WIRELESS MFI MONITORING NETWORK
Hardware Description
Introduction
The test module for the wireless monitoring network incorporates systems for impedance
measurement, communication, high voltage supply, rail power supply and user input for
programming. It consists of three PCBs that each support one or more of these functions.
This description will address each of these circuits and illuminate how each operates within
the system.
Andrew P. Bishop
andrew@brertechnical.com
MCU:
The MKL15Z32VLH4 is the primary computing system that directly controls
almost every part of the test module. It’ll be responsible for gathering all
information from the sensors, processing it and passing it wirelessly to the Zigbee
network.
The only thing the MCU doesn’t directly control is the PWM signal that drives
the boost converter channels, this is controlled by a separate switching converter
controller (MAX1771). It does however communicate by SPI (SPI1) with a digital
potentiometer that acts as gain control for this controller. It selects the boost
converters channels using 6 GPIO pins (PTC0-5) and receives a linearly attenuated
voltage from the output of whatever channel is selected to an ADC (ADC0_SE0).
The MCU has an attached 32.768kHz crystal oscillator for driving an RTC
that’ll track the duration of tests and send that information back with other data
collected. Pins 17, 20 & 26 will be connected to a 2x5 header that has a pitch of
1.25mm x 1.25mm. This’ll be for programming the MCU.
The Zigbee transceiver on a separate PCB will connect through a header to
the other SPI port (SPI0) on the MCU. This header is identical the programming
header.
The MCU uses GPIOs (PTB0-3) to control RC selection for the measurement
circuit. This outputs a variable DC voltage level into a second ADC (ADC0_SE1)
through an instrumentation amp that’ll be explained later. This recovered value,
matched with the value obtained from the boost converter output allows the MCU to
predict the unknown impedance that is above the measurement system.
A thermometer (MCP9700) is attached to ADC0_SE23 so the unit can
estimate environmental temperature. This could potentially be used to sense for fire
as an added potential.
Measurement Systems:
This circuit is essentially the bottom half of a voltage divider that can adjust
its impedance between four values (1k, 10k, 100k, 1M). It uses relays driven by the
MCU through MOSFETS to open each impedance value one at a time. These resistors
are called RC. The signal above RC but below the unknown load is obtained using an
instrumentation amplifier (INA333) with a gain of 2. This amp outputs to the MCU’s
ADC and inputs from the voltage divider. Since the minimum output of the boost is
12v and the maximum input of the INA333 is 3.3v. There is a possibility of damaging
this amp if the unknown load is a short. A resistor (R13) is placed in series with the
unknown load that is large enough (2.87k) to prevent the voltage into the IN333
from exceeding 3.3v if the boost is at 12v. The MCU can then ensure that the voltage
into the INA333 will not break it by starting with VB0 = 12v & RC = 1k.
The second measurement system is a thermometer that will send an analog
output voltage to the MCU’s ADC0_SE23. It’ll power off the 3.3v rail that is disabled
in sleep mode so it won’t draw power.
Wireless System:
The Zigbee transceiver is set up in a simple configuration. It sits on its own
PCB alongside an antenna cut out of the same board, a header for SPI
communication with the MCU, and a lot of decoupling caps. The antenna uses both a
N & T configuration of caps and inductors for both filtering and impedance
matching. The IC runs off a 20MHz crystal. The header is a 2x5 pin with a pitch of
1.25mm x 1.25mm.
Power Sources:
This entire module is powered by two 9v batteries whose voltage is
regulated by two LDOs and 6W boost converter. The boost converter will output a
constant 12v source for the high voltage boost converter and will include an enable
pin so the MCU can disable both boost converters entirely. This will handle all
analog systems. The LDOs will power all digital circuits by outputting 3.3v. The
reason that there must be two is because one has to have extremely low quiescent
current and the other must be capable of deliviering high current. One will be
disabled by the MCU during sleep and the other will be running all the time. The
smaller one will have a low max output current and will power only the MCU. All
other 3.3v circuits will be powered by the other LDO which will include an enable
pin that the MCU can use to disable it.
High Voltage Source:
This circuit is an adjustable, multichannel 12v to 12-500v boost converter
controlled by six GPIOs and an SPI signal. The six channels are selected through the
GPIOs and the SPI controls gain of the channel selected. All the channels use the
same feedback amplification and control system but have their own separate base
components like diodes, capacitors and inductors. To isolate a single channel, you
have to terminate the others at the power source and at the feedback loop. This
circuit uses a p-ch/n-ch mosfet switch configuration to disable power at each
channel source, then an analog switch to select which feedback reaches the
controller. This same feedback is also sent to the MCU because it’s linearly
proportional to the boost high voltage output. The high voltage from all the channels
output through a 2x3 header to the separate MFI sites. The feedback loop consist of
a non-inverting amplifier circuit that will apply gain to the controller IC depending
on the impedance of the digital potentiometer that is part of the amps own feedback
loop. The pot can adjust between 0-100k but it’ll only need to adjust to about 40.1k
max.
User I/O:
A single pushbutton tied between GND and PTE18 will allow the user to set
the module into different states, and a single LED tied to PTE19 will indicate which
state the module is currently in. These two functions will only work when the
module is awake and in programming mode. Once in operating or sleep mode, these
will be disabled by writing their pins as inputs in order to preserve energy. The
switch will not need a pull-up resistor because the internal one will be used instead.
It’ll be enabled only when the module is in programming mode, the only time it’ll be
needed.
END OF DOCUMENT
Parts List:
MCU PCB:
Item
Quantity
Part Description
Designators
1
2
HDR-2X5, 1.25mm x 1.25mm
J1, 2
2
1
HDR-2X8, 2.54mm x 2.54mm
J3
3
1
HDR-1X2, 2.54mm x 2.54mm
J4
4
1
Switch-SPST, Off-(On), SMD
SW1
5
1
LED, GRN 20mA 2.2v, 0805
LED1
6
1
IC-Dig, MCU, KL15Z32VLH4, QFN-48
U1
7
1
Crystal, 32.768kHz, 12.5pF
X1
8
2
Cap, Ceramic, 18pF, 50V
C1, 2
9
1
Res, 1%, 10M, 125mW, 150V
R1
10
3
Cap, Ceramic, 0.1uF, 25V
C4, 6, 10
11
1
Cap, Ceramic, 10uF, 10V
C3
12
1
Cap, Ceramic, 1uF, 10V
C5
13
4
Relay-SPST, Off-(ON)
K1, 2, 3, 4
14
4
MOSFET-N-CHANNEL, 2N7002, 1.2Ohms, 120mA, 60V
Q1, 2, 3, 4
15
1
IC-Lin, LDO Supply, 3.3V, 150mA
U2
16
1
IC-Lin, LDO Supply, 3.3V, 50mA, Iq = 3uA
U3
17
1
IC-Lin, Boost Converter, 12V, 6W
U4
18
2
Cap, Ceramic, 0.47uF, 10V
C7, 8
19
1
Cap, Ceramic, 10uF, 16V
C9
20
4
Res, 1%, 1k, 125mW
R8, 9, 10, 11
21
1
Res, 0.1%, 1k
R7
22
1
Res, 0.1%, 10k
R6
23
1
Res, 0.1%, 100k
R5, 12
24
1
Res, 0.1%, 1M
R4
25
1
IC-Lin, Instr Amp, INA333, MSOP-8
U5
26
1
Res, 1%, 2.87K, 125mW,
R13
27
1
Res, 1%, 100Ohm, 100mW,
R2
28
1
Res, 0Ohm
R3
29
1
IC-Lin, Thermometer, MCP9700, 3.3V
U6
Boost Converter PCB:
Item
Quantity
Part Description
Designators
1
6
Res, 5%, 100M, 100mW
R1, 4, 7, 10, 13, 16
2
6
Res, 1%, 10M, 125mW
R2, 5, 8, 11, 14, 17
3
6
Res, 1%, 30.1K, 100mW
R3, 6, 9, 12, 15, 18
4
1
Res, 0.1%, 100k, 63mW
R19
5
1
Res, 1% 4.02k, 125mW
R20
6
1
Res, 1% 0.02Ohm
R21
7
6
Cap, 2.2uF, 630V
C1, 2, 3, 4, 5, 6
8
1
Cap, Ceramic, 0.001uF, 25V
C7
9
7
Cap, Ceramic, 0.1uF, 25V
C8, 9, 10, 11, 12, 13, 14
10
12
Diode-Schottky, Vf = 1.15V, Vr = 1000V, SMD
D1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
11
6
MOSFET-N-CHANNEL, 2N7002, 1.2Ohms, 120mA, 60V
Q1, 3, 5, 7, 9, 11
12
6
MOSFET-P-CHANNEL, FDN5618P, 170mOhms, 1.2A, -60V
Q2, 4, 6, 8, 10, 12
13
1
MOSFET-N-CHANNEL, 1.95Ohm, 3.8A, 620V
Q13
14
3
IC-Lin, Analog Switch, ADG721
U1, 2, 3
15
1
IC-Lin, Instr Amp, INA333
U4
16
1
IC-Dig, Digital Pot, MCP4132
U5
17
1
IC-Lin, Op Amp, AD8571
U6
18
1
IC-Lin, Switching Controller, MAX1771CSA+
U7
19
1
HDR-2X8, 2.54mm x 2.54mm
J1
20
1
HDR-2X6, 2.54mm x 2.54mm
J2
21
6
Ind-Power Inductor, 150uH, 340mOhm, 2A
L1, 2, 3, 4, 5, 6
Zigbee PCB:
Item
Part Description
Designators
1
Quantity
1
Ind, 6.8nH, 220mOhm
L1
2
1
Ind, 5.6nH, 220mOhm
L2
3
1
Ind, 8.2nH, 220mOhm
L3
4
1
Ind, 3.3nH, 220mOhm
L4
5
3
Cap, Ceramic, 1pF, 10V
C1, 2, 5
6
2
Cap, Ceramic, 0.5pF, 50V
C3, 4
7
2
Cap, Ceramic, 0.1uF, 25V
C6, 9
8
5
Cap, Ceramic, 47pF, 10V
C7, 8, 15, 16 , 17
9
2
Cap, Ceramic, 18pF, 50V
C11, 12
10
1
Cap, Ceramic, 100pF, 50V
C10
11
2
Cap, Ceramic, 0.01uF, 50V
C14, 18
12
1
Cap, Ceramic, 1uF, 10V
C13
13
1
HDR-2X5, 1.25mm x 1.25mm
J1
14
1
Crystal, 18pF
X1
15
1
IC-Dig, Zigbee Transceiver, MRF24J40, QFN-40
U1