Handy voltage transition indicator circuit
You simply need a simple way to test the voltage on battery-operated circuits. On the other hand, complicated circuits don’t look very attractive. So to keep things simple and low-cost, Einar Abell suggests his single transistor voltage indicator circuit. It is able to detect the transition between two voltage levels on battery. Simply speaking, if the circuit is powered from 9V battery, then it will transition from green to red indicator when voltage foes from 7.1V to 5.8V. One downside of this indicator is that it drains about 1mA of current. For any battery, this is nearly not acceptable. To avoid constant drainage, drop a push button to test voltage when needed. Pros and cons of discrete voltage transition indicator circuits Pros: Cons:
