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Switching cells between parallel and series Many Li-ion/LiPo charging modules are designed to handle either a single cell or multiple cells in parallel. That’s because they often run off 5V (eg, from a USB port or charger), and that’s ideal for linear charging of a single cell which ranges from about 3.3V when flat to 4.2V when fully charged. In many cases, you need two or three cells in series to get a high enough voltage to power a device. While it’s possible to generate higher voltages for charging multiple cells in series from 5V, that requires a more complex switch-mode boost converter, and they can generate EMI. When charging cells in series, you also need to consider how to keep their voltages balanced (equal). This circuit shows a much simpler solution. A single DPDT or 4PDT switch can be used to switch two or three cells between series and parallel connections, for powering a circuit and charging, respectively. When connected in parallel for charging, the cells are automatically balanced. To avoid the need to manually switch between charging mode and usage mode, a DPDT or 4PDT relay can be substituted for the switch, with the coil powered from the charging socket. A diode or similar can be used to ensure the coil cannot be energised by voltage back-fed from the battery. This way, the cells are automatically switched between the two modes. Use the relay’s NC contacts for the parallel connections (marked P) and NO contacts for the series connections (marked S). Benabadji Mohammed Salim, Oran, Algeria ($80) Editor’s note: while this should work in theory, we do not recommend this approach. That’s because any voltage imbalance which builds up in the cells as they discharge (due to differing cell capacities etc) will cause very high currents to flow as soon as the cells are switched into the parallel configuration. These currents could easily destroy or weld the switch/relay contacts. If you decide to use this configuration, we strongly recommend inserting PTC thermistors (or similar currentlimiting devices) into the connections between cells in the parallel configuration, switched out when the cells are connected in series. These can be connected inline where the red asterisks are shown. While adding such PTC thermistors will affect the rate at which the different cells charge, they should not affect the end-of-charge detection very much, so all cells should still charge fully (given enough time). The PTC thermistors should be chosen to have a ‘hold’ current rating that’s somewhat above the maximum charge current and a ‘trip’ current below the switch/ relay current rating for DC. Connecting two pushbuttons to an input-only pin Some PICs have a pin that can only be used as an input, eg, GP3/RA3 on the PIC10F200. In many cases, this is because that pin has other functions like MCLR. Sometimes you need to connect more devices to a micro than it has pins; many common techniques for making a pin dual-purpose require it to be switchable between being a digital input and output, which is not the case here. Other tricks use an internal ADC (analog-to-digital) converter with several external resistors connected as a divider to measure a different voltage generated by each pushbutton being pressed. For more on these techniques, see Microchip application note AN234 and Tips ‘n Tricks DS40040C. This simple circuit shows how the input-only GP3/RA3 can be used to 80 Silicon Chip sense two different pushbuttons being pressed, despite not being able to act as an output or an analog input. This relies on the pin having a selectable internal pull-up current, as is the case in both this chip and most of Microchip’s midrange family (the new XLP generation). According to their datasheets, the internal pull-up resistor has a typical value of about 23kW when Vdd = 5V. As GP3 is a TTL type input, we must choose a value for the external resistor so that when the internal pull-up is enabled, we have more than 2.4V as the idle voltage at GP3, giving an idle high-level state. This allows us to sense when S1 is pressed, as the pin will idle high but will go low when S1 is pressed. To sense a press of S2, the internal pull-up is periodically disabled. When we Australia’s electronics magazine disable the pull-up, the input will be pulled low by the 27-47kW resistor if S2 is not pressed or will be held high if it is pressed. The 1kW resistor avoids a short circuit between Vdd and Vss if both buttons are pressed simultaneously. Amine Houari, Oran, Algeria. ($70) siliconchip.com.au