We have a phone in-use indicator with a flashing red LED. The 9 volt alkaline battery in the indicator lasts only a month or two. I wanted to replace it with a NiCad rechargeable battery. But, I did not want to give any thought to recharging the battery. The goal was to feed a trickle of current to the battery at all times so it would stay charged by itself.
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
I have a basic circuit simulation program on my computer and can "build" a virtual circuit without buying components.
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
Here you see the components as connected. The drawing is part schematic and part pictorial. The connections inside the in-use indicator simply go to the battery connection leads on the circuit board. I tapped into the wires from the wall wart to the phone machine's power jack.
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
The extra wires are next to and behind the answering machine. The white electrical tape encloses the solder connections and the diodes with the resistor.
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.