Building clocks are one of the interesting and easiest projects that can be seen among hobbyists and makers. Presented here is a multifunction rechargeable clock unit that not only functions as a clock but also as a rechargeable light lantern and a mobile charger. The clock has all the basic functions like date-time display, alarm and room temperature display. The clock is powered through a lead acid battery which can be recharged through solar power or mains power supply so that the clock never ever runs out of power.
The clock is also provided with a female USB socket which can be used to charge most of our mobile phones and gadgets. It has a dedicated battery charging and load cut-off circuitry that takes care of the battery through exact charging topology and prevents it from being overcharged or getting undercharged.
The functional unit of the clock is divided into two units namely power unit and the main clock unit. The block diagram of the power unit is shown in figure 1.
The main aim of the power unit is to provide continuous power to the clock and further circuits. In this process, a lead acid battery is being charged which powers the circuits ahead.
The lead acid battery can be charged via two sources namely solar power and mains power.
Battery management circuit
The battery management circuit is so designed that it accepts power from two sources at the same time but gives priority to the main power such that if solar power is low or unavailable, user can plug it into the mains wall supply to charge it. A solar photovoltaic panel is used to directly convert solar light and heat energy into electricity.
To harness the mains power into meaningful power, a step down transformer is used which converts high voltage AC into low voltage AC. This AC voltage is rectified and converted into DC, and then further smoothened by using capacitors as low pass filters. After this, the power becomes acceptable to be fed to the battery management circuit.
The lead acid battery is directly connected to the battery management circuit which takes cares of charging the battery and maintaining its charging voltage and current to acceptable limits that ensures longer battery life, capacity and optimum usage.
Next we have used a comparator circuit which checks the battery voltage against its internal reference voltage and shuts-off the load thus preventing battery from further discharging. Then a linear low drop out regulator is used to derive regulated 5v rail from the battery rail. This 5v rail is non-current limited and powers most of the circuitry ahead like lightening LEDs, etc.
A current limited load switch is used to derive a 5v rail which is current limited to 500mA for USB charging applications. We have current limited this rail to 500mA for USB charging applications to prevent USB application from immediately discharging the clock battery.
The block diagram of the main clock unit which comprises of the microcontroller section is shown in figure 2.
Here the brain of the circuit is the microcontroller which keeps track of all the sensors and switches connected to it, makes decisions and calculations, and finally updates the display section. The non-current limited 5v rail from the power section is converted into 3.3v rail in this section via a 3.3v LDO regulator which powers the microcontroller and other peripherals. To sense the room temperature, a temperature sensor is provided whose output is an analog signal proportional to the temperature. This signal is fed to one of the analog to digital converter channel of the microcontroller.
The user inputs are sensed by three momentary push to on switches which are used to set/check various parameters of the clock like alarm, date, etc. The time keeping is done by a separate real time clock (RTC) chip which is interfaced to the microcontroller via I2C bus. The RTC is also powered by a backup battery which keeps the time running even if the main power fails.
The display section comprises of the four 7-segment displays which are multiplexed to efficiently use the limited pins available on the microcontroller. The two center LEDs between the two pairs of seven segment displays are driven by the RTC clock. A RGB LED is also provided to display indications of AM/PM and date display. At last, a buzzer is provided to acknowledge user button presses and to depict alarm states.