How many times do we use an electric switch in a day? So often that it has become secondary nature and no one keeps a count. The mechanical switch, which is flipped or pushed to make or break a connection, has been the standard all these years. But times have changed and touch switches are the new trend. Unlike the mechanical switches which are easily damaged by dirt, moisture or mishandling, touch switches are encased making them resilient to the toughest environment. The current designs of touch switches make them aesthetically pleasing and blend with the room decor.
Touch switches can be of different types
Resistive – The switch operates by measuring the change in resistance between two contacts when they are touched simultaneously. Such switches are comparatively easy to make and can be implemented by having touch plates connected to the base of a Darlington pair transistor (see project in EFY December, 2017 issue) or the gate of a N-channel Mosfet.
Piezo ceramic – Piezo touch switches sense displacement of charge due to compression of the piezo element and can be embedded in any kind of material including stiff materials like stainless steel.
Capacitive – A capacitive sensor works by detecting a change in capacitance due to the influence of an external object.
A capacitive sensor can be made on a double sided PCB by regarding one side as the touch sensor and the opposite side as the second plate of the capacitor. When voltage is applied across these plates, the two plates get charged. In the equilibrium state the plates have the same voltage as the power source. Capacitance is a measure of charge per volt (Cs=Q/V). When a finger is moved near the sensor plate, the electric field around the capacitor plate polarises the human body and energy gets transferred to it. This causes more charge to flow to the sensor plate till another equilibrium state is reached. This increases the charge concentration on the sensor plate causing a change (increase) in capacitance.
It is not necessary to physically touch the sensor plate as a disturbance of the electric field near the sensor is enough to induce a change in capacitance. The sensor plate can, therefore, be placed behind a protective overlay. The induced capacitance is determined by the equation
where, Er is the dielectric constant of the insulating plate and d its thickness. The dielectric constant of glass ranges between 3.8 and 14.5 while that of plexiglass (acrylic) is between 2.6 and 3.5 Wood has a dielectric constant between 1.4 and 2.9. As such, glass is the best material for the protective overlay followed by plexiglass.
While designing the touch sensor PCB, the following points should be kept in mind
- The ground plane plate should not overlap the sensor pad but shifted from under the touch pad. This results in lower capacitance but higher sensitivity as the electric field is projected further in the air.
- The touch pad should not be unnecessarily large as a smaller plate has a higher charge density and therefore a stronger electric field than a larger plate. A recommended size is the size of a finger tip, say 15mm * 15mm.
- The length of the track from the touch pad to the input pin of the detector should be as short as possible. The tracks should also not be parallel or cross other tracks.
- The detector circuit should have a low signal to noise ratio.
- If more than one switch is accommodated on a panel, the space between two pads should be large enough so that two adjacent touch switches are not triggered with one touch.
The detector circuit has an oscillator, the frequency of which is dependent on the capacitance of the touch pad. When a finger is moved close to the touch pad, the additional capacitance induces a change of frequency of the internal oscillator. The detector circuit tracks the oscillator frequency at timed intervals and when the shift crosses the threshold change, the circuit triggers a key press event.
Keeping the above design considerations in mind, it is better to uses a specialised detector IC. Tontek Design Technology Ltd, a Taiwanese company makes a series of touch pad detector ICs. TTP223 is a one touch key pad detector IC while TTP224 and TTP225 can manage 4 or 8 pads respectively.
The block diagram of the TTP223 IC is shown below
The equivalent connection diagram for the IC is as under
The capacitance Cs is added to adjust the sensitivity. The lower the value of Cs, the higher is the sensitivity. The range of Cs value is 0 to 50pF.