Typical, see figs 1, 2; Note 1, 2

fF

28

-

1,000

Sensitivity range

S

Cs = 10nF to 100nF

A

700

Supply current, slave mode

Cs = 10nF to 100nF

A

60

Supply current, master mode

Note 1: All percentage thresholds have been eliminated and replaced with a fixed threshold w.r.t. reference level

Note 2: Sensitivity depends on value of Cx and Cs. Refer to Charts 1, 2.

The piezo acoustic driver has been removed, as the duration required to operate the beeper would interfere with

the sensing interval and slow down the device.

The device has a fixed threshold point of 6 counts of deviation. Gain pin adjustment (pin 5) has been eliminated

and replaced with a strap option for Master / Slave mode:

The sensitivity of the circuit is governed by the relative sizes of Cs and Cx. A detection is made if the signal

rises by 6 counts from the reference level; this amount, unlike the QT110, is not ratiometric to the signal level

and therefore the sensitivity can be altered by simply changing Cs. To provide a consistent level of sensitivity,

only stable types of capacitors are recommended for Cs, such as NPO, C0G, PPS film, and certain types of

polycarbonate when used over normal room temperature ranges.

Larger values of Cs will make the sensor more sensitive, while larger amounts of Cx will desensitize it (see

Charts 1, 2). Minimizing stray Cx is crucial if high levels of sensitivity are desired. By using values of Cs around

0.47uF (470nF), proximity distances of several centimeters can easily be obtained from small electrodes.

NOTE: It is extremely important to maintain stable levels of Vdd, as the supply is used as a

reference. Minor fluctuations in Vdd WILL cause false triggers or rapid swings in gain. DO NOT use

bench power supplies or supply circuits shared with other digital functions. Ordinary 78L05 class

regulators are fine in almost all cases. The QT115 is an extremely sensitive device... do not take

power supply issues lightly.

The QT115 pins are defined as shown:

Ground, 0V

Gnd

8

QT Sense pin 2

SNS2

7

QT Sense pin 1

SNS1

6

Vdd = Master mode, Gnd = Slave mode (strap option)

Master/Slave select

5

>10us, <50us negative pulse input to trigger in slave mode

In Master Mode: Connect to either Gnd or Vcc.

Sync Pulse In (slave mode only)

4

20us nominal negative sync pulse

Sync Pulse Out (master or slave mode)

3

Active-low output

Out

2

Power, +3V to +5V

Vdd

1