Myalarm V-LED1 Bedienungsanleitung PDF herunterladen (Seite 5)

Issue 194 September 2006

CIRCUIT CELLAR

www.circuitcellar.com

The minor exception I mentioned

earlier is power management. One of

the great things about the 802.15.4

wireless networking standard and the

protocols that build on it (e.g.,

Freescale’s Simple MAC and Zigbee) is

low-power consumption. Units can

sleep the majority of the time and

thereby reduce the average power.

Unfortunately, with the hardware pro-

vided for the Wireless Design

Challenge, there was no way to put

the accelerometers or serial port chip

on the MC13192-SARD board into

Power Down mode, so I couldn’t

implement a low-power Sleep mode.

With a custom PCB, I could have

included power control for these com-

ponents as well.

WATER LEVEL SENSOR

The water level sensor module

enables you to monitor the level of

water in a sump pump hole. You

could also use it to detect water

levels in a bathtub or hot tub.

The water level sensor is

designed around a Freescale

MPXM2010G pressure sensor. This

sensor works well because it has

on-board temperature compensa-

tion and calibration circuitry,

which allows for a simple and reli-

able design. One end of a tube is

connected to the pressure sensor

and the other is placed in the sump

pump hole. The use of a pressure

sensor gives you the flexibility to

calibrate the trip level and avoid elec-

trical contacts that may corrode when

repeatedly exposed to water.

The water level sensor design is

based on a Freescale MC13192-SARD

PCB, a MPXM2010 breakaway board

that cam

e with the contest kit, and an

additiona

l board for signal amplification

and conditioning (see Figure 2, p.33). The

signal amplification circuit is an adapta-

tion of the one described in Michelle

Clifford’s 2004 application note, “Water

Level Monitoring” (Freescale).

The Analog Devices AD8544 is

available only in a surface-mount

package, so I used two AD822 dual op-

amps instead. The AD822 is a DIP

package, which makes construction

easier, and I already had some on

hand. The AD8544 should work equal-

ly well, as should any 5-V single sup-

ply capable rail-to-rail op-amp. The

other change I made to the application

note design was to double the gain to

increase the sensitivity in the range

that is useful for this application. The

application note describes the process

of measuring the water level in a

washing machine tub, which is much

deeper than the range necessary to

detect a filling sump pump hole.

I will leave it up to you to read the

AN1950 application note for a more

detailed description of the amplifier

circuit’s operation. But let me summa-

rize it here. The pressure sensor’s out-

put is a differential signal not refer-

enced to ground. The amplifier con-

verts this differential voltage to a

ground-referenced, single-ended volt-

age appropriate for the MC9S08GT60

microcontroller’s ADC. An op-amp is

connected to each output of the sensor

to buffer the signals and add a small

offset to the positive sensor output.

The difference is amplified by a third

op-amp circuit with a gain of 1,000 to

scale the sensor’s several millivolt

range (in this application) to a range

useful for the microcontroller. A

fourth op-amp is a simple voltage fol-

lower to drive the ADC.

Five-volt power for the pressure sen-

sor an

d amplifier circuitry is obtained

by a connection to TP104 on the solder

side of the MC13192-SARD PCB.

Ground is obtained from the J101 BDM

port pin 2. The signal is connected to

the ATD2 input on pin 3 of J105.

The MC13192-SARD PCB, pressure

sensor, amplifier circuit board, and

9-V

battery

were all mounted in a black

plastic project case for protection.

Instead of a 9-V battery, power may be

supplied by an external power source,

such as the wall plug power supply

supplied with the kit, by plugging

power into the J106 power plug on the

SARD. Photo 2 shows the completed

water level sensor.

TEMPERATURE SENSOR

The temperature sensor module is

another simple design. With the high

level of hardware integration on the

MC13192-SARD PCB, all you need is

a National Semiconductor LM34 pre-

cision Fahrenheit temperature sensor.

The measurable temperature

range is 0° to 256°F. The upper

limit is the result of the 8-bit data

field in which the temperature is

transmitted to the base. And

although the LM34 can measure

below 0°, this requires a negative

supply. That would have complicat-

ed the design so it wasn’t necessary

for the application (to measure the

proper operation of an HVAC

sys-

tem or watch for temperatures that

may cause water pipes to freeze and

burst). Measurement from 0° to 256°F

is sufficient for these purposes

Connections for the LM34 sensor

are shown in Figure 3. Like the

water level sensor, 5-V power for

the temperature sensor is obtained

LM34

GND

OUT

5 V

S101

LED 1

S102

LED 2

S103

LED 3

S104

LED 4

Freescale 13192 SARD

5 V

J101 BDM

Digital/analog I/O J105

TP104

RS-232

Power

–

9-V Battery

–

J101/2

J105/3

Figure 3

—The temperature sensor’s layout is fairly simple. The

LM34 is connected to the MC13192 SARD PCB’s 5-V supply,

ground, and analog input. The pressure sensor hardware is

complete.

Photo 2

—Check out the completed water level sensor

unit. The MC13192 SARD PCB, pressure sensor PCB,

battery, and signal conditioning circuit are mounted in a

standard black project box.

2609016 Smith.qxp 8/4/2006 2:19 PM Page 34

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