Of course, you can buy a ready-made memory. On sale there are now a great many for every taste. But their price is unlikely to satisfy a beginner hobbyist or someone who is able to make a charger himself do it yourself. I decided to repeat this scheme, but make a charger to charge two batteries at once. The USB 2.0 output current is 500 mA. So you can safely connect two batteries. The modified scheme looked like this.
I also wanted to be able to connect an external 5 V power supply. The circuit contains a total of eight radio components.
The tool will require a minimum set of amateur radio: soldering iron, solder, flux, tester, tweezers, screwdrivers, knife. Before soldering the radio parts, they must be checked for serviceability. For this we need a tester. Resistors are very easy to check. We measure their resistance and compare with the nominal. On how to check the diode and LED there are many articles on the Internet. For the case I used a plastic case measuring 65 * 45 * 20 mm. The battery compartment was cut out of a Tetris children's toy.
I’ll tell you more about the alteration of the battery compartment. The fact is that initially the pros and cons of the battery power terminals are installed the opposite. But I needed that in the upper part of the compartment there were two insulated positive terminals, and at the bottom one common negative. To do this, I moved the lower plus terminal to the top, and cut the total negative terminal out of tin, soldering the remaining springs.
The battery compartment was fixed on the case cover with three small screws. The board was cut out from the old Dandy game console modulator. Deleted all unnecessary parts and tracks of printed wiring. He left only the power socket.I used thick copper wire as new tracks. I drilled holes for ventilation in the bottom cover.
The finished board sat tight in the case, so I did not fix it.
After installing all the radio components in their places, we check the correct installation and clean the board from flux. Now let's deal with the wiring of the power cord and setting the charging current for each battery. I used a USB cable from an old computer mouse and a piece of power cable with a plug from the Dandy as a power cord.
The last setting step is to set the charging current. To do this, we break the circuit of the diode VD1 and the "+" battery. In the gap we connect the tester in the mode of measuring the current turned on to the limit of 200 mA. Plus a tester for the diode, and minus to the battery.
We insert the battery into place, observing the polarity, and apply power. In this case, the LED should light up. It indicates that the battery is connected. Next, by changing the resistance R1, we set the required charge current. In our case, it is approximately 100 mA. With a decrease in the resistance of the resistor R1, the charging current increases, and with an increase it decreases.
We do the same for the second battery. After that, we twist our case and the charger is ready to use. Since different finger-type batteries have different capacities, it will take different times to charge these batteries. Batteries with a capacity of 1400 mA / h with a voltage of 1.2 V will need to be charged using this circuit for about 14 hours, and batteries of 700 mA / h will take only 7 hours. I have 2700 mAh batteries. But I did not want to charge them 27 hours from the USB port. Therefore, I made a power socket for an external 5 volt 1A power supply, which I was lying idle.
