This design is a type of indicator that is used in voltage reading. It operates as an indicator rather than a meter that drives LED for voltage indication. Voltage thresholds are 1.5, 3, 4.5 and 6V. Above each incremental threshold an additional LED turns on.
Many voltage indicators rely upon the measurement source voltage for power, but in this voltage indicator case using the MCP6L94T-E/ST, 1.5V is simply too low for satisfactory operation. Vcc is higher than what we usually see because it is necessary to have sufficient headroom to bias shunt zener regulator D3.
Vcc could have been reduced to 4.5V by dividing all the threshold reference voltages by a factor of 2, but that would preclude using the MCP6L94T-E/ST because its input common mode range is specified no lower than 1.5V from the negative rail—It would definitely not work at 0.75V. To conserve battery life a “Push to Test” pushbutton switch is used. Another way of doing it would be to use ±9V, and then it could sense voltages down to zero volts if desired. However, this would require the same number of batteries and complicates the pushbutton switch.
There are some applications that an exact value is better than providing a detailed value like in automotive. Providing reading using LEDs is simple and compact that it would fit to mobile applications. It is best suited for systems integration.
For more detail: LED DC Voltage Indicator
