Constant current source by op
In this project, we're going to make a constant current circuit, using the Operational Amplifier and the transistor. This circuit can maintain a constant current not just when the source voltage is changed but also when the load is changed.
Look at the schematic. When the current is about to change, the voltage across R1 changes. This is fed back to the operational amplifier. The output of the operational amplifier changes in accordance with the feedback signal, which in turn controls the base voltage of the transistor Q1 to maintain the current constant.
Let's get to the experiment.
Set the select switch to 9V(up) side first, and press S1 ON and OFF while watching the brightness of LED 1. LED 1 becomes dimmer when S1 is ON.
Now, set the select switch to 6 V, with S1 turned OFF. As you've perhaps guessed already, when S1 is turned ON and OFF, the current flows to both LED 1 and LED 2, thereby increasing the load to the circuit.
The transistor Q1 's current tends to increase, but it is held constant as explained above. Because the load increases but current doesn't, LED 1 becomes dimmer.