Solar Mailbox project
Francois DRUILHE - francois.druilhefree.fr
Final external Realization
The purpose of this project is to develop a self sufficient Mailbox
(real one) that will be powered only by the sun and that will display
the number of the house, but only in accordance with the battery level.
The system must work autonomously when there is or not enough light to
charge the battery.
Central Digit On, other one in PWM Modes
The Mailbox is powered by a 5V/80mA Polysilicon solar
cell. The sun energy is used to charge a 3 AA NiMH battery.
At night, when there is no light, the PIC is driving the 3 Digit
according with a sequence which is defined in its program given in
click on image to download
full resolution schematic
The Solar Cell is charging the 3 AA NiMH cell trough the “Sziklai pair”
composed by the T5 (2N2907) and T4 (1N1711). This is necessary to ensure
a very low reverse current when the sun is off and the battery at full
charge. Control of the charge can be applied on D5 with a "1" level from
the PIC , which will reverse the T6 that define the current in T5 base.
For Battery protection purpose, the value of Zener diode DZ6 must be
4.6V to prevent the battery for over-charging which will degrade
significantly its life time. This
function is not yet managed by the PIC program and is reserved for
The 3 digits are controlled by 3 separate 2N1711 (each digit is compose
about 20 white LED). The control signal is the OR between a PWM signal,
that ensure a constant background level of light plus a "blinking" part
which is the sequence generated by the PIC.
Sun_Sense: Just a low pas filter
composed of R8 and C6. Beware that leakage current from the PIC can
affect the level. This prevent R8 to be bellow 39KOhms.
Vbat_sense: These 2 diodes in serial
create a 1.3V constant voltage that can be measured by the PIC to
determine the level of the battery.
This function is not yet managed by the PIC program and is reserved
for further use.
The PIC16F628 operates with a 32.768KHz crystal oscillator. This
frequency have been selected, not to consume too much. In this
condition, the PIC is able to operate down to 3V.
Apart when the Battery is
totally low, the PIC is running and infinity loop which period is
approximately 1 second, the red led is blinking accordingly.
During day light the
signal is high and the PIC is not performing any
operation (than the 1 second blinking loop). The Green led is on. If the
battery voltage is low enough, the Solar cell is charging it. If the
Battery voltage is above 4.6V (3 times 1.3V), then the DZ6 is drawing
the current to ground protecting the battery cells.
In the future
Stop_Charge should be used.
night the SunSense signal get low and the PIC is programmed to:
Generate a PWM signal (100Hz, Duty Cycle of 5%) on the PWM pin
- Generate a
"blinking" sequence on the 3 separate control signals (1 minute period)
Download this project PDF format
Download schematic in PDF format