Electromechanical devices like relays will usually ignore this

short pulse. The pulse also has too low a duty cycle to visibly

affect 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 2-5).

The QT113H variant has an active-high output; the heartbeat

signal of the QT113H works in exactly the same manner.

The QT113’s `output is active low and can sink up to 5mA of

non-inductive current. If an inductive load is used, such as a

small relay, the load should be diode clamped to prevent

damage. When set to operate in a proximity mode (at high

gain) the current should be limited to 1mA to prevent gain

shifting side effects from occurring, which happens when the

load current creates voltage drops on the die and bonding

wires; these small shifts can materially influence the signal

level to cause detection instability as described below.

Care should be taken when the QT113 and the load are both

powered from the same supply, and the supply is minimally

regulated. The QT113 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, the load is turned off, the supply rises and the object

is reacquired, ad infinitum. To prevent this occurrence, the

output should only be lightly loaded if the device is operated

from

an

unregulated

supply,

e.g.

batteries.

Detection

‘stiction’, the opposite effect, can occur if a load is shed when

Out is active.

The output of the QT113 can directly drive a resistively

limited LED. The LED should be connected with its cathode

to the output and its anode towards Vcc, so that it lights when

the sensor is active. If desired the LED can be connected

from Out to ground, and driven on when the sensor is

inactive.

The QT113H variant has an active-high output.

3 - CIRCUIT GUIDELINES

3.1 SAMPLE CAPACITOR

Charge sampler Cs can be virtually any plastic film or

medium-K ceramic capacitor. The acceptable Cs range is

from 10nF to 500nF depending on the sensitivity required;

larger values of Cs demand higher stability to ensure reliable

sensing. Acceptable capacitor types include polycarbonate,

PPS film, or NPO/C0G ceramic.

3.2 OPTION STRAPPING

The option pins Opt1 and Opt2 should never be left floating.

If they are floated, the device will draw excess power and the

options will not be properly read on powerup. Intentionally,

there are no pullup resistors on these lines, since pullup

resistors add to power drain if tied low.

The Gain input should be connected to either Vdd or Gnd.

Tables 1-1 and 2-1 show the option strap configurations

available.

3.4 POWER SUPPLY, PCB LAYOUT

The power supply can range from 2.5 to 5.0 volts. At 3 volts

current drain averages less than 600µA in most cases, but

can be higher if Cs is large. Increasing Cx values will actually

decrease power drain. Operation can be from batteries, but

be cautious about loads causing supply droop (see Output

Drive, previous section).

As battery voltage sags with use or fluctuates slowly with

temperature, the QT113 will track and compensate for these

changes automatically with only minor changes in sensitivity.

If the power supply is shared with another electronic system,

care should be taken to assure that the supply is free of

- 6 -

Figure 2-4

Using a micro to obtain HB pulses in either output state

Figure 2-3

Getting HearBeat pulses with a pull-down resistor

3

46

5

1

+2 .5 to 5

7

2

OUT

OPT 1

OPT 2

GAIN

SNS1

SNS2

Vss

Vd d

8

Ro

H eartBeat™ P u lses

Microcontroller

PORT_M.x

PORT_M.y

3

46

5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Figure 2-5 Eliminating HB Pulses

3

46

5

7

2

OUT

OPT 1

OPT 2

GA IN

SN S 1

SN S 2

CM O S

100p F

G AT E OR

MIC RO INPU T