Courtesy Light

15 seconds delayed switch-off, A good idea for bedroom lamps

This circuit is intended to let the user turn off a lamp by means of a switch placed far from bed, allowing him enough time to lie down before the lamp really switches off.
Obviously, users will be able to find different applications for this circuit in order to suit their needs.

Circuit diagram :

Large View

Parts :

R1 = 470R 1/2W
R2 = 100K
R3 = 1M5
R4 = 1K
C1 = 330nF-400V
C2 = 100µF-25V
C3 = 10nF-63V
C4 = 10µF-25V
C5 = 10nF-63V
D1 = 1N4007
D2 = 1N4007
D3 = BZX79C10
D4 = TIC206M
Q1 = BC557
IC1 = 7555 or TS555CN CMos Timer IC
SW1 = SPST Mains suited Switch

Circuit operation:

Due to the low current drawing, the circuit can be supplied from 230Vac 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.

Notes:

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 = 10µF, R3 increases timing with about 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! The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.

Source : www.redcircuits.com