Indicator Circuits
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Last Updated on: Wednesday, June 02, 2021 01:45 PM

1 Line Telephone Line Status Indicator This circuit monitors one phone line. . . Circuit by David Johnson P.E.-June, 2000

1.5v LED Flasher Version A  Many published circuits that flash LEDs need 3 volts or more.  This circuit uses only a single inexpensive C-MOS IC and flashes the LED for a full year on a single 1.5 volt AA alkaline battery cell.  The circuit uses a charge pump technique to provide. . . Circuit by David Johnson P.E.-June, 2000

120 VAC / 240 VAC Indicator  Two LEDs indicate if a power line has 120vac or 240vac voltage on it . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2006

125khz Wireless Smart Key Detector  This circuit will turn on an indicator light whenever it detects a smart car key containing an RFID chip. . . Circuit by Dave Johnson P.E.-July, 2006

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 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 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 Indicato  rMany 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

Indicator Circuits
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