Projects from my workbench

Doubler/Charger Project     This post describes a project to trickle charge NiMH batteries as part of an UPS circuit.    The supply voltage for the UPS comes from a 12 V wall adapter.  The battery pack of 8 NiMH batteries provides roughly 10.5 volts as the backup power.  There's not enough voltage "headroom" to connect a constant current circuit to charge the batteries.  Rather than use a separate supply, I decided to build a circuit to increase the voltage to allow the increased value to run the charger circuit.    A well-known circuit uses a LM555 timer IC as an astable oscillator feeding a diode-capacitor network to double the input voltage to roughly 22 V.  This supplies an LM317 wired as a constant current source applied to the NiMH batteries.    The oscillator function is controlled by a 3.3 volt logic signal from the UPS3 control board.  A logic low input from the UPS board allows the LM555 oscillator to run and the batteries to charge. Clic

Uninterruptible Power Supply for Raspberry Pi    This is a blog to share my UPS project.  You'll see pictures, code, schematics, and test information.  I have never written a blog before so good luck to me! Introduction    This uninterruptible power supply was designed to provide operating power to a Raspberry Pi (RPi) computer.  It uses a diode switch  to select wall power if available, but switch to battery backup automatically should there be a power failure.  Battery voltage is supplied by 8xAA Nickel-Metal Hydride batteries.  To ensure that power will be available, I selected Tenergy brand Centura Low Self Discharge cells, such as these available from Amazon.  They are intended to maintain charge over long periods of storage.    The Tenergy AA size battery has built in low self discharge technology that prevents it from losing its power after long storage periods. Once charged, it can be stored for 12-24 months with a high capacity percentage left. Unlike typica