How interesting it will be if the lights come on when we touch a spot and goes off when we touch that again. This also does away with the mechanical contacts, switches and we can control different appliances from a single place. Here we have circuit for four individual touch switches, which can be used for different applications such as lights, fans, etc. With one touch, the relay fires, contacts close and another touch opens the contacts. They are fashionable, concealed and replace bulky switches. It adds a novelty and beauty in the interior decoration. As many as four switches can be configured from a single IC. Touch sensors could be just small dots of conducting plates, say copper, just three dots for each sensor. Relay contacts can be used in place of fan regulator contacts to make a touch control of fan speed.
Using CMOS device for the job simplifies the design, as they can be triggered at extremely low currents. Their normal current consumption is also very low making them ideal for continuous operation. CD 4066 is such an interesting CMOS switch IC, which has four individual bilateral switches inside the package, each capable of switching analogue or digital signals. Pin assignment
of CD 4066 is given in Figure 29.
Take for instance one single switch (A) at Pins 1,2 and 13. Pins 1 and 2 are switch contacts ana iJini3 is the control terminal. As long as the control terminal (13) is low, there will he infinite resistance between 1 and 2 pins, which means that the circuit is open. If the 13 Pin goes high, the contacts close and there will be negligible resistance across them. If it goes low, the contacts open again.
Switches can operate at high speed up to a minimum frequency of 40 MHz. However they can only handle 10mA of current, which must necessarily be amplified for higher currents. All the other three switches at (B) Pins 3,4 and 5, (C) 6,9 and 8, and (D) 12,10 and 11 are similar and independent. Being CMOS devices they need extremely small currents to operate the switches (10nA is typical). This feature is made use of in making the touch switches. We can make use of a single IC to control four different switches. All the four switches act similarly and independently.
Touch plates are three contact plates, which can be very small dots of 1mm copper but adequately spaced. One dot is connected to positive supply rail and center one is connected to the control terminal through a 100K resistance (R3, R7, Rll, and R15). Third one is connected to the ground.
Now when fingers touch control dot and positive dot, the control terminal goes high and brings the required switch into conduction. Control switch is kept in high state by a feedback of 10M resistor (R4, R8, R12, and R16). Switch now drives a transistor (Qv Q2, Q3, and Q4), which fires a corresponding relay. LEDs (Dl, D3, D5, and D7) show which of the switch has been closed. But now when we touch the control dot and ground dot, control pin is pulled down and switch goes off. Control pin stays low with the feedback.
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