As soon as the battery voltage level falls below the threshold level the output of 741 goes high. This causes the Q1 to conduct and consequently the relay coil is energized and the main circuit and LED is disconnected from the battery. At the same time, the circuit shown in blue is now connected to the battery through NO contacts of the relay. The blue circuit is a flasher, built around a pair of an NPN and a PNP transistor. This flasher is connected to the same LED. The flashing rate can be adjusted by varying the values of R8 and/or C1. D3 and D4 are used for isolation.
For the flasher, Q2 should be rated 300mA or more otherwise the flasher won't work. Please email me if you don't want to disconnect the main circuit when the battery level falls.
|Circuit Exchange International||Return to Miscellaneous Circuits||http://www.zen22142.zen.co.uk|