The following simple circuits use an adjustable voltage regulator LM317T as a current limiter. The capacitor smoothes the half-wave rectified DC 55mA output from a wall charger, allowing the voltage regulator to work properly, and the LED is a kind of continuity tester, telling you your battery is in fact being charged. The LED also will prevent your battery from discharging if the power is disconnected (you will need a LED or other diode if you use a lamp timer). Don’t use a LED in this way if you plan on charging at a higher rate than 30 mA (or the max mA rating of the LED)—you will blow the LED. If your wall charger has a continuity light, you can leave out the LED altogether as in the simpler circuit. If you use a LED and resistor in parallel across the input leads (instead of in series with one of the input leads as here), you will have no indication of continuity and no clue if you are charging (done that—crashed). One circuit here includes a switch between a 15 mA and a 27 mA rate, but you can select your own values, or skip the switch and just use one resistor (definitely easiest to build).
Choose your own resistors based on the charge rate you want. You can calculate the resistance as follows: R=1.25/I, where I is amperage, R is resistance. For example, for a 15 mA charge rate (0.015 amp) requires 1.25/0.015 = 83.3 ohms. Unfortunately, Radio Shack carries only certain values, and this is not one of them. Fortunately, you need only be approximate, and by measurement of the circuit, a 82 ohm resistor gives 15.2 mA. For a 110 mAh pack, you could be compulsive and use in series a couple of resistors to get the required 114 ohms, or be reasonable with a single 100 ohm resistor (giving a measured 12.5 mA).
Mount the parts on a piece of proto board, or make your own circuit board, solder on your connectors, and you will be set.
Radio Shack part numbers:
|276-1778||$1.99||LM317T adjustable voltage regulator|
|272-956||$0.99||220 uF axial electrolytic capacitor|
|275-635||$2.99||SPDT switch (or similar)|
|276-026||$0.99||LED mini red (make sure it has a max mA rating greater than your intended charge rate)|
|271-312||$7.99||¼ watt 5% carbon film resistor assortment, 500 pieces (a lifetime supply!)|
|276-149||$1.19||small project board|
Rob Crockett 1/97