This page relates the parallel advances of the Power of the Mud project made respectively by Paul Granjon in Cardiff and Michka Melo in Brussels between their intensive work session which took place in Cardiff in May 2017 and September 2017.

• I connected and tested the last version of our Power Harnessing System (PHS) circuit of our last working session in Cardiff.
• We saw that the energy bursts provided by the one-transistor version of the PHS circuit to the BBC microbit were quite short because of a mismatch between the nominal charge voltage of the supercap (2.7 V), the minimal power supply voltage of the BBC microbit (1.7 V), and the voltage drop across the circuit (about 1 V).
• The duration of the BBC microbit activity bursts seems to increase linearily as the capacity of the supercaps increase.
• We also observed that the two-transistor version of the PHS circuit in which the second transistor is activated by the BBC microbit itself does not seem to work, and probably never will, because it seems that the output voltage provided by the BBC microbit cannot be higher than the voltage it is supplied with.

• 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.

• 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.

• 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.

• 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).

• 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.

• 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 3 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.

• 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 8 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.

• 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.

• 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.

• 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.

• 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.
  • Would the digital output mode of the BBC microbit go higher than the analog output mode - and higher than its own input ? We can try out.

• We could also try out what happens when we change the 1381E to 1381J.

• Results to be transcribed from notes.
• (Spoiler : the KSD5041QTA is the best transistor I have for this regard.)

• The 1381J voltage detector does not trigger at 2.8 V, probably because the voltage is too low, we therefore get a 0 voltage on the 1381J output, and a 0 voltage on the BBC microbit 3V power supply pin.
• Conclusion : using the 1381J does not improve the autonomy of the BBC microbit.

• I modified the code of the BBC microbit to have set a '1' digital output on pin 1.
• When powered through USB, we obtain 3,11 V on pin 1, the same as on (analog) pin 0.
• When powered through our one-transistor circuit, we obtain 1,75 V on pin 1, the same as on (analog) pin 0, input power pin 3V being powered at 1,84 V.
  • We therefore cannot hope that the BBC microbit will open the gate of the second transistor controlling the line powering itself, as the gate voltage will always be a little bit lower than the emitter voltage in this version of a two-transistors circuit.
• Conclusion : using the digital output of the BBC microbit does not improve the autonomy of the BBC microbit.

• For some with higher nominal charge voltage - Experiment still to be performed.
• For some with higher capacity - Experiment still to be performed.
• Elak needed one week delivery to provide supercaps, so experiment cancelled for now…
• But, in the shower, a light bulb sprouted on top of my head. To increase the operation voltage of the circuit or the overall capacity, I could just pile up supercapacitors !!!

• I put two 2.7 V 10F supercapacitors in series, and charged them up to the point where the power-feeding 3V pin of the BBC microbit would be fed with 3.02 V. This means that :
  • The voltage at the output pin of the 1381E would be 3.68 V.
  • The voltage at the + pin of the power supply would be 3.72 V.
  • The voltage between the two supercaps would be at 2.65 V, just below the its nominal charge voltage of 2.7 V.
  • The voltage across one of the supercaps would only be 3.72 - 2.65 = 1.07 V, which will mean that we would not use it at its full 10F capacity.
• The BBC microbit discharged for a beautiful 45 sec before it went off.
• The BBC microbit went off because of lack of power (below 1.70 V on its 3V power-feeding pin), but not because the 1381E shut down. We checked, and the 1381E output drops to 0 only several seconds after the BBC microbit went off.
• Nota bene : this test has been performed with a KSD5041, our best transistor so far, and not with a 2N3904.
• Another measurement of voltage across each of the supercaps gave a 0.48 + 3.20 repartition, the one closer to the - pin of the power supply being the most charged.
  • The charge repartition is therefore quite different than the one measured on the first test.
  • It therefore seems quite a random repartition…
• Autonomy of blinking BBC microbit (+ max analog output on P0 and max digital output on P1) with 2N3904 is 37 sec, 46 s with KSD5041.

• I put two 2.7 V 10F supercapacitors in series, and charged them up to the usual 2.8V minimal voltage of the power supply.
• We had a brilliant 60 seconds autonomy of the blinking BBC microbit.
• As the supercaps might start to discharge already when there voltage reaches 2.55 V, which seems to be the light-up “rising” voltage of the 1381E according to our (lousy) characterization (Test #9), we measured the autonomy of the blinking BBC microbit from 2,55 V to shut-down : 30 seconds.
• Autonomy of blinking BBC microbit (+ max analog output on P0 and max digital output on P1) with 2N3904 is 13 sec, but reaches 58 s with KSD5041.

• The BBC microbit seems to light up when the voltmeter shows 2.67 V on the + pin of the power supply.

Experiment still to be performed.

Experiment still to be performed.

Experiment still to be performed.