At our breakfast counter you see the in-use indicator (red LED lighted in the photo), the phone, the answering machine, and the wall wart power converter for the answering machine.
Step 1: Planned Circuit
The 13 volt current from the answering machine power supply needed to be dropped to about 9 volts for the battery. There may be other, perhaps even better ways to do it. But, I chose to use a string of five diodes to lower the voltage. Each diode drops the voltage about 0.6 volt. In simulation the drop was 0.8 volt.
A circuit that constantly recharges a battery should feed 1/100th of the amp-hour rating of the battery to the battery at all times. The battery is rated at 150 milli-amp hours. The 6 K Ohm resistor brings the current flow down to 1.52 mA.
Step 2: The Connections
The first band on the resistor (blue) is the color code for "6." The second band (black) is the color code for "0." The third band (red) is the color code for "multiplied by 100." Resistors usually have a fourth band that is metalic in color (gold, silver). This band indicates the range of latitude plus or minus the nominal value that is acceptable for that resistor, its tolerance from specifications as in 20 percent or 5 percent, etc.
Step 3: The Actual Splice
I made this modification about three years ago. We have not needed to replace the battery in the in-use indicator since. If we are on the phone a lot, the LED may flash a bit more slowly for a few days.
We relied on the in-use indicator a lot when we used a dial-up Internet connection. Now it is helpful to know if a phone somewhere in the house is ajar on its cradle or if someone is on the phone in another part of the house.
What was invovled in working out this project could be appied to other projects.