QT301 Touch Sensor
Extended sensor report from Min by Sol Huh.April 18, 2006

I bought this QProx QT301 from DigiKey for about 4 dollors. Since Min explained alot about this sensor, I'm not going to introduce the information that she already mentioned.

QT301 data sheet

Note: On VDD(8), 0.1nF capasitor is required. For Cs, 0.1nF capasitor is a good start before testing different size of capasitors.When Cs is small, the sensor gets more sensitive but unstable.
When calibrating, for the upper calibration, hold the electrode tight and connect the switch for 1 second. For lower calibration, do not touch the electrode and swtich on for a second.
I made a mistake understanding Cx at the begginning. I thought the CX is a distance of the electrode and finger when it is touched, but it was actually the distance between the finger and the electrode when it is not touched and it goes to the ground.

PWM & Low Pass Filter

Output of many chips is PWM because it can have similar output with analog in the digital IC. For example, digital IC cannot get the 2.5V output. In this case, give the sign below.

The average is 2.5. This averaging filter is low pass filter, also known as integrator.

Integrator adds the signal.

Cut Off frequency (Hz)=1/2pi *Rc = 100/2pi = 15Hz
QT301 has 100kHz which is much bigger than 15 Hz.
It cannot pass but it adds to the effective voltage.

Low voltage=0, high voltage=5, Average 2.5

Detail

LM2904 Op Amp

Issue1. Electrode Design

How to control the detecting direction which goes every direction?

Experiment

Measure the amount of water in the cup. The sensor must detect only water in the cup. When I hold the cup, the sensor should not detect my hand.

How can I do?

Reference from QT320

The electrode spread out its field to the all direction. Field must come out to the inside of the cup where the water is.
Place the sheilding material(metal) on the direction where the sensor should not detect. Then connect the metal to the ground.
I attatched the sheiding material(metal) to the outside of the cup and connected to the ground.

Planning
1. Coopper tape on the outside of the cup.
2. Wrapping up the paper tape
3.
4. Surrounding metal is connected to the ground.
5.

Result

When the cup is full of water, voltage is 5V. When it is empty, the voltage is 0. The level of water and the voltage in between was unlinear.

Application for the other touch sensors

Systems using lower voltage is recommended t use QT118. It the voltage doesn't matter, using QT113 is recommended. QT112: Fast responding use, such as a joystick for game machine. QT113:Very sensitive When PCB is stable and voltage is 5, it senses about 3 inches. Voltage uses is 900uA , the highest. QT115: In the several QT115 sensors, it can pass only the highest one signal by composing. QT118: It has direct PIEZO connection pin. It's good for a battery using system.

Link to the other project from somewhere