An interesting circuit suggested to indicate when a device is turned off. In the figure we have its simple configuration, which uses a general-purpose diode, a low current indicator lamp and a high value electrolytic capacitor. In operation, the capacitor is charged through the diode with the voltage of the power supply, thus remaining indefinitely and no longer consuming energy from the source. When the device is turned off, the lamp is placed in the circuit, in parallel with the capacitor. The discharge of the capacitor causes the lamp to flash or even stay on for a few seconds, indicating the device is switched off. Once the capacitor is discharged, there will be no power consumption until the device is turned on again. The lamp must have an operating voltage according to the source the system is used with. Likewise, the working voltage of the capacitor must be higher than that of the source.
