Wireless charging for a HP-25 calculator

[Maximilian Hohmann]

Hello!

An update on my "poor man's wireless Woodstock charger made from readily available components".

I have received more parts from China, unfortunately not yet the small LiPo battries that fit inside the battery holders, so that at least I can test my setup outside the calculator.

1. I have thorougly tested the small battery protection board. It reliably turns off the charging current when the battery reaches 4,2V. I left it connected in a safe place for three days in a row. As can be seen in the picture I power my wireless charging pad through a "smart" USB cable with an integrated Volt- and Ampere-meter. The current drops to 0,1A once the battery is fully charged. Those 0,1A seem to be what is required for the communiation between the transmitting and receiving coils. It is the same when I place a fully charged phone on the transmitter.

2. I got a better voltage converter for the calculator now (Pololu 2.5V, 500mA Step-Down Voltage Regulator D24V5F2). When the calculator is turned off it only draws 1 microAmpere, which means no battery drain is to be expected. The maximum current is 170mA with the calculator on and displaying all "8"s. Alternatively I have bought some AMS1117 2.5V low drop voltage regulator chips. I have not tried them out yet.

3. As can be seen in the pictures the circuit works with and without the coil receiving engery just as it should. Other than with an "normal" Woodstock it should also be possible to use this contraption without a battery safely, because the voltage regulator will protect the calculator form overvoltage.

The only thing left to do once I have the batteries is to "make" proper battery connector springs that also hold the battery pack in place.

Regards
Max