The following scheme is a courtesy light circuit you can use to delay an electronic device to be disabled. For example, you can connect this circuit to the lamp in your room. When you turn off the lamp with a switch on the circuit, the lamp will remain lit in a certain period of time, before the lamp really switched off.
|C4 10uF /25V
IC1 7555 or TS555CN CMos Timer IC
SW1 SPST Mains suited Switch
How the circuit work:
Due to the low current drawing, the circuit can be supplied from 220Vac mains without a transformer. Supply voltage is reduced to 10Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1 is a CMos 555 timer wired as a monostable, providing 15 seconds on-time set by R3 & C4. When SW1 is closed, IC1 output (pin 3) is permanently on, driving Triac D4 which in turn feeds the lamp. Opening SW1 operates the monostable and, after 15 seconds, pin 3 of IC1 goes low switching off the lamp.
- The circuit is wired permanently to the mains supply but current drain is negligible.
- Due to transformerless design there is no heat generation.
- The delay time can be varied changing R3 and/or C4 values.
- Taking C4=10uF, R3 increases timing with approx. 100K per second ratio. I.e. R3=1M Time=10 seconds, R3=1M8 Time=18 seconds.
- Low Gate-current Triacs are recommended.
- Use a well insulated mains-type switch for SW1.
WARNING!!! This circuit using high voltage source. Please Make sure you use a safety device before trying to activate this circuit.