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


  1. Simple and easy to design: Discrete voltage transition indicator circuits typically use only a few components, making them relatively simple and easy to design.
  2. Low cost: Because they use only a few components, these circuits are generally less expensive than more complex voltage indicator circuits.
  3. Customizable: Discrete voltage transition indicator circuits can be easily customized to suit specific requirements, such as indicating the number of voltage levels.
  4. Reliable: These circuits are generally more reliable than more complex voltage indicators because they have fewer components that could fail.


  1. Limited functionality: Discrete voltage transition indicator circuits typically provide only a limited amount of information about the voltage level, such as whether it is above or below a certain threshold.
  2. Limited accuracy: The accuracy of these circuits is generally limited by the accuracy of the voltage divider used in the circuit.
  3. Limited range: These circuits may not be suitable for applications that require a wide range of voltage levels to be indicated.
  4. Limited scalability: Discrete voltage transition indicator circuits may not be easily scalable to handle a large number of voltage levels, as this would require more components and increase the complexity and cost of the circuit.

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