To turn fig 3 into a fully functioning nightlight, all that is required is an LED, limiting resistor and on/off switch. See fig 4.
The nightlight is switched on by closing SW1. If C1 has been previously charged, then D1 will illuminate and remain lit until the terminal voltage across C1 drops to around 3V (for a white LED). Colour LEDs for example red, will remain lit for longer because they have a lower forward voltage drop. The problem with this circuit is that if SW1 remains closed during the day, then the capacitor charging current from the joule thief will bypass the capacitor and not be stored for use at night.
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