Temperature Monitor Circuit
This Simple Temperature Monitor Circuit gives an indication using an LED when the battery voltage falls below 9 volts. This circuit is ideal to monitor the level of charge in 12V small batteries. These batteries are used in burglar alarm systems and portable devices. The working of this circuit depends on the biasing of the base terminal of the T1 transistor.
Simple Temperature Monitor Circuit
When the voltage of the battery is more than 9 volts, then the voltage on base-emitter terminals will be same. This keeps both transistors and LED off. When the voltage of the battery reduces below 9V due to utilization, the base voltage of T1 transistor falls while its emitter voltage remains same since the C1 capacitor is fully charged. At this stage, the base terminal of the T1 transistor becomes +ve and turns ON. The C1 capacitor discharges through the LED