Here is a simple circuit to monitor the voltage level in Solar panels. Usually a volt meter test is done during the installation of solar panel and then after, the panel remains unattended. If the solar panel is not providing sufficient voltage for charging the battery, that will permanently destroy it. A 12 volt Solar panel gives maximum of 22 volts DC in bright sunlight, which drops to around 16 volts when the load is connected. So a voltage level around 16 volts is necessary for charging the 12 volt Lead Acid battery. But the voltage output from the solar panel may reduce due to the accumulation of dust on the panel and corrosion of terminal contacts. These remain unnoticed ,so only when the battery fails ,we note this. This simple circuit solves the problem. If the solar panel gives above 15 volts with the battery connected, the LED remains off. When the voltage from solar panel drops below 15 volts, LED turns on to indicate low voltage. This usually happens when the sunlight is low, but if the LED remains on in bright sunlight also, it indicates low output from the solar panel. The circuit can also be used as Battery level monitor.
Working of the circuit is simple. IC1 ( uA 741 ) is wired as a voltage comparator. Its inverting input (Pin 2) gets a Reference voltage of 15 V provided by the Zener diode ZD. At the same time, the Non Inverting input (Pin 3) gets a variable voltage through VR. So if the voltage from the solar panel is above 16 volts, Zener conducts and provides 15 volts to the Inverting input. The voltage at Pin3 is set slightly higher than 15 volts by adjusting VR. In this condition, output of IC1 will be high because pin 3 of the comparator gets higher voltage than pin 2. LED is connected from the positive rail to the output of IC1. So both the sides of LED will be high and it remains off.
When the voltage from the solar panel reduces below the level set by VR, Voltage at pin 3 drops below pin 2 and the output of IC1 turns low and LED lights due to the current sinking in to the output of IC. Diode D1 separates the circuit from battery so that, it indicates only the solar panel voltage.Connect this circuit to the output of solar panel observing correct polarity.
The same circuit can also be used as Voltage level monitor for different types of batteries. Select appropriate value of Zener as shown in the table below.