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 Dave Johnson P.E.,  06/10/00]

Alarm Sounds when Phone Line is Cut  -  Burglars, rapists and other criminals will often cut a phone line before they break into a home.  The cut phone line will prevent many alarm systems from alerting the police.  The battery powered circuit below will sound an alarm when the unit . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/16/05]

Cell Phone Charger  -  It seems that as cell phones become more sophisticated, their operating times become shorter and shorter.  If you are on the go all the time and have difficulty finding AC or DC outlets to charge your phone, you might consider this simple scheme . . . [Hobby Circuit designed by David A. Johnson P.E.,  08/24/09]

Cell Phone Solar Powered Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas Instruments . . . [Hobby Circuit designed by David A. Johnson P.E.,  01/02/11]

Cut Phone Line Alarm  -  Burglars, rapists and other criminals will often cut a phone line before they break into a home.  The cut phone line will prevent many alarm systems from alerting the police.  The battery powered circuit below will sound an alarm when the unit . . . [Circuit by Dave Johnson P.E.,  10/16/05]

Display RF Signal Strength  -  Many cordless telephones, computer networks and cell phones use a RF transmitter lefted on the 2.45GHz band.  Using a microwave diode detector and 100uA meter, this passive circuit will display the strength this signal.  The drawing shows how a bent piece of wire can be used as the antenna. . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

DIY Cell Phone Charger  -  It seems that as cell phones become more sophisticated, their operating times become shorter and shorter.  If you are on the go all the time and have difficulty finding AC or DC outlets to charge your phone, you might consider this simple scheme . . . [Hobby Circuit designed by David Johnson P.E.,  08/24/09]

Do It Yourself Portable Cell Phone Charger  -  It seems that as cell phones become more sophisticated, their operating times become shorter and shorter.  If you are on the go all the time and have difficulty finding AC or DC outlets to charge your phone, you might consider this simple scheme . . . [Circuit by Dave Johnson P.E.,  08/24/09]

Four 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 Dave Johnson P.E.,  06/10/00]

Line Status Indicator for 1 Telephone Line  -  This circuit monitors one phone line . . . [Hobby Circuit designed by Dave Johnson P.E.,  06/10/00]

One-Line Telephone Line Status Indicator  -  This circuit monitors one phone line . . . [Circuit by David A. Johnson P.E.,  06/10/00]

RF 2.45Ghz Signal Detector  -  This passive RF indicator is made from a few simple parts.  A 100uA moving coil meter is used to display the relative intensity of the RF signal.  This circuit can be used with some cell phones and many cordless telephones . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

RF Signal Detector at 2.45GHz  -  Many cordless telephones, computer networks and cell phones use a RF transmitter lefted on the 2.45GHz band.  Using a microwave diode detector and 100uA meter, this passive circuit will display the strength this signal.  The drawing shows how a bent piece of wire can be used as the antenna. . . . [Hobby Circuit designed by Dave Johnson P.E.,  07/09/06]

Solar Powered Cell Phone Charger  -  The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v.  That voltage can be fed to any cell phone or USB connected portable device, to charge its battery.  The circuit uses a Reg113-5 voltage regulator from Texas Instruments . . . [Circuit by David A. Johnson P.E.,  01/02/11]

Telephone Status Indicator for 4 Lines  -  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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/10/00]

Two 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 . . . [Hobby Circuit designed by David Johnson P.E.,  06/10/00]