Solar Mailbox project

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. : The Solar Cell is charging the 3 AA NiMH cell
Solar Mailbox project - schematic

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 further use. : 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.

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