Yes, voltage can be constant in a circuit under certain conditions. This occurs when a voltage source is connected to the circuit that maintains a stable, fixed voltage across its terminals, regardless of changes in the current flowing through it or other elements in the circuit.
One common example of a circuit with constant voltage is a battery-powered flashlight. Inside the flashlight, there is a battery that serves as the voltage source. The battery has a specific voltage rating (e.g., 1.5 volts for a typical AA battery). As long as the battery has enough charge, it will maintain a constant voltage of 1.5 volts across its terminals.
When you switch on the flashlight, the voltage supplied by the battery remains constant, and this voltage drives the current through the flashlight's circuitry, allowing the light bulb to illuminate. The current may vary depending on the resistance of the light bulb, but the battery's voltage remains unchanged during operation until the battery is depleted or turned off.
In summary, a circuit with a constant voltage source (like a battery) can maintain a constant voltage across its terminals, even if the current flowing through the circuit changes.