This voltage monitor circuit is based on an Elektor design with a 555 timer IC in the book 302 Circuits, which uses two LEDs (red and green) to indicate whether the voltage is within range (bad or good). However, in practice this circuit has some shortcomings because the change in the indicated color when the voltage drops below the threshold often goes unnoticed.
The circuit described here is designed to monitor a 12-V supply voltage (such as voltage of the electrical system in a car) and signals an undervoltage situation with a blinking green LED, which is more likely to be noticed. The small red LED also lights up in case of undervoltage to provide confirmation.
The 556 IC used here contains two 555 timers. One of them detects the switching threshold, and the other provides the blinking function. The threshold voltage for the undervoltage warning can be set to the desired value with potentiometer P1.
The current consumption of the circuit depends on the type of LED used. If a low current LED is used as the blinking indicator, the value of the series resistor (here 330 ohms) must be increased significantly.