In this project, we make a constant current circuit, using an operational amplifier and a transistor. This circuit maintains a constant current even when the source voltage changes because more energy is used up in the circuit. Look at the schematic.
When the current changes, the voltage transmitted through R1 changes. The output of the operational amplifier changes according to the feedback signal from R1. This output from the amplifier controls the base voltage of transistor Q1 allowing it to maintain the constant current. Let's get to the experiment.
First set the switch to position A, and press the key while watching LED 1. It is dimmer when the key is closed. This is because both LED 1 and LED 2 are in the circuit when the key is closed. The load-the amount of energy used by the LEDs in this circuit-increases, but the current remains constant. So the LED becomes dimmer.
Now, set the switch to position Β with the key off. Do you see any change in LED brightness from position A to position B?
Setting the switch to Β changes the supply voltage from 9V to 6V. But the current remains constant again, so the LED brightness does not change.