Battery Charger LM393
The project makes the USB port a convenient source of power for devices such as battery charger for a pair of AA Nickel Metal Hydride (NiMH) and Nickel Cadmium (NiCd) cells.
The USB standard specifies that a device may not use more than 100mA until it has negotiated the right to use 500mA, but it seems that no USB ports enforce this requirement because any USB port can supply 5V at up to 500mA.
Two AA NiMH or NiCd cells of any capacity at about 470mA are being charged by this project in about 1½ hours while giving 700mAh for NiCds and about 3½ hours while giving 1500mAh for NiMHs. An automatic charge cut-off circuit is incorporated by the charger based on cell temperature. This makes the cells be left in the charger indefinitely after cut-off.
Battery Charger LM393 Circuit
One half of an LM393 dual voltage comparator is the heart of the charger where the output can be in one of two states either floating or low. The cells being charged are in direct contact with a thermistor with a resistance that varies with temperature. The progress of charging process is indicated by an LED.