EE16A Lab: Touchscreen 3WED 2-5

GSI: Andy WangLA: Ashley, Carol, Eric

Last Week: Resistive Touchscreen

✘ Touching the screen yields a voltage we can measure

✘ Have to push hard enough to make contact

✘ Need to take one measurement for each coordinate

✘ No multi-touch

Capacitive Touchscreen

✘ Touching the screen changes the capacitance✘ No moving parts✘ Multi-touch is easy

How to measure capacitance?

Measuring Capacitance

Discharge everything by closing clean switch:

Measuring Capacitance

Charge touchscreen ( + finger?) to a known voltage

Measuring Capacitance

Share charge with known capacitor:

Op Amp as a Comparator

Essentially does:if V_in > V_ref:

return V_ddelse:

return 0

✘ Student-built circuit from this semester’s EE 16B ----->

✘ Wires are straight and as short as possible

✘ IC’s (black chips) placed across middle of board so each side of pins is disconnected from the other

✘ Power supply connected to blue (-) and red (+) rails

Clean Circuits!

Notes

✘ Materials handed to you: 2 copper strips, glass slide, tape

✘ Entire circuit grounded by same ground

✘ Use multimeter to measure capacitance

✘ Peel off tape from copper strips

✘ HW7 #5 Part d

✘ Finished capacitive touchscreens on TA desk as examples