Convertor mA and V for cellphone

[garf]

Hi,
I'm very new to adjusting own electronic devices but I want to make my old cellphone work on a 9V battery.
The purpose is a game that I want to make that opens the lock of a box using a cellphone. The only problem is the battery will die quite fast.
Adjusting the battery attachment with an enclosure for a 9V battery will fix this, asking the participants to bring their own power

My cellphone works on 3,7V 750mA AC and a standard 9V battery has 300 to 500 mAh I think ...
How can I make this work?

 

[SherpaDoug]

Try https://www.ti.com/ use their WEBENCH tool to design a supply. Tell it what power you have and what you need and it will suggest TI chips and schematics to get you there.

 

[jiripolivka]

A 9v battery is not good for anything that needs so much current like you old cell phone. You can certainly learn how to build a DC/DC converter but even with that the current needed from a 9V battery will exceed 100 mA while 20 mA is a heavy load for such battery.
All that is why cell phones use special Li-ion rechargeable batteries no other battery type can compete. For 3.7 V you can use an emergency (short-life) power supply of three AA alkaline cells but a regulator to 3.7 V is needed. Find some suitable 1A voltage regulator at Mouser or Digi-Key.