By Tina Kamat
How the circuit Works.
Referring to the circuit above, the four diodes at the input form the basic bridge rectifier circuit for rectifying the mains AC into DC. This rectified DC is stabilized by the 12V zener and filtered by C2 to acquire a fairly clean DC for the accompanying touch switch circuit.
R5 is used for limiting the input mains current to a much lower level suitable for operating the circuit safely.
An LED can be seen connected with this supply which ensures a dim light is always ON near the circuit for facilitating quick location of the touch switch pad.
The IC used in this transformers touch lamp with delay circuit is a double D flip-flip IC 4013, which has 2 flip flop stages built inside it, here we make use of one of these stages for our application.
Whenever the indicated touch pad is touched by finger, our body offers a leakage current on the point causing a momentary high logic on pin#3 of the IC, which in turn causes the pin#1 of the IC to go high.
When this happen the attached triac is triggered via R4, and the bridge rectifier completes its cycle powering the series lamp. The lamp now illuminates brightly.
Also in the meantime, the capacitor C1 gradually starts charging via R3, and when it gets fully charged pin#4 is rendered with a high logic which resets the flip flop in its original condition. This instantly turns pin#1 low switching OFF the SCR and the lamp.
The value of the R3/C1 produces a delay of about 1 minute, this can be increased or decreased by suitably increasing or decreasing the values of these two RC components as per individual preference.