The circuit is designed using an N-channel enhancement-mode field effect transistor BS170 (T1). It does not draw any power when inactive and less than 15 microamperes while functioning. A car key 12V fob battery would be an ideal choice in terms of simplicity.
Momentarily pushing switch S2 charges capacitor C1 rapidly via resistor R1. When the voltage on capacitor C1 exceeds the threshold voltage at the gate of T1, it starts charging the reservoir capacitor C2. Transistor T1 remains in conducting state as long as the voltage on C1 is greater than FET gate’s threshold voltage. The time period of the alarm depends on the value of capacitor C1 and resistor R2, which governs the capacitor’s discharge current.
The component values provided here give an on-time of about ten minutes. You can increase or decrease the time period of alarm by changing the values of capacitor C1 or resistor R1.
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Press switch S2 along with your electrical appliance’s switch to enable the alert alarm. Switch S1 is power-on switch.