Building the pretty straight forward light harvester described in a previous post, I learned that that design is somewhat passive: the harvester just charges the whole day and then discharges for 1-2h (depending on the capacitor) once it gets dark.

To explore a more active harvester that periodically emits energy whenever the capacitor is charged, I reimplemented the VxSE3 solar engine by WLF Rigter: Reproduced here with my specific adaptations:

- used a green LED with forward voltage of 3.2V
- a solar panel with 3V output (max 4V)
- \(10\mu F\) storage capacitors

The circuit above is pretty clear for the charging cycle: the capacitors are charged in parallel each through its own diode. I used Schottky diodes with a forward drop of 0.4V. Given the 3V solar cell this means they will each be charged to about \(U_{c0} = U_{c1} = 2.6V\) .

Once the capacitors reach their full charge the transistor \(T_0\) goes into conduction. We can see that by looking at the voltage \(U_{BE0}\) :

$$ U_{BE0} = U_{c0} - U_{d1} - U_{d2} - U_{R1} = 2.6V - 1.8V - U_{R1} = 0.8V - U_{R1} $$which is \(>0.7V\) since the resistor just acts to limit the current into the transistor.

With \(T_0\) saturated and in conduction it is instructive to look at this re-drawing of the circuit to understand the voltage doubling feature of this circuit:

As soon as \(T_0\) is in conduction, the output of the green LED \(D_3\) is pulled to the potential of the lower capacitor \(C_1\) . The base of the pnp transistor \(T_1\) is pulled to the same potential as well. The saturates \(T_1\) as can be seen from the base-emitter voltage of \(T_1\) :

$$U_{BE1} = -U_{R0} - U_{c1} + U_{BE0} \approx -2.4V -U_{R0} < -0.7V $$The saturated npn transistor \(T_1\) then connects the two capacitors leading to an approximate doubling of the input voltage. More precisely, the following voltage is applied over the green LED and its resistor:

$$ U_{D3} + U_{R2} = U_{c0} + U_{c1} - U_{CE1} - U_{CE0} \approx 2 U_{c0} - 0.4V = 4.8V $$This doubling of the voltage reminds of a charge pump circuit.

The resistor \(R_2\) limits the discharge current through the green LED \(D_3\) as usual, in the most simple LED circuit. As the capacitors discharge the voltage eventually drops too low to keep transistor \(T_0\) in conduction and the circuit resets to charging mode.

The initial version used the parts as described in the original engine:

- \(R_0=4.7k\Omega, R_1=4.7k\Omega, R_2=68\Omega, C_0=C_1=1mF\) .
- \(D_0=D_1=\) Schottky
- \(T_0=\) 2N3904 and \(T_1=\) 2N3906
- \(D_2=\) low power red LED
- \(D_3=\) one ultra-bright green LED.
- 4V max - rated for 3V solar cell.