Cellular Phone Calling Detector:  Flashes a LED when detecting an incoming call.  Powered by one 1.5V cell  (added 4/05)

Charge Indicator Gauges Lead Acid Batteries:  05/27/99 EDN-Design Ideas / (added 11/05)

Circuit Detects Current Pulses:  01/06/94 EDN-Design Ideas / (added 3/03)  The pulse detector provides a visible indication of positive and negative current pulses. The pulses amplitudes can vary from20 to150 mA. The pulses durations can range from10 to40 msec, and their repetition rate can span40 to180 pulses/minute.

Circuit Forms Low Frequency Circulator:  10/11/01 EDN Design Ideas  /  (added 1/05)  The electronic circulator made its debut ten years ago (Reference 1). It functioned at VHF as a three-port unit using a Comlinear (now part of National Semiconductor, www.nsc.com) CLC 406 operational amplifier. The circuit in Figure 1 extends the circulator's performance to four-port operation at low frequencies, using the readily available 941 (equivalent to the ubiquitous 741) and LM3... ...

Circuit Forms Peak/Clipping Indicator:  11/22/01 EDN Design Ideas  /  (added 1/05)  The simple peak detector in Figure 1 is the result of a need for a single-5V-supply, level/clipping indicator for a multimedia-PC sound system. The design is unique in that it detects both stereo channels on a single peak-hold capacitor. All the adjustments in the circuit simultaneously apply to both left and right stereo inputs. ...

Contactless Mains Voltage Indicator :  (circuit / schematic design added 11/06)

Cut Phone Line Detector #2:  (schematic / circuit added 10/02)

CW Zero Beat Indicator:  (electronic Circuit / Schematic added 10/04)

CW Zero Indicator:  (electronic circuit added 7/03)

Data Multiplexer Adds Cursor to LED Display Driver:  03/30/95 EDN-Design Ideas / (added 03/03)

Digital Mains Voltage Indicator:  (electronic Circuit / Schematic added 10/04)

Doorbell for the Deaf:  (schematic / circuit design added 9/02)

Expanded Scale Indicator Revisited:  08/08/02 EDN Design Ideas  /  (added 1/05)  The visualization aid that a previous Design Idea describes allows only the expansion of the upper end of the scale (Reference 1). But what can you do if, according to your project requirements, you need to expand the middle region of the scale? Figure 1a illustrates the challenge. A voltmeter comprises a 100-µA dc meter and a series resistor....

Fastest Finger First Indicator:  (electronic Circuit / Schematic added 10/04)

Free Line Indicator Stops Interruptions:  12/07)/00 EDN-Design Ideas / (added 11/05) / When only one phone line is available for two phones, each time you want to make a call, someone may be using the second phone. A simple circuit lights an LED, which indicates whether the line is free (Figure 1a). Batteries are unnecessary; the phone line powers the circuit, and an accumulator saves energy for an "in use" indication. A rectifier bridge ensures that the voltage is positive for the circuit. You can safely use this circuit on a private phone line, but you may need authorization before connecting it to your operator line....

Fuse Blown Indicator:  It came to me in a flash. A simple way to see if a fuse has blown without removing it from its holder. Its not often you can design a circuit using just two components, but with just one resistor and an LED this circuit provides visual indication of when a fuse has blown....   (added 1/06)

Fuse Monitor/Alarm:  a simple way to see if a fuse has blown without removing it from its holder (added 4/02)