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=====MFCitizen @Brussels=====

**[[http://michkamelo.wordpress.com|Michka Mélo]], September 13th-14th, 2017**

====Abstract====

====Tests with resistor-based voltage divider====

   * FoAM's power supply does not go below 2.8 V of voltage.
   * In order to control the voltage to a lower threshold, I built a resistor-based voltage divider.
   * First version of the resistor-based voltage divider was made of a 100 ohms and a 10 ohms resistors in series. It worked but, smelled funny. After few runs, the 100 ohms had turned a nasty shade of black. It seems that the amount of current flowing in the resistor was too important because the resistor value was too low.
   * The second version of the resistor-based voltage divider was made of a 2.2 kohms and 1 kohms resistors in series. No weird smells, it worked.
   * However, I kept having funny results. I tested many of the components, and finally concluded with a test without the voltage divider that it might be perturbing the correct working of the circuit.
   * I will therefore not present here the hectic results of these first tests.

====Tests directly powered by the power supply====

===Test #1 - 1381E only===

   * When powering the 1381E voltage detector directly with the power supply (power supply + pin on 1381E pin 2, power supply - pin on 1381E pin 3, we obtain a 2.8V voltage on its output pin (pin 1), which is the same value as the one measured between the + and - pins of the power supply.

===Test #2 - 1381E powering BBC microbit directly===

   * When connecting the BBC microbit 3V pin to the output pin of the 1381E (the GND pin of the BBC microbit being connected to the - pin of the power supply), the BBC microbit does not light up.  The measured voltage on the output pin of the 1381E is 0.64 V.
   * It seems that the 1381E does not supply enough current to its output pin to power the BBC microbit.

===Test #3 - 1381E controlling 2N3904, which powers BBC microbit===

   * When connecting the gate of the 2N3904 to the output pin of the 1381E, then 2N3904's collector to the + of the power supply, and the 2N3904's emitter to the + of the BBC microbit, it does light up.
   * The voltage measured at the + end of the 1381E is 2.80V.
   * The voltage measured at the output of the 1381E is 2.64V.
   * The voltage measured at the emitter of the 2N3904 is 1.82V, which is sufficient to bleakly but steadily light up the LEDs of the BBC microbit.
   * We can measure an overall -0.98 V voltage drop between the power supply + pin and the BBC microbit + pin, through the 1381E (2.80-2.64=0.16 V) and the 2N3904 (2.64-1.82=0.82 V).

===Test #4 - Power supply feeding 1F supercap, powering the circuit===

   * When connecting a 1F 2.7 V supercap in parallel of the power supply, we obtain the same values as in test #3.
   * When turning off the power supply, once the supercap charged, we have rougly 1 sec autonomy of the BBC microbit. The BBC microbit goes off when :
     * the voltage on its input pin goes roughly below 1.76 V.
     * the voltage on the output pin of the 1381E goes rougly below 2.6 V.
     * the voltage on the + pin of the power supply goes below 2.76 V.
   * 
===Test #5 - Power supply feeding 3F supercap, powering the circuit===

   * When connecting a 3F 2.7 V supercap in parallel of the power supply, we obtain the same values as in test #3.
   * When turning off the power supply, once the supercap charged, we have rougly 5 sec autonomy of the BBC microbit. The BBC microbit goes off when :
     * the voltage on its input pin goes roughly below 1.72 V.
     * the voltage on the output pin of the 1381E goes rougly below 2.52 V.
     * the voltage on the + pin of the power supply goes below 2.67 V.

===Test #6 - Power supply feeding 10F supercap, powering the circuit===

   * When connecting a 10F 2.7 V supercap in parallel of the power supply, we obtain the same values as in test #3.
   * When turning off the power supply, once the supercap charged, we have rougly 15 sec autonomy of the BBC microbit. The BBC microbit goes off when :
     * the voltage on its input pin goes roughly below 1.71 V.
     * the voltage on the output pin of the 1381E goes rougly below 2.52 V.
     * the voltage on the + pin of the power supply goes below 2.67 V.

===Test #7 - Adding an additional 2N3904 transistor controlled by the BBC micro itself===

   * We add up a second NPN 2N3904 transistor, which base is connected to the 0 output line of the BBC microbit. The BBC microbit outputs a 1023 (max value) analog signal to its 0 output line to activate the gate of this transistor. The collector of this transistor is connected to the + of the power supply, and the emitter to the + of the BBC microbit. 
     * The goal of this second transistor is to allow the BBC microbit to feed itself directly from the power supply (the supercap) once the voltage of the power supply (the supercap) drops below the one activating the output pin of the 1381E. Indeed, if the power supply (supercap) voltage drops below the lower activation limit of the 1381E, its output signal will drop to 0, which will shut the gate of the first transistor, thereby cutting the power line to the BBC microbit, even though the power supply voltage is still (theoretically) around 2.2 V. The BBC microbit being able to run as low as 1.71 V, as our prevous results show, it would be a pity not to use the burst of energy delivered by the supercap between 2.2 and 1.7 V.
   * (A.) When connecting this two-transistors version of the circuit to the power supply, we obtain the following values :
     * 2.80 V at the + pin of the power supply, as in test #3.
     * 2.63 V at the output pin of the power supply, very much as in test #3.
     * (1.) 1.82 V at the emitter of the first 2N3904 transistor, as in test #3, and therefore the same at the emitter of the second 2N3904 transistor.
     * (2.) 1.78 V at the 0 output line of the BBC microbit, and therefore the same at the gate of the second 2N3904 transistor.
   * (B.) We know that current flows through an NPN transistor when :
     * (1.) It has a relative positive voltage on its gate.
     * (2.) Its gate voltage is at least 0.6 V superior to its emitter voltage.
     * (3.) The collector voltage is superior to the emitter voltage.
   * We can therefore deduce from the values (A.1) and (A.2) that the second transistor cannot open, because the voltage difference between the gate (1.78 V) and the emitter (1.82 V) of the transistor is below 0.6 (criteria B.2).
   * WAs the voltage provided by the power supply (supercap) drops, the voltage at the gate of the first 2N3904 transistor drops, and so does the voltage supplied to the BBC microbit, which in turn affects the voltage supplied to the gate of the second 2N3904 transistor, which will never opens.
   * This circuit typology therefore cannot work.

===Test #8 - Higher power supply voltage===

   * We clearly see from tests #3 and following that one of main issues reducing the autonomy of the BBC microbit is the voltage drop between the  power supply and the BBC microbit (about 1 V). When charging the supercap to its nominal voltage of 2.7 V, this brings us to 1.7 V at the 3V pin of the BBC microbit, at which the BBC microbit barely lights up.
   * We tried to overcharge the supercap at 4.0 V to measure how much the voltage drop across the circuit would be, but the supercap overheated quite a bit, so we stopped the experiment to prevent explosion.

===Test #9 - 1381E characterization===

   * Our power supply not going below 2.8 V, it was tricky to check the max and min activation threshold of our 1381E.
   * Using a slowly discharging 10F supercap allowed us to observe that :
     * The output of the 1381E would jump to the value of rougly 2.35 V when the input + pin of the component would reach 2.55 V.
     * The output would sink down to 0 at around 2.35 V on the input + pin.
     * The voltage drop between the + input pin voltage and the output voltage seems to be around -0.15 to -0.2 V.

====Discussion, conclusion, future work====

   * The current typology of the one-transistor version of the circuit does not give the BBC microbit a long autonomy because there is a mismatch between the nominal charge voltage of the supercap, the minimal power supply voltage of the BBC microbit, and the voltage drop across the circuit. Possible options to overcome these pitfalls include :
     * Increasing the capacity of the supercap to a higher value.
       * We already observed that going from 1F to 10F increased the autonomy of the BBC microbit about 10 folds.
     * Changing the supercap to a higher nominal charge voltage to allow for longer bursts
       * but whould we adapt the 1381x too to have appropriate triggering thresholds ?
       * this means increasing the amount of microbial fuel cells to stack ahead of the supercap to charge it...
     * Changing the transistor to have a lower voltage drop across the circuit.
       * do NPN transistors with a lower voltage drop exist ?
     * Accept the shortness of the BBC microbit bursts...
       * Which might be even shorter once the BBC microbit powers a motor, or any higher load than a LED...
   * The two-transistors version of the circuit displayed here cannot work, as the output given by the BBC microbit does not seem to go above the power supply it gets. 
     * To overcome this pitfall, an option could be to try again the two-transistor version similar to the one displayed in the 1381 solarbot circuit by Beam robotics.