Michka Mélo, September 13th-14th, 2017

• 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.

• 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 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.

• 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.

• 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.

Explain why the second 2N3904 does not seem to light up. Tell the values of voltage drops

* Try out with higher