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 |