Small Printed Circuit Board


Power supply

Power supply board

W.Bot is battery powered. It uses 8 x 1.2V NiMH AA batteries (HR6).

The power board outputs 2 different voltages: 5V, used by logic circuits, and 7.4V used by motors. Voltages and current consumptions are monitored, and informations about the power status can be obtained via the I2C bus.

The power board also controls the battery recharge process: the power station can provide two different recharge currents: 240mA for the fast recharge (when the battery level is low), and 45mA for the trickle charging. The recharging currenct is selected by the power board, and the effective recharging current is also monitored.

Power statuses

W.Bot could be in 4 different power statuses:

  • Battery powered: the status led has two fast flashes, and then a long pause
  • Connected to the power station, not charging: the status led has slow speed blinks
  • Connected to the power station, low current charging: the status led has medium speed blinks
  • Connected to the power station, not charging: the status led blink has fast speed blinks

When W.Bot is conneted to the power station, the robot is powered via the external source, and battery is disconnected from the robot. If recharge is required, the charging current will flow from the power station to the battery.
Low or high charging current is selected by the power board sending a different signal to the power station. The power station sends a feedback to the power board about the effective charging current.

Battery charge process

If the battery voltage drops below 10.5V the high current charging is selected, otherwise the low current is selected.

If the high charging current is active, the charging process is suspended every 10 minutes to check the battery voltage: when the battery is full, the low current is selected.

The battery full event is identified when the battery voltage reach a peak, and then decreases a bit.

Charging current is selected sending to the power station a different signal via the RA5 pin:

  • No current: 0Hz signal (logic level 0)
  • Low current: 1,500Hz signal
  • High current: 3,000Hz signal

The effective charging current is monitored reading the signal on the RA3 pin: the current is calculated measurng the high level of the square signal sent by the power station, where 1ms is equal to 1mA. So, if for example the length of the high level is 250ms, it means the charging current is 250mA.

PIC I/O Pins

Port Pin number Analog channel Direction Description
RA0 13 AN0 In Current on 7.4V power supply line
RA1 12 AN1 In Current on 5V power supply line
RA2 11 AN2 In Voltage on 7.4V power supply line
RA3 4 - In Charging current monitor
RA4 3 AN3 In Current on battery
RA5 2 - Out Charging current selector
RC0 10 AN4 - I2C Clock
RC1 9 AN5 - I2C Data
RC2 8 AN6 In Voltage on 5V power supply line
RC3 7 AN7 In Voltage on battery
RC4 6 - Out Charging status LED
RC5 5 - Out External power detection