# The capacitive touch

### The capacitive touch

The capacitive touch

C.P. Ravikumar, Texas Instruments

Anirban Chatterjee is an undergraduate intern at Texas Instruments, India. When a design contest was announced for the interns, he submitted a proposal called “DrishTI” (meaning “Sight”) for aiding a blind person to type characters into a computer.

DrishTI uses the capacitive touch booster pack for the MSP430 launchpad [1].  How can the capacitive touch help the blind? The idea is intriguing. A blind person can hold the booster pack in his/her hand and click in three areas, marked Left (L), Center (C), and Right (R). This is shown in Figure 1.  If we look at a sequence of three successive clicks, there are N = 3 x 3 x 3 = 27 possibilities; these are LLL, LLC, LLR, …, RRL, RRC, RRR.  The number of characters in the English language is 26, just one short of the number N.

DrishTI maps each unique sequence of clicks to an English character. For example, “LLL” can be “A,” “LLC” can be “B,” and so on. This is also explained in Figure 1 using the character tree. At the root of the tree is the set of 26 characters. The first click will select a subset of the alphabet; e.g. a left click will select the subset “{A, B, …I}. The next click will further divide the subset to {A,B,C} or {D,E,F}, or {G,H,I} depending on whether the user selected L, C, or R. The third click will uniquely identify the character.  I find it intriguing that L, C, and R also happen to be the symbols for the three passive elements of Electrical Engineering.

Figure 1 - The Capacitive Sense Booster Kit and the Character Resolution Tree using three clicks on the kit

Anirban demonstrated the idea by building a user interface program that works on the PC and will display the characters that are indicated by the user through a trio of clicks.

“If we use four types of clicks from the booster kit, namely, left, centre, right, and the down, we can support 4 x 4 x 4 = 64 characters. If we use five types of clicks, we can support the entire ASCII character set,” says Anirban. “I tested the system on some blindfolded users. They were able to achieve a speed of about 30 characters per minute after about 20 sessions. The number of errors made by the user reduces from 5 in the first session to Zero in the 20th session.”

You can watch a video demonstration of DrishTI at [2].

References

• DrishTI was a prize winning entry in the design contest that was conducted for summer nterns.

• thankyou this was very helpful

• how to avoid the Radio interfernce to capacitive touch

• Hi Chen,

The cap touch is designed to reduce interference to minimum.

The techniques involved are explained in the following PDF file www.ti.com/.../slaa379.pdf

There are also some test results for Radio-Frequency (RF) Emission and Susceptibility in the pdf link.