulse Receiver -  This circuit is discussed in more detail in the Experimenters Journal.  The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provided enough amplification for the signal to be easily viewed on an oscilloscope. . . Circuit by Dave Johnson P.E.-July, 2000

1uS Light Pulse Receiver + Post Amp -  This circuit is designed to detect very weak light pulses lasting 1uS.  It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity.  A post voltage amplifier is included with a gain of about X20.  The circuit is described in more detail in the receiver section of my Handbook of Optical Through the Air Communications.    Note: The LF357 op amp is no longer available, this circuit is for reference only.. . . Circuit by Dave Johnson P.E.-March, 1999

2 Cell Lithium Ion Battery Charger -  This circuit was build to charge a couple series Lithium cells    (3.6 volts each, 1 Amp Hour capacity) installed in a portable transistor radio.  The charger operates by supplying a short current pulse through a series resistor and then monitoring the battery voltage to determine if another pulse is required.  The current can be adjusted by changing the series resistor or adjusting the input voltage __ Designed by Bill Bowden

2 Line Telephone Line Status Indicator -  Many home businesses use multiple phone lines.  This circuit gives you a visual indication when a line is in operation.  The two AA battery cells should provide enough power for about one year of operation.  The circuit is line polarity insensitive. . . Circuit by David A. Johnson P.E.-June, 2000

2 Transistor Oscillators -  This page has two unusual two-transistor oscillators.  I set the component values for a low frequency application.  Both circuits draw only about 1 micro amp of current. . . Circuit by Dave Johnson P.E.-December, 2002

3 Level Audio Power Indicator -  Battery-operated3 LED display that connect it to loudspeaker output __ Contact: Flavio Dellepiane, fladello @ tin.it

3 Volt Low Battery Voltage Flasher -  This circuit is designed to monitor two alkaline cells    (3v) that form the battery often used in portable electronic equipment. . . Circuit by David A. Johnson P.E.-January, 1998

36-Hour Watchdog Timer -  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. . . Circuit by David A. Johnson P.E.-April, 2010

4 Line Telephone Line Status Indicator -  Many home businesses use multiple phone lines.  This circuit gives you a visual indication when a line is in operation.  The two AA battery cells should provide enough power for about one year of operation. . . Circuit by David A. Johnson P.E.-June, 2000

50 MHz Frequency Counter Voltage Meter & SwR/PWR Indicator -  This is a successor of the PIC16C71 4-digit LED f-counter & V-meter.  Some hard to find parts used in the previous version, which are out of production for some time, has been omitted.  A rather early PIC16C71 has also been replaced by 28-pin device PIC16F876 __ Designed by Aleksander Stare

7-segment Display Water-Level Indicator -  This water-level indicator uses a 7-segment display, instead of LEDs, to indicate the water level  (low, half and full) in the tank.  Moreover, a buzzer is used to alert you of...__ Electronics Projects for You

AC Current Indicator LightThis 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 David A. Johnson P.E.-October, 2005

AC Indicator-Self PoweredOften 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, 2007

Acceptable Voltage IndictorI have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is. . . Circuit by David A. Johnson P.E.-February, 2002

Active AC Power Line IndicatorThe circuit below is designed to detect when a power cable is active   (hot) .  When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow.  It is powered by a 3v CR2032 lithium coin cell.  I. . . Circuit by David A. Johnson P.E.-May, 2005

Alarm Beeps when the “Check Engine” Light Turns OnWhen 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 Johnson P.E.-July, 2006

Alkaline Low Battery Voltage IndicatorIn many battery powered systems a user would like to know when it was time to replace the batteries  before they were completely discharged, causing the device being powered to fail completely.  Alkaline batteries have an open cell voltage of about 1.5 volts.  As they are discharged, the voltage slowly drops.  When the cell voltage reaches about 1.25v, they have delivered about 90% of their stored energy. . . Circuit by David A. Johnson P.E.-June, 2012

Automotive Electrical Voltage Indicator This circuit will indicate if an automotive electrical circuit has zero volts or 12 volts present.. . . Circuit by David Johnson P.E.-July, 2006


Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

A 'Tiptronic-Style' Gear Indicator -  It's easy to build and indicates the selected gear in both manual and automatic cars on a digital readout.__ SiliconChip

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 David A. Johnson P.E.-October, 2005

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, 2007

AC Power Indicator -  The AC powerline indicator presented here has a complete galvanic isolation from the grid.  The indicator is an LED that lights up when a current flows, although the current can be measured more accurately with an AC voltmeter set to its mV rangemust register on this site __ Designed by Published in Elecktor July/Aug, 2010

AC Power Line Voltage Indicator -  The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter.  The status of the mains voltage is available in the form of an LED bar graph__ Electronics Projects for You

Acceptable Voltage Indictor -  I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is. . . Circuit by David A. Johnson P.E.-February, 2002

Active AC Power Line Indicator -  The circuit below is designed to detect when a power cable is active    (hot) .  When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow.  It is powered by a 3v CR2032 lithium coin cell.  I. . . Circuit by David A. Johnson P.E.-May, 2005

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 Johnson P.E.-July, 2006

Alkaline Low Battery Voltage Indicator -  In many battery powered systems a user would like to know when it was time to replace the batteries  before they were completely discharged, causing the device being powered to fail completely.  Alkaline batteries have an open cell voltage of about 1.5 volts.  As they are discharged, the voltage slowly drops.  When the cell voltage reaches about 1.25v, they have delivered about 90% of their stored energy. . . Circuit by David A. Johnson P.E.-June, 2012

Audio Indicator -  This circuit can be used to remotely monitor a loudspeaker, alarm, or audio source for presence of an audio waveform.  It can also be directly connected across loudspeaker terminals used as a peak indicator.   __ Designed by Andy Collison

Audio Level Indicator -  Audio Amplifier output relay delay __ Designed by Tomi Engdahl

Audio Visual Indicator for Telephones -  Many a times one needs an extra telephone ringer in an ad joining room to know if re is an incoming call.  For example, if  telephone is installed in  drawing room you may need an extra ringer in  bedroom.  All that needs to be done is to connect  given circuit in parallel with  existing telephone lines using twin flexible wires.  This circuit does not require any external power source

Automotive Electrical Voltage Indicator -  This circuit will indicate if an automotive electrical circuit has zero volts or 12 volts present.. . . Circuit by David Johnson P.E.-July, 2006

Automotive Voltage Indicator -  Monitors battery voltage; Three-LED Display __ Contact: Flavio Dellepiane, fladello @ tin.it

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Indicator Circuits
Indicators:  #'s - A       B - C       D - F       G - I       J - L       M - P        R - S       T - Z


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