On-Duration expires, whichever occurs first. If the latter occurs

first, the sensor performs a full recalibration and the output

becomes inactive until the next detection.

In this mode, two Max On-Duration timeouts are available: 10

and 60 seconds.

This makes the sensor respond in an on/off mode like a flip

flop. It is most useful for controlling power loads, for example in

kitchen appliances, power tools, light switches, etc.

Max On-Duration in Toggle mode is fixed at 10 seconds. When

a timeout occurs, the sensor recalibrates but leaves the output

state unchanged.

This mode generates a negative pulse of 75ms duration with

every new detection. It is most useful for 2-wire operation, but

can also be used when bussing together several devices onto

a common output line with the help of steering diodes or logic

gates, in order to control a common load from several places.

Max On-Duration is fixed at 10 seconds if in Pulse output

mode.

Note that the beeper drive does not operate in Pulse mode.

A piezo drive signal is generated for use with a piezo sounder

immediately after a detection is made; the tone lasts for a

nominal 95ms to create a ‘tactile feedback’ sound.

The sensor drives the piezo using an H-bridge configuration for

the highest possible sound level. The piezo is connected

across pins SNS1 and SNS2 in place of Cs or in addition to a

parallel Cs capacitor. The piezo sounder should be selected to

have a peak acoustic output in the 3.5kHz to 4.5kHz region.

Since piezo sounders are merely high-K ceramic capacitors,

the sounder will double as the Cs capacitor, and the piezo's

metal disc can even act as the sensing electrode. Piezo

transducer capacitances typically range from 6nF to 30nF in

value; at the lower end of this range an additional capacitor

should be added to bring the total Cs across SNS1 and SNS2

to at least 10nF, or possibly more if Cx is above 5pF

Piezo sounders have very high, uncharacterized thermal

coefficients and should not be used if fast temperature swings

are anticipated, especially at high gains. They are also

generally unstable at high gains; even if the total value of Cs is

largely from an added capacitor the piezo can cause periodic

false detections.

The burst acquisition process induces a small but audible

voltage step across the piezo resonator, which occurs when

SNS1 and SNS2 rapidly discharge residual voltage stored on

the resonator. The resulting slight clicking sound can be greatly

reduced by placing a 470K resistor Rs in parallel with the

resonator; this acts to slowly discharge the resonator,

attenuating of the harmonic-rich audible step (Figure 2-3).

Note that the piezo drive does not operate in Pulse mode.

The output has a full-time HeartBeat™ ‘health’ indicator

superimposed on it. This operates by taking 'Out' into a 3-state

mode for 350µs once before every QT burst. This output state

can be used to determine that the sensor is operating properly,

or, it can be ignored using one of several simple methods.

The HeartBeat indicator can be sampled by using a pulldown

resistor on Out, and feeding the resulting negative-going pulse

into a counter, flip flop, one-shot, or other circuit. Since Out is

normally high, a pulldown resistor will create negative

HeartBeat pulses (Figure 2-4) when the sensor is not detecting

an object; when detecting an object, the output will remain

active for the duration of the detection, and no HeartBeat pulse

will be evident.

If the sensor is wired to a microcontroller as shown in Figure

2-5, the controller can reconfigure the load resistor to either

ground or Vcc depending on the output state of the device, so

that the pulses are evident in either state.

Electromechanical devices will usually ignore this short pulse.

The pulse also has too low a duty cycle to visibly activate

LED’s. It can be filtered completely if desired, by adding an RC

timeconstant to filter the output, or if interfacing directly and

only to a high-impedance CMOS input, by doing nothing or at

most adding a small non-critical capacitor from Out to ground

(Figure 2-6).

The QT110’s output is active low ; it can source 1mA or sink

5mA of non-inductive current.

Care should be taken when the IC and the load are both

powered from the same supply, and the supply is minimally

regulated. The device derives its internal references from the

power supply, and sensitivity shifts can occur with changes in

Vdd, as happens when loads are switched on. This can induce

detection ‘cycling’, whereby an object is detected, the load is

turned on, the supply sags, the detection is no longer sensed,

5

QT110 R1.04/0405

Figure 2-2 Powering From a CMOS Port Pin

0.01µF

CMOS

microcontroller

OUT

P O RT X .m

P O RT X .n

Vdd

Vss

QT110

3

46

5

1

7

2

OUT

OPT2

GAIN

SNS2

SNS1

Vss

Vdd

8

OPT1

SENSING

ELECTRODE

Rs

+2.5 ~ +5

10s

Vdd

Gnd

Pulse

10s

Gnd

Gnd

Toggle

60s

Gnd

Vdd

DC Out

10s

Vdd

Vdd

DC Out

Max On-

Duration

Tie

Pin 4 to:

Tie

Pin 3 to:

Table 2-1 Output Mode Strap Options