When I plug the usb charger into the socket, we see that the lights come on. Now I connect the negative of the usb to the negative battery contact (the one with the spirng that we identified earlier), and the positive to the input of the diode. I soldered the output of the diode to the positive of the battery holder. The diode has an input and an output, and the output is marked with the white line. In this case we will use a 1N4001, which has a voltage drop of 0.7v. However, one of my followers suggested an alternative solution, which is to use a diode. Here again we could connect the voltage regulator from before to the two battery contacts we identified earlier and set it to 4.5v.
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Now let's see how to transform to work with USB lights that run on 3 batteries. If we have a 2 battery light string the voltage will be 3 volts, while if we have a 3 battery light string the voltage will be 4,5 volts. With a small screwdriver, I turned the voltage regulator potentiometer until I saw the voltage needed by the light string on the display. I set the multimeter to measure the voltage, and connected the test leads to the two output wires. Now I connected the USB cable to the power supply, and plugged it in. Then I soldered two more wires to the output of the voltage regulator. I soldered the two red and black wires to the positive and negative input of the voltage regulator. To lower the voltage we can use a voltage regulator board like an LM2596, which costs about 1€. There is 5v on these wires, so we cannot connect them directly to the battery holder, because the lights need 3v. The white and green wires are for data and we can cut them, while the power is carried by black and red. To this cable I have cut off one end, leaving the one that connects to the power supply, which generally is an USB type A. To connect the lights to a USB power supply, such as that of a phone, we need an old USB cable. However always look which contacts are connected where to individuate the right ones, as their order may vary. The contacts are positioed like shown in most of the cases. Since we have 3 batteries in series, the multimeter will show 4,5v. In the case of a 3 battery light string, the two contacts connected to the lights are the top-left one and the bottom-right one. We do not need the two contacts at the bottom, because they are the ones that were used to connect the batteries in series. The one with the spring is the negative, and the flat one is the positive. In the case of a 2 battery light string thay are these two at the top, which are soldered to the circuit board. To connect a power supply to the lights we need to see which contacts in the battery holder are electrically connected to the lights.
So if I measure the voltage with the multimeter we have a total of 3v. The batteries produce about 1.5v, and in the battery holder they are connected in series. Later we will also look at one with 3 batteries. Let's start with a light string that works with 2 batteries. Generally 2 or 3 batteries should be put in the battery holder.
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