Practical Electronics


Introduction: Practical Electronics

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.



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    19 Discussions

    I find it strange a phone in use indicator doesn't draw it's power from the pnone lineline

    Three years.... What took you so long to write the instructible? Just kidding. Nice project, glad it works. A possible improvement would be to not have a battery, but have a capacitor (4700uF perhaps?) in its place. Just a thought....

    7 replies

    The three year delay also involves the fact that I did not discover Instructables before July 2008. A large capacitor would also have a large physical size. The flasher already has a cavity for a 9 volt radio battery. Thank you for the suggestion and for looking at my Instructable.

    Understandable. How does your circuit, being apparently hooked directly into a power supply shared by another device, detect the phone line?

    My circuit simply provides power to a rechargeable battery inside the white plastic cube with the flashing red LED shown in the Introduction. Inside the cube is an integrated circuit with additional discrete components. It is from Radio Shack and is designed to detect the phone line.

    Oh, now it makes sense. I don't mean to be greedy, but may I see a schematic? One of these may be useful for my house.... heheeee!

    Please do not feel you are being greedy. The idea was to be shared so you and others can make use of it. Are you able to view steps 2 and 3? Both of those, especially step 3, are pretty much a schematic. The circuit has worked out very well for me and continues to do so. You might want to check the actual output of your circuit with your meter, rather than just by the numbers working with things like Ohm's Law.

    What was the integrated circuit, and does Radio Shack still sell them? I saw the step 2 and 3 schematics, and they are noce, but the schematic I was interested in was how the device actually detects the phone line using the IC. That, I do not see.... Thus, the greed.

    I meant to reply to your question earlier and forgot. I searched Radio Shack's site for a phone line in use indicator, but found nothing. They were an item when dial-up Internet connections were more common. There are circuits for building such things. Usually they consisted of a means of detecting the voltage difference between the normal 48 VDC on the line and what it drops to when the line is in use. Then a circuit was closed that turned "on" an LED. I built one of these once to cancel an automatic ringer on a church bell. It chimed the hour right outside my office window and made talking on the phone impossible. The circuit used a common NPN transistor biased to conduct when the phone line voltage drop occurred. The problem was that any changes in the impedance of the phone line changed the value of the biasing resistor. Do an Internest search for DIY phone in use circuits. There were even different types of circuits to do this same task.

    What software are you using as i hav been looking around for some software to test circuits out and have been unable to find any thanks

    4 replies

    The web page listing for the simulation software says it is for Windows 3.1 and 95. It also came on a floppy disk. I had problems with it when I upgraded from Windows 95 to 2000. I sent an e-mail to Global Specialties. They sent me a version of the program file by e-mail for Windows 2000 and up. I now use that version on Windows XP and it works fine. Try to contact Global Specialties directly and buy from them. See if you can get a version for Windows XP from the start.

    Update: Go to and download their product catalog (8 pages in PDF). Search it for "Protolab." They sell a version for Windows 98 and up.

    If you "borrowed" power from the answer machine wall-wart to charge the battery, why not simply use this supply to directly power the ciruit...

    3 replies

    The voltage difference between the output of the answering machine power supply is four volts higher (more than 140 percent) than the voltage required by the battery. It might have worked, but I wanted a voltage going to the battery that was closer to the battery's stated voltage. In addition, this Instructable seemed a good opportunity to teach people just starting in electronics about using diodes to drop the voltage in a circuit.

    OK but I still don't see why you cannot use your ciruit, but just not use a battery anyway.

    You mean use the power from the AC/DC adapter to power the flasher directly. I did not really think along those lines. It might be possible, probably is. A battery in a circuit like the one I have does tend to act as a voltage regulator. That could be a good thing. We once had a wireless phone that had quite a few cross talk problems. The AC/DC adapter was putting out nearly 14 volts in a circuit that nominally called for 12 volts. I added a 12 volt zener diode and the problems went away. If there were surges and spikes coming through the AC/DC adapter, the battery protects the rest of the circuit a bit. Thank you.