I am at early stage(although its 2 years now) of building a WISP(Wireless Internet Service Provider) network. And since I am deploying the WISP network in rural mountainous area. But I have realized that I need UPS(Uninterruptible Power Supply) for the routers and antennas for delivering good internet service. UPS are quite expensive so, I started to looking at building diy Uninterruptible Power Supply(UPS) for my telecommunication equipment. I know how to build diy inverter and also had a inverter for car, so I looked at whether an existing inverter circuit could be converted to UPS and what extra circuit was needed. I liked to use SAA-1000A Inverter for car which has input of 12V DC and output of 230V AC. After little research, I came to know that to convert an existing inverter to UPS I need to add Automatic Changeover Switch(ATS).
So what is automatic changeover switch circuit. An Automatic Changeover Switch (ATS) is a device that automatically switches the power supply from the main source (like the grid) to a backup source (like a generator or inverter) when a power failure is detected. Once the main power is restored, the switch automatically changes back.
While I could find many automatic changeover switch(ATS) circuit I build a DIY Automatic Changeover Switch (ATS) using Arduino. The circuit is shown below.
The input to the circuit comes from 12V DC adapter connected to wall ac socket. The switch following the DC jack barrel is only to cutoff the voltage source derived from AC wall socket. When the switch is closed 12V DC is applied but when then switch is open the automatic changeover switch circuit is completely cutoff. Arduino is powered from +5V output of the LM7805 linear voltage regulator connected to the 12V power supply. An equal voltage divider circuit formed by two 10kohm resistors R1 and R2, is used to make the analog input read 2.5V. The Arduino is used as the input voltage detector and depending upon whether there is any voltage on analog pin, the switches of the main power supply and the battery power backup is turned. When the voltage read is greater than 1V, then it means there is power from the main and Arduino is programmed to turn on the relay switch connected between the main power supply and the load. If the voltage read is less then 1V, it implies that there is no main power voltage and so Arduino is programmed to switch on the relay connected to the battery power backup supply. There whether there is any main power supply or not, the load bulb continues to receive power supply. So this circuit effectively is an automatic changeover switch.
By replacing the battery supply by an arduino based inverter we will have a diy UPS.
