Sundial Smart Meter

A networked device that monitors usage of off-grid residential PV systems in Senegal

Project Role: PCB design, wireless networking.
The Sundial Solar Monitoring System is a mesh network of sensors that collect usage data for solar systems deployed off-the-grid in rural Senegal. Measurements for each village are stored on an SD card, collected once per month by an Oolu Solar agent. This data is combined with customer and payment information and uploaded to our secure cloud platform. Users can then view up-to-date analytics on their devices, helping them provide better service for their customers.

Technical Details

A direct current sensing topology was used to measure the power consumption of the various inputs as well as the power generated by the 25W solar panels. This topolgy utilizes extremely precise and low value sense resistors on the high-side of each of the loads requiring measurement. The voltage developed across these resistances is then amplified and read into an ADC to determine the current. The advantage of this design as opposed to indirect sensing using hall-effect sensors is that lower currents can be measured more accurately and at a lower cost.

The above schematic shows the current sensing circuit based on the MAX4173 Current Sense Amplifier, with differential and common-mode noise filtering. Low gain error is ensured with the LMV358 op amp configured as a current buffer. Minimal offset error due to offset current and input bias currents is created by limiting resistance seen by the inverting and non-inverting inputs and ensuring well-matched multiplexer channels.

Load Switching is implemeneted using a logic level NMOS transistor as the gate driver for the "pass" transistors. The circuit also constains inrush current limiting and over-current protection using PPTC solid state fuses. Inrush current protection is provided by C14 and R15, which limit the turn-on speed of the PMOSFETS and prevent spikes due to capacative loads such as consumer electronics with significant power supply filtering.

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Solar Powered MPPT Lithium-Ion Battery Charger

This project combines an entirely analog MPPT controller using a wide-input range buck-boost converter as the input stage with a switched CC/CV single cell Lithium-Ion battery charger circuit centered around the LM3420 charge controller. A photovoltaic array’s delivery of useful power to the load is a non-linear function of load-line voltage. The I-V curve of a solar panel depends on insolation and array temperature and has a single point, denoted by the maximum power point resistance, at which the power (current multiplied by voltage out) is greatest. Thus, an MPPT controller finds that operating point by adjusting its input resistance until it matches the continually-changing maximum power-point resistance. The preturb and observe method was chosen; a method which involves dithering the duty cycle of a voltage-mode converter and putting into the feedback loop circuitry that samples the output power, slowly adjusting the duty cycle to reach the maximum power-point.

The second stage was a challenging design because it required interfacing a high-efficiency non-synchronous switching converter with a charge controller circuit. To obtain high-efficiency, a buck converter with high-effiency at low input voltage ranges first had to be found. Then came the matter of selecting the switching frequency to obtain the optimal balance between inductor and capacitor sizes and power loss due to higher-frequency switching. Higher frequency switching is advantageous because it means lower current ripple for the inductor, which means the inductor can be smaller and have a lower peak current rating. It is also advantageous because it allows the output capacitor to be smaller for the same amount of voltage ripple. Since the draw back is power loss and component size wasn't a top priority a happy medium was found around 750kHz. Another complication was the way in which Li-Ion batteries need to be charged. There are two stages to charging a Li-Ion battery: One, the constant current stage, and two, the constant voltage stage, which sees the charge current into the battery drop slowly up until the end of the charge cycle. Because the current at the end of the cycle is two orders of magnitude less that the 0.2C - 0.5C current most Li-Ion batteries are charged at caculating a proper value for the inductor to avoid discontinuous current mode is somewhat tricky.

Lastly, designing the type 2 compensator for the buck converter first required first selecting an open-loop crossover frequency that was low enough to attenuate switching noise and high enough to avoid voltage-droop with steps in load current. In order to find this value an ac-equivalent circuit model of the buck converter was constructed so that the output-impedance could be found. The closed loop output impedance was then used to determine what minumum crossover frequency was acceptable along with the rule of thumb that switching frequency should be atleast 10 times higher the crossover frequency. Finally, the mid-frequency gain and poles and zeros of the error amplifier were chosen to cancel out the double pole of the buck's power-stage and to provide proper phase margin for stable operation.

After the voltage regulator and MPPT tracker were designed the only other task was to put together a current-sensing current using an LMV358 op-amp.

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Theremin Musical Instrument

The theremin was invented in 1919 by a Russian physicist named Lev Termen (Theremin is the westernized version of his name). The theremin is unqiue in that it is operated without physical contact; two antennas, one for pitch and the other for volume, allow the musician to play simply by moving his hands in proximity to the device.

The theremin I designed with partner Nate Michener is made entirely of analog discrete components with no digital signal processing. The theremin produces various pitches and has volume control using only the capacitance created in the air, which acts as a natural dielectric, between two conductors: the users hand and an antenna made out of a loop of copper wire. The voltage-controlled oscillator at the input takes advantage of the old UA741 op-amp's extremely limited slew-rate.

Technical Details

As mentioned above, the design is centered around the 741 op-amp and its slew-rate limitation. The ua741 is limited to 0.5V/us - so, if it oscillated from -15V to +15V, the maximum frequency it can reach without limitation is 16.7kHz. Above this frequency the amplitude of oscillations will decrease. The design uses a topology in which the oscillation frequency is adjusted by a small capacitance between an antenna (a loop of copper wire) and ground (a person's hand). The oscillator is similar to a Wien Bridge Oscillator with its frequency-dependent positive feedback gain, and a static negative feedback gain. The signal coming out of this oscillator is rectified by a diode bridge, so that it can be used as a DC-control signal for the next stage: the voltage controlled oscillator.

The next stage is the VCO. This is where the pitch is generated; it takes the DC control voltage from one of the proximity sensors and uses it to modulate the frequency that is generated in this oscillator. The first stage is an integrator, which builds up charge and ramps its output voltage into a Schmitt Trigger. The slope of this ramp signal is directly proportional to the voltage of this DC signal. The schmitt trigger is a bistable multivibrator, so using the output form the RC integrator it will create a variable-frequency square wave. The output of the Schmitt Trigger is then fed back into the gate of a MOSFET activating it, which causes the capacitor in the integrator circuit to dump its voltage to ground, thus reseting the circuit. An in-depth circuit analysis is detailed in the document below.

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GPS Display

A simple GPS display using 8-bit pic16f873a MCU, communicating through the UART module. Shows, longtitude and latitude and will show when the receiver has a fix on satellites.

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500W Brushless DC Motor Speed Controller

A motor controller for use on an e-bike. Utilizes sensored trapezoidal control to drive a 500W BLDC motor. The inverter power stage uses a soft-switching PWM scheme, with trapezoidal commutation. Trapezoidal commutation is the simplest scheme, though more torque ripple is produced in comparison to a Field-Oriented Controller, which requires the measurement of phase currents. The soft chopping approach allows not only control of the current and of the rate of change of the current but a minimization of the current ripple as well. In this soft chopping mode the low side transistor is left ON during the phase supply and the high side transistor switches according to the pulsed signal. This also reduces EMI and is the method used on this board. Hall effect sensors are used to detect the position of the rotor.

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200W Offline Forward-Converter for Battery Charging Applications

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