Nokia 3310 can be charged directly from the USB port of any PC or using a common adapter with USB output connector.
This makes it, together with very simple and intuitive controll using just 3(!) special keys, very popular even in the age of smart superphones.
One just has to prepare a simple two-wire cable where he connects pin 1 of the USB (+5V) with the core and pin 4 (Ground) with the shielding
of the Nokia 3.5mm connector.

However, some younger Nokias e.g. 5140 can't be charged this way and even an oficial charger for Nokia 3310 can't be directly used
although the connector is the same. Even if one has a regulated power source he still can't find a voltage which Nokia 5140 would swallow.
It will always annoy him by responding: 'Not Charging'.

One might be suspicious that there is some secret information running between the original adapter and the cell phone or its voltage
must have some special time-profile. The truth is far simpler: The voltage must be 'soft'!

There is a very simple hack to charge Nokia 5140 directly from any power source giving ~> 8V :
Prepare a cable with the Nokia 3.5mm connector on one side and e.g. two banana connectors on the other. Again + pole must be in the core.
Then solder a 5 Ohm resistor into one of the branches so that it is in series with the mobile phone.
Connect your Nokia to circa 8.1 V 'hard' via this cable and that's it!

Of course, you can use this new cable with the serial resistor and 8 V for the older Nokias e.g. 3310 as well.
You can reach the same effect by simply limiting the power if you have an appropriate power source with this capability.

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