4 Best Touch Sensor Switch Circuits Explored

The post details 4 methods of building touch sensor switch circuits at home, which can be used for 220 V appliances with mere finger touch operations. The first one is a simple touch sensor switch using a single IC 4017, the second one employs a Schmidt trigger IC, the 3rd one work with a flip flop based design and there's another one which uses the IC M668. Let's learn the procedures in detail.

Using a 4017 IC for the Relay Touch Activation

Referring to the below given circuit diagram for the proposed simple touch activated relay circuit, we can see that the entire design is built around the IC 4017 which is a 10 step johnson's decade counter divider chip.

The IC basically consists of 10 outputs, starting from its pin#3 and randomly ending at pin#11, constituting 10 outputs which are designed to produce a sequencing or shifting high logics across these output pins in response to every single positive pulse applied at its pin#14.

The sequencing does not need to finish at the last pin#11, rather could be assigned to stop at any desired intermediate pinout, and revert to the first pin#3 to initiate the cycle afresh.

This is simply done by connecting the end sequence pinout with the reset pin#15 of the IC. This makes sure that whenever the sequence reaches this pinout, the cycle stops here and reverts to pin#3 which is the initial pinout for enabling a repeat cycling of the sequence in the same order.

For example in our design pin#4 which is the third pinout in the sequence can be seen attached to pin#15 of the IC, implies that as the sequence jumps from pin#3 to the next pin#2, and then to pin#4 it instantly reverts or flips back to pin#3 to enable the cycle again.

How it Works

This cycling is induced by touching the indicated touch plate which causes a positive pulse to appear at pin#14 of the IC each time it's touched.

Let's assume at power switch ON the high logic is at pin#3, this pin is not connected anywhere and is unused, while pin#2 can be seen connected with the relay driver stage, therefore at this moment the relay stays switched OFF.

As soon as the touch plate is tapped, the positive pulse at pin#14 of the IC toggles the output sequence which now jumps from pin#3 to pin#2 enabling the relay to switch ON.

The position is held fixed at this point, with the relay in the switched ON position and the connected load activated.

However as soon as the touch plate is touched again, the sequence is forced to jump from pin#2 to pin#4, which in turn prompts the IC to revert the logic back to pin#3, shutting of the relay and the load and enabling the IC back to its standby condition.

Modified Design

The above touch operated flip flop bistable circuit might show some oscillation in response to finger contact, leading to relay chattering. To eliminate this issue, the circuit should be modified as given in the following diagram.

Or you may also follow the diagram which is shown in the video.

2) Touch Sensitive Switch Circuit Using IC 4093

This second design is another accurate touch sensitive switch can be built using a single IC 4093 and a few other passive components. The shown circuit is extremely accurate and fail-proof.

The circuit is basically a flip-flop that may be triggered through manual finger touches.

Using Schmitt Trigger

The IC 4093 is a Quad 2-input NAND Gate with Schmidt trigger. Here we employ all the four gates from the IC for the proposed purpose.

How the Circuit works

Looking at the figure the circuit may be understood with the following points:

All the gates from the IC are basically configured as inverters and any input logic is transformed into an opposite signal logic at the respective outputs.

The first two gates N1 and N2 are arranged in the form of a latch, the resistor R1 looping from the output of N2 to the input of N1 becomes responsible for the desired latching action.

Transistor T1 is Darlington high gain transistor which has been incorporated for amplifying the minute signals from the finger touches.

Initially when power is switched ON due to the capacitor C1 at the input of N1, the logic at the input of N1 is pulled to ground potential making N1 and N2 feedback system latch with this input producing a negative logic at the output of N2.

The output relay driver stage is thus rendered inactive during initial power switch ON. Now suppose a finger touch is made at the base of T1, the transistor instantly conducts, driving a high logic at the input of N1 via C2, D2.

C2 charges instantly and blocks any further faulty triggers from the touch, making sure the de-bouncing effect does not disturb the operation.

The above logic high instantly flips the condition of N1/N2 which now latches to produce a positive at the output, triggering the relay drive stage and the corresponding load.

So far the operation looks pretty straightforward, however now the next finger touch should make the circuit collapse and return to its original position and for implementing this feature, N4 is employed and its role becomes truly interesting.

After the above triggering is done, C3 gradually gets charged (within seconds), bringing a logic low at the corresponding input of N3, also the other input of N3 is already held at logic low through the resistor R2, which is clamped to ground. N3 now becomes stationed in a perfect stand by position “waiting” for the next touch trigger at the input.

Now suppose the next subsequent finger touch is made at the input of T1, another positive trigger is released at the input of N1 via C2, however it does not produce any influence over N1 and N2 as they are already latched in response with the earlier input positive trigger.

Now, the second input of N3 which is also connected to receive the input trigger via C2 instantly gets a positive pulse at the connected input.

At this instant both the inputs of N3 goes high. This generates a logic low level at the output of N3. This logic low immediately pulls the input of N1 to ground via the diode D2, breaking the latch position of N1 and N2. This causes the output of N2 to become low, switching OFF the relay driver and the corresponding load. We are back into the original condition and circuit now waits for the next subsequent touch trigger in order to repeat the cycle.

Parts List

Parts required for making a simple touch sensitive switch circuit.

• R1, R2 = 100K,
• R6 = 1K
• R3, R5 = 2M2,
• R4 = 10K,
• C1 = 100uF/25V
• C2, C3 = 0.22uF
• D1, D2, D3 = 1N4148,
• N1---N4 = IC 4093,
• T1 = 8050,
• T2 = BC547
• Relay = 12 volts, SPDT

The above design can be further simplified using just a couple of NAND gates, and a relay ON OFF circuit. The entire design can be witnessed in the following diagram:

3) 220V Electronic Touch Switch Circuit

It may be now possible to convert your existing mains 220V light switch circuit with the electronic touch switch circuit explained in this post. This third idea is built around the chip M668 and it employs just a handful of other components for implementing the proposed  mains touch switch ON/OFF application.

How this simple mains electronic touch switch circuit works

The indicated 4 diodes form the basic bridge diode network, the thyristor is used for switching the mains 220V AC for the load, while the IC M668 is used for processing the ON/OFF latching actions whenever the touch switch is touched.

The bridge network rectifies the AC into DC through R1 which limits the AC current to safe level for the circuit, and VD5 regulates the DC suitably. The final outcome is a rectified, stabilized 6V DC which is applied to the touch circuit for the operations.

The touch plate is connected with a current limiting network using R7/R8 so that no shock sensation is felt by the user while putting finger on this touch pad.

The various pinout functions of the IC can be learned from the following points:

The supply positive is applied to pin#8 and ground to pin#1 (negative) The touch signal on the touch pad is sent to pin#2, and the logic is transformed into an ON or OFF at the output pin#7.

This signal from pin#7 subsequently drives the SCR and the connected load into either ON or OFF states.

C3 makes sure that the SCR is not false triggered due to multiple pulses in response to an improper or inadequate touching on the touch pad. R4 and C2 forms an oscillator stage for enabling the required processing of the signals within the IC.

A synchronization signal from R2/R5 is divided internal through pin#5 of the IC. Pin#4 of the IC has a very crucial and interesting function. When connected with the positive line or Vcc, the IC enables the output to alternately toggle ON/OFF, allowing the light or the load to switch ON and OFF alternately in response to every touch on the touch pad.

However when pin#4 is connected to the ground or the negative line Vss, it transforms the IC into a 4 stage dimmer circuit.

Meaning in this position every touch on the touch pad causes the load ( a lamp for example) to reduce or increase its intensity sequentially, in a gradually dimming or gradually brightening manner ( and OFF at the ends). If you have any questions regarding the functioning of the above discussed mains touch switch circuit please write them down through the comment box...

4) Touch Activated Lamp Circuit with Delay Timer

The fourth design is a transformerless touch activated 220V delay lamp switch circuit enables the user to momentarily switch ON a table lamp or any other desired bed lamp during night time.

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.

Touch using a Single MOSFET

Just a single MOSFET and few additional passive parts are all that's needed to build this touch sensor circuit. The MOSFET gate can be seen connected with a 22M resistor, and a touch sensor built using a PCB with copper mesh on it.

With the indicated 22 M resistor, even breathing on the sensor mesh will be enough for the MOSFET to turn OFF momentarily. If you find this sensitivity to high, you can reduce the 22M to 10M then it will allow the MOSFET to turn OFF with a finger touch on the copper sensor board.

During the absence of a touch on the sensor, the MOSFET remains in the switched ON condition, due to the positive voltage arriving from R1. During this period the base voltage to Q2 via R2 remains grounded through the MOSFET drain which causes the Q2 transistor to remain switched OFF. With Q2 switched OFF, the relay also remains switched OFF.

As soon as the sensor plate is touched, it instantly causes a grounding of the R1 voltage through the finger, forcing the MOSFET to turn OFF.

When the MOSFET is turned OFF, the Q2 transistor gets the access to the potential from the R2, and it turns ON now, When Q2 turns ON, the relay coil gets the required amount of power, and it also turns ON, switching ON any load that may have been configured across its contacts.

Removing the finger from the touch sensor, restores the circuit to its original form, and the relay switches OFF.

Using IC 4049

A very neat little touch switch circuit can be built using a single CMOS IC 4049 which contains 6 NOT gates. Therefore, by using six NOT gates, an SCR, and a few passive parts we can make a precision device that will respond to touch signals without any spurious results.

The complete working of the above 4049 based circuit can be studies through the following instructions

PCB Design

Using IC 4011

Working with just one 1 / 2 of an IC 4011, along with a few general purpose BJTs, a touch operated switch could be designed which can be well suited for numerous battery powered circuits. Considering that all the inputs to the leftover gates of the IC 4011 are connected to the ground line, the current consumed by the IC in the off state is practically zero which means the battery life will be long and unaffected.

When the 'on' contacts are touched with a finger causes pin 3 to become high, which turns on the darlington pair and power is supplied to the load. When the 'off' contacts are touched, the opposite happens, and the action switches OFF the load.

Q1 needs to be a high gain transistor, and Q2 picked out according to the current specifications of the load