Just Encase Solar Super Capacitor - Design - Blog 3

Interface Circuit Design

Power Management Circuit Description
The power management circuit allows periperal power to be turned off when the peripherals are not being used.
Q3 controls power to the temperature sensor and the capacitor voltage monitor while Q1 controls power to the Bluetooth module.
Q2 allows V+ (which is 3.3V) to turn on Q4 (which is switching super capacitor voltage) when peripheral power is active. Q4 connects super capacitor voltage to a voltage divider that connects to an A/D input. The voltage divider is needed because the capacitor voltage can exceed the maximum A/D input voltage.
U3 is a low current, low drop-out linear regulator that consumes 2 microamps while outputing 3.3 volts. This isn't terribly efficient (63%) when the super capacitors are fully charged, but efficiency increases as the capacitor voltage decreases, reaching about 94% when they are sufficiently discharged. I actually purchased a switching regulator that is slightly more efficient on average than this one, but decided this performance was adequate for my application.
The Schottky diode (D1) protecting the solar cell for external chaging of the super capacitor probably isn't needed and will be shorted out if this is the case.
J4 allows the voltage of one of the super capacitors to be directly monitored, eleminating the need for Q2 and Q4, but is not needed if those transistors are installed.
J 6 allows an external source to initially charge up the super capacitors before turning on power to the circuit using switch S1.
Hopefully the serial Tx pin driving R2 can be set low when the Bluetooth module is turned off, but the R2 resistor is there to limit current in case it isn't easy to do this.

Note that the interface includes connections for a Bluetooth module.

Power Management Booster Pack PCB Layout

Video of Main Project Components & Preliminary Firmware

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Preliminary Firmware - LCD

//
//  Super Capacitor Weather Station
//
//
//  Author :  Doug Wong
//  Date   :  2022
//  Version:  1.00
//  File   :  SuperCapWeather.ino
//
//

// Include application, user and local libraries
#include "SPI.h"
#include "OneMsTaskTimer.h"
#include "LCD_SharpBoosterPack_SPI.h"

// Variables
LCD_SharpBoosterPack_SPI myScreen;
uint8_t myOrientation = 0;
uint16_t myCount = 0;
uint16_t TMP = 23;
uint16_t MV = 5400;

#define LCD_VERTICAL_MAX    myScreen.getSize()
#define LCD_HORIZONTAL_MAX  myScreen.getSize()


// Add setup code
void setup()
{
    Serial.begin(9600);

    myScreen.begin();
    myScreen.clearBuffer();

    myScreen.setFont(0);
    myScreen.text(4, 4, "Super Capacitor");
    myScreen.text(4, 20, "Weather Station");
    myScreen.text(26, 40, "T: 22");
    myScreen.text(20, 60, "mV: 5300");
    myScreen.text(64, 84, "DougW");
    myScreen.flush();

    for (uint8_t i = 0; i < 20; i++)
    {
        delay(100);
    }

}
void loop()
{
     myScreen.text(44, 40, String(TMP));
     myScreen.text(44, 60, String(MV));
     myScreen.flush();
     
    for (uint8_t i = 0; i < 20; i++)
    {
        delay(100);
    }
}

Super Capacitor Protection Circuit Description

The TL 431 is a shunt regulator that abruptly conducts when the voltage at its reference pin exceeds 2.5V. This turns on both transistors, clamping the voltage at about 2.7 volts in this case. R1, R2 an Q1 dissipate any power needed to clamp the voltage. In my system, the current won't exceed 100 mA, so the power that converts to heat in these components at each capacitor will be less than 0.27W. I still need to confirm the accuracy of this protection circuit in my application.

Project Status
The interface schematic is complete.
The PCB layout is complete.
The PCB has been fabricated and it has arrived.
The solar cell has been tested and characterized.
The wiring diagram is complete.
The super capacitors have been tested.
The capacitor protection circuit has been analyzed and tested.
Preliminary firmware to place data on the LCD has been written and tested.

Next Steps
Design the chassis to hold all components in place
Assemble the power management PCB
Test the power circuitry
Write firmware to test circuit connections and sensors

Relevant Links

Just Encase Solar Super Capacitor Experiment - Blog 1
Just Encase Solar Super Capacitor - Project Outline - Blog 2
Just Encase Super Capacitor Solar Weather Station - Design - Blog 3
Just Encase Super Capacitor Solar Weather Station - Assembly - Blog 4
Just Encase Super Capacitor Solar Weather Station - Bluetooth - Blog 5
Just Encase Super Capacitor Solar Weather Station - Indoor Operation - Blog 6
Just Encase Solar Super Capacitor Weather Station - Outdoor Tests - Blog 7

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