Electrical potentials are generated by muscle when there is movement. This Signal can be pick up by electrode to form EMG. For my Final Year Project, I did an amplifier and signal conditioning circuit for EMG. Attached below is the schematics for the Amplifier.

Disposable surface electrode are used to pick up the signal. The amplitude of the signal pick up is low, so amplification is needed to measure the detected signal. A differential amplifier is needed because of the high CMRR. 2 electrodes are connected in between the region of interest and the ground electrode is connected to a reference point. A driven feedback is implemented in the design to improve the signal quality. 2nd order active low pass and active high pass are used to further amplify the signal and filter off the unwanted part of the signal. As the design uses single supply, a 5v to -5v converter is used to power the active amplifier. Isolation amplifier and supply isolation is added as some basic safety implementation in the circuit.

Few years back, I had been working on a project for solar auto lighting. The light will turn on when the the solar panel voltage drop below a set point indicating that it is dark. The detection is done by using D2 in the schematic. The zener diode value will be adjusted so that the transition between the switch depends on the amount of light the panel receive. Relay K1 is used to switch the load and the battery. If the battery power fall below a set threshold, the relay will cut the supply to the load. Relay K2 is used to control the charging of the circuit. When a Set point is reach the charging will be cut off. A Comparator is used for the voltage comparison.