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Backup battery for cordless phones By LEO SIMPSON Do you have a cordless phone in your home? Isn’t it annoying when you have a blackout and you can’t use your phone? This little project is about adding a backup battery to the base station so that it keeps going regardless of blackouts. 16  Silicon iliconCChip hip Y OU KNOW the situation. You come home in the evening and the meal is on the stove cooking and then the lights go out. You start scrabbling around for a torch or candles and then you can look up the phone number for the local electricity authority. Next problem: your cordless phone is not working and the regular handset is in another room or doesn’t work or you gave it to your sister. So there you are, in the dark, no power and without a phone! Sure the handheld phone has its own battery but the base station doesn’t, so when the mains power goes, so does the phone. This situation has occurred in the author’s house on several occasions recently and I decided to do something about the phone – I wish I could do something about the blackouts but that’s another story. So what can you use for a backup battery? My older Telstra Freedom 400 cordless phone used a 12V plugpack and so the obvious answer was to use a 12V SLA (sealed lead acid) battery as the backup. The circuit would be real simple, involving just a diode and a resistor, as shown in Fig.1. But most cordless phones on the market now seem to use any voltage but 12V. For example, the Telstra Freedom 320 cordless phone pictured uses a 9V plugpack while Panasonic models use 13.5V. Other brands use 6V or 8V or you name it. Not to be deterred, I sat down and nutted out the circuit of Fig.2. It is more complicated than Fig.1 but it has the advan­tage that it can be tailored to suit almost any cordless phone/ plugpack combination possible. It uses an LM317T adjustable 3-terminal regulator and by changing just one or two resistors, you can make it suit your particular situation. The catch is that you won’t be able to use your existing plugpack if it puts out less than 14V. Most 12V DC plugpacks easily exceed 14V and can deliver quite a lot more, depending on the load current. Now let’s have a think about the required voltage and cur­ rent for a typical cordless phone base station. While they may all come with a DC plugpack which is labelled 6V, 9V, 12V or whatever, the actual voltage delivered is usually about 15% more, depending on the mains voltage itself. For example, the Freedom 320 comes with a 9V plugpack but it voltage close to 10.5V, depending on circuit tolerances. If you want other voltages, the table on the circuit shows the required values for R1 and R2. The 0.1µF capacitor at the input to the LM317 and the 10µF capacitor at its output are included to ensure stability. Fig.1: use this circuit if your cordless phone base station runs at 12V. You will be able to use your existing 12V plugpack to charge the 12V battery and power the phone. When the power goes down, the 12V battery will keep the phone powered up. Construction We built the prototype onto a small piece of Veroboard, as pictured, but we have also designed a small PC board to do the job and its component overlay is shown in Fig.3. Fig.5 shows you need to install it and the battery inside a suitable plastic case. We used a 12V 2.3A.h battery available from Oatley Electronics and we installed it in a standard plastic utility case measuring 197 x 63 x 102mm. This is just the right length for the selected battery. A smaller battery would enable you to use a smaller case but then its backup time would be shorter. We mounted a DC socket, to suit the connector of the 12V plugpack, at one end of the case and a 4-way terminal block for the output and battery terminations, at the other end. The bat­tery itself was secured in the case with nothing more than a few short lengths Parts List Fig.2: this circuit uses an LM317T regulator to produce 10.5V to power a 9V cordless phone base station. When the power goes down, the 12V battery keeps the base station still powered at 10.5V. We use 10.5V instead of 9V because that is typically what the 9V plugpack would deliver. delivers around 10.5V and the load current is typically around 80mA, regardless of whether the unit is recharging the phone battery, on a call or just sitting there waiting for something to happen. Since the cordless phone is likely to have more range and more noisefree operation at 10.5V than at the nominal 9V, it makes sense to have the backup supply produce a similarly higher voltage. So for the 6V case, you would design for an operating voltage of about 7V and so on. Referring back to the circuit of Fig.2, the power from the 12V DC plugpack is fed to the input side of the LM317 3-terminal regulator and to the 12V battery via a 100Ω resistor which has a diode, D1, connected across it. The 100Ω resistor allows a trick­le charge current of about 10mA or so into the battery while ever the power is applied. When the power goes off, the battery then supplies the 3-terminal regulator via diode D1. The output voltage of the 3-terminal regulator is set by the three resistors connected to its Adjust terminal. The values shown will give an output a component layout using Veroboard and while it may not be the simplest possible, it does have the advantage that no tracks need to be cut. Assembling the PC board is straightforward. Just remember to put the diode and 10µF capacitor in the right way. If you decide to use a piece of Veroboard make you sure wire the LM317 in the right way and note that it does have a rather odd pinout, as shown on the circuit of Fig.2. Once the circuit board is assembled 1 12VDC plugpack 1 panel-mounting DC socket to suit plugpack 1 DC female power line plug to suit cordless phone base station 1 12V SLA battery (see text) 1 PC board, 33 x 25mm, code 12510991 1 plastic utility case, 197 x 63 x 102mm 1 4-way insulated terminal block 1 LM317T adjustable 3-terminal regulator 1 1N4001 diode 1 22µF 16VW electrolytic capacitor 1 0.1µF MKT polyester capacitor Resistors (1%, 0.25W) 1 56kΩ 1 2.2kΩ 1 270Ω Note: for other voltages, see table on Fig.2. Fig.3 (left): this small PC board accommodates all the components. Make sure you put the LM317T in the right way around. Fig.4 (below) shows the actual size artwork for the PC board. October 1999  17 This view inside the plastic case shows the 12V SLA battery and the small circuit board (in this case, the Veroboard version) which accommodates the LM317T adjustable 3-terminal regulator. Fig.5: trickier to assemble, this Veroboard layout will do the same job as the PC board version shown in Fig.3. of double-sided foam adhesive tape. The battery can be expect to last for many years so it probably will never need to be removed. Connect two wires from the DC socket to the appropriate points on the circuit board and apply power. Check that the output of the circuit is 10.5V DC or whatever your required voltage is. Remember that if the voltage is a little higher or lower than you expect, say 10.7V instead 10.5V, it does not matter. The phone will still work happily. After the various wires were at18  Silicon Chip The Veroboard version will save you the expense of a PC board. tached to it, the cir­ cuit board was itself attached to the base of the case, again with a length of double-sided foam adhesive tape. Complete all the wiring inside the case and then make up an output lead with a DC connector on it to suit the DC socket on your cordless phone base station. It is most important that you get the exact type to fit. If you get the wrong type it may not fit at all or it may be such a sloppy fit that its operation will subsequently be unreliable. Testing When all the wiring is complete, connect the DC plugpack to the case and the DC output to the cordless phone base station and check that it all operates as it should. Check that the battery takes over when the DC plugpack is disconnected. Finally, check that all functions of the phone work exactly as they did when powered directly by the original DC plugpack. Finally, here’s another thought: the same system could be used as a battery backup system for a phone answering machine. That way, if you have a blackout, you will not miss out SC on any phone messages.