Electronic Touch Triggered On/Off Switch

The circuit illustrates the function of a switch that responds sensitively by means of a touching mechanism with the utilization of two CMOS with models 4027 and 4011.

• JK Flip Flop – the most versatile of the basic flip flops because it can be configured to work as an RS flip flop, a T flip flop or a D flip flop
• CMOS 4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• CMOS 4027 – a dual JK flip flop that has independent clock, set, and reset inputs for each flip flop used in toggle, register, and control functions due to its features such as capability of driving two low power TTL loads, logic edge clocked flip flop design, logic swing independent of fanout, toggle rate of 3 MHz at 5 Vdc, supply voltage range of 3 V to 6 V, protection of diodes on all inputs, noise immunity, and quiescent current of 2 nA at 5 Vdc

One of the characteristics of CMOS is its ability to provide high resistance in its input. Although it prevents current flows, there is still a small portion of current that will pass which will constitute to the voltage drop that can be used to drive certain types of circuits. The operation of this touch switch circuit is made possible with the utilization of four NAND gates by using CMOS 4011 and a JK flip flop by using CMOS 4027. In the operation NAND gates, the output is high if either of the inputs is high, while a low output is a result of both high inputs.

The initial stage of the circuit consists of the touch switch where the logic gate U1 connected to it acts as a voltage amplifier. Other circuit like the U1 is also connected to other Schmitt trigger which is a comparator circuit that incorporates positive feedback. During the idle stage of the circuit, a logic zero is in the input of U1 through the resistor R2. With the low input, the output of U1 will be high which will cause diode D1 to conduct. Simultaneously, a high logic one will be the input to U2 through the resistor R4. On the other hand, the output of U2 will be low and will be fed to U3 which will cause a high output of U3. U4 then will have an output of low because of the high input.

As the contact T is touched by a finger, a signal with frequency of 50 Hz will be applied to U1. Contact T can be made of round or square metal surface that will not easily corrode. The input signal will be converted to square pulses with the negative half cycle period causing the diode D1 to conduct. This will then charge the capacitor via resistor R4. The time it would take to transfer the signal to the input of U2 is a few tenths of a second to prevent any creation of unwanted noise. The output of U1 will produce a positive forehead that will change the state of IC2 4027. The sensitivity of the circuit can be varied by decreasing the value of resistor R2.

Upon removing the finger on the contact, the 50 Hz signal is suppressed in the input of U1 while this pertains to the cutoff operation of D1 while capacitor C4 discharges via R3 which will eventually produce a low output on U4.

The operation of a JK flip flop is that it will not change its current state unless the contact T has been touched again. Several circuits can benefit from the output of IC2 operation which has a complemented output where the high state of pin 1 will cause a low state of the other pin 2. A trimmer TR1 and a relay RL1 is being driven by the circuit but it can also be incorporated with a triac through an optocoupler or any other circuit that can be suitable for this operation. A triac is a 3-terminal transistor that controls and conducts current flow on both alterations of an AC cycle. An optocoupler is a solid state component which utilizes a light emitting diode for transmitting light via an optical transparent barrier between two circuits. If the circuit is not in use, the input of IC2 should be low. A stabilized voltage ranging from 6 V to 15 V DC can be supplied to the circuit at around 1 mA of current.

The touch switch can be of two types, the capacitance touch switch which requires only one electrode to function and the resistance touch switch which requires two electrodes to produce physical contact to an electrically conductive material to be able to operate.

The touch switch function can be used in as a vehicular touch switch activated LED illumination module where a touch switch antenna is used for detecting the user input. Some touch switches are designed for tuning a wide range of antenna capacity. This will give the user enough freedom in application and does not restrict the touch surface to a small area or size which makes touch switches virtually limitless in its applications like the level detector, limit switch, machine stop, parts detector, and conveyor line control.

Source:users.otenet.gr/~athsam/touch_button.htm