design
ideas
Edited by Bill Travis
Temperature-measurement scheme
uses IR sensor and sigma-delta ADC
Albert O Grady and Mary McCarthy, Analog Devices, Limerick, Ireland
any noncontact temperature-
EXCITATION VOLTAGE 5V
measurement systems use infrared
Msensors, such as thermopiles,
REF192/
which can detect small amounts of heat
AD780
AVDD DVDD
radiation. Biomedical ther-
Figure 1
mometers that measure the
REFIN2
CONTROLLER
temperature of an ear or a temple use
REFIN1 ( )
DIN DATA OUT
noncontact temperature measurement,
as do automotive-HVAC systems that ad-
DATA IN
REFIN1 ( ) DOUT
just temperature zones based on the
SCLK
body temperature of passengers. House- SERIAL CLOCK
MLX90247
hold appliances and industrial processes
AD7719
OUTIR
AIN1
can also benefit from the use of noncon-
VSS
tact temperature measurement. Infrared
thermometers can measure objects that
OUTIR
AIN2
move, rotate, or vibrate, measuring tem-
26k
perature levels at which contact probes
AIN5
RSENSE
either would not work or would have a
15k
shortened operating life. Infrared meas- AIN6
urements do not damage or contaminate AGND DGND
the surface of the item being measured.
Thermal conductivity of the object being
measured presents no problem, as would
be the case with a contact temperature-
measurement device. The circuit in Fig- Using an infrared sensor and a sigma-delta ADC, you can make noncontact temperature measure-
ure 1 provides a design for a high-reso- ments.
lution digital thermometer that uses a
thermopile sensor and a sigma-delta ADC. The design provides high resolu- cision signal-conditioning components
tion and response times of approximate- preceding the ADC. The MLX90247D
ly 1 msec, and it eliminates the need for sensor comprises a thin, micromachined
Temperature-measurement scheme
high-performance, low-noise signal con- membrane embedded with semiconduc-
uses IR sensor and sigma-delta ADC........65
ditioning before the ADC. tor thermocouple junctions. The See-
The high-accuracy, noncontact digital beck-coefficient thermocouples generate
Automotive link uses single wire ................66
temperature measurement system uses a dc voltage in response to the tempera-
Novel idea implements
the MLX90247D thermopile from ture differential generated between the
low-cost keyboard..........................................69
Melexis (www.melexis.com) and the hot and the cold junctions. The low ther-
Get more power with
AD7719 high-resolution, sigma-delta mal conductivity of the membrane al-
a boosted triode ............................................72
ADC from Analog Devices (www.ana- lows absorbed heat to cause a higher tem-
log.com). The AD7719 provides differ- perature increase at the center of the
Anticipating timer switches
ential inputs and a programmable-gain membrane than at the edge, thus creat-
before you push the button ........................74
amplifier; thus, you can connect it di- ing a temperature difference that is con-
Publish your Design Idea in EDN. See the
rectly to the sensor, allowing the temper- verted to an electric potential by the ther-
What s Up section at www.edn.com.
ature-measurement system to provide moelectric effect in the thermopile
high accuracy without the need for pre- junctions. The MLX90247D also con-
www.edn.com April 3, 2003 edn 65
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design
ideas
tains a thermistor, allowing you to con- cy, low-noise external-signal condition- from 15.207 k at 40 C to 38.253 k
figure a temperature-compensated sys- ing. The AD7719 simultaneously con- at 85 C with a nominal impedance of
tem in relative-measurement mode. verts both the thermopile and the ther- 26 k at 25 C. Again, you can directly
The AD7719, a dual-channel, simulta- mistor sensor outputs. The main channel measure voltages from the thermistor, as
neously converting ADC with an internal with its programmable-gain amplifier Figure 1 indicates. Biomedical ther-
programmable-gain amplifier is an ideal monitors the thermopile, and the auxil- mometers generally have a measurement
ADC when you use it with the iary channel monitors the thermistor. range of 34 to 42 C. In this range, the
MLX90247D sensor in temperature- You can use on-chip chopping and cali- thermopile s differential output is 336
measurement applications. The main bration schemes in optimizing the de- V. Operating the AD7719 in its 20-
channel is 24 bits wide, and you can con- sign. The AD7719 features a flexible se- mV input range with a 5-Hz update rate
figure it to accept analog inputs of 20 mV rial interface for accessing the digital data allows temperature measurement with a
to 2.56V at update rates of 5 to 105 Hz. and allows direct interface to all con- resolution of 0.05 C.
The auxiliary channel contains a 16-bit trollers.
ADC and accepts full-scale analog inputs The sensitivity of the thermopile is 42
of 1.25 or 2.5V with an update rate equal µV/K; thus, it produces an output voltage
to that of the main channel. The AD7719 of 9.78 to 15 mV over the industrial tem-
accepts signals directly from the sensor; perature range of 40 to 85 C, an out-
the internal programmable-gain ampli- put that the AD7719 can directly meas- Is this the best Design Idea in this
fier eliminates the need for high-accura- ure. The thermistor s impedance ranges issue? Select at www.edn.com.
Automotive link uses single wire
Anthony Smith, Scitech, Biddenham, Bedfordshire, UK
12V
n the automotive industry, in VB (NOMINAL)
which the goal is to produce cars with
R1 5V
LED2 VS
Isimpler, lighter wiring looms, any in-
43 (TYPICAL)
terface that uses just one wire instead of
two offers a distinct advantage. Q1
C1
Figure 1
10 nF R5
The circuit in Figure 1 imple-
1k
Q2
TX
ments a bidirectional link using a single
R2
wire, with the car s chassis or ground con-
R6 R7 MICROCONTROLLER
ductor providing a negative return path. 330k 330k
D1
RX
The microcontroller communicates with
REMOTE SWITCH
R4
AND INDICATOR LED
10k
the driver of the car by illuminating
Q3
LED1. The driver communicates by op-
S1
D2 R3 C2
LED1
430
erating switch S1. Detecting the switch 10 nF
0V
closure requires no current sensing: The
circuit simply exploits the fact that the
CHASSIS OR
GROUND CONDUCTOR
forward voltage drop of a properly biased
This circuit implements a bidirectional link using a single wire and a ground return.
LED is usually two or three times the VBE
of a bipolar transistor. Q1, LED2, and Q2
form a semiprecision current source. Q3 across it to fairly precise and constant val- With S1 open, R6 biases Q3 on, pulling
in the receiver path detects the switch clo- ues. For example, with R3 430 , the the receiver pin, RX, low. RX remains low,
sure. When the microcontroller s TX pin current in LED2 is approximately 10 mA regardless of whether LED1 is on. When
goes high, Q2 illuminates LED2 and bias- with 5V at Q2 s base (TX high). If you use the switch closes, the values for R4 and R6
es Q1 on. Q1 sources a constant current to a device such as the HLMP-1000 for ensure that Q3 s base pulls down to ap-
LED1 via R2 and D1. LED2, its forward voltage remains con- proximately 150 mV (with VS 5V),
LED2 constitutes an inexpensive but ef- stant at approximately 1.6V, putting ap- thereby turning off Q3 and allowing RX
fective voltage reference, which imposes proximately 0.9V across R1. The resulting to go high. As long as the switch remains
a constant voltage across current-setting 20 mA or so flowing in Q1 provides ade- closed, RX stays high, whatever the state
resistor R1. Provided that you choose R3 s quate brightness for LED1 and remains of the TX pin. Powering the current
value to suit Q2 s base drive, you can set acceptably constant with changes in VB or source directly from the car s battery volt-
the current in LED2 and the voltage temperature. age, VB, rather than from the microcon-
66 edn April 3, 2003 www.edn.com
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design
ideas
troller s supply, not only relieves the bur- counter in the harsh automotive envi- the external LED breaks a feature that
den on the low-voltage regulator, but also ronment. C2 with R4 provides a degree of may be useful for troubleshooting pur-
ensures that LED1 receives proper bias, noise filtering and has negligible effect on poses. In the event of a broken wire, lit-
even with a very low value for VS. Thus, the switching of Q3.You may need C1 and tle collector current flows in Q1, and its
provided that R3, R4, and R6 have appro- R5 to roll off Q2 s frequency response to base-emitter junction shunts LED2; pro-
priate values, the circuit functions with avoid the possibility of high-frequency vided that R1 is much smaller than R3, the
VS as low as 3V or even lower. A further oscillation. The transistor types are not shunt steals LED2 s bias current, thereby
advantage is that you can replace LED1 critical; most devices with respectable turning it off. Although the circuit was
with several LEDs connected in series. current gain and adequate power rating developed for an automotive product,
With VB 12V, the current source has ad- are satisfactory. LED2 provides a triple you could easily adapt it for use in other
equate compliance to drive four or five function. As well as acting as a voltage ref- applications in which a simple user in-
LEDs. erence for the current source, it also pro- terface must operate on a single line.
R2 is a nonessential component, but it vides local indication of the external LED
reduces the power consumption in Q1. D1 status by illuminating in synchronism
provides positive overvoltage protection with LED1. Additionally, it provides
for the current source, and voltage-sup- open-circuit (broken-wire) indication by
Is this the best Design Idea in this
pressor D2 can protect against the harm- turning off completely (even when TX is
issue? Select at www.edn.com.
ful transients that systems often en- high) if the connection between D1 and
Novel idea implements low-cost keyboard
Jean-Jacques Thevenin, Thomson Plasma, Moirans, France
any applications that use a mi-
KEYBOARD BUTTONS
crocontroller also use a keyboard.
MIf your application uses a
RS
Figure 1 I/O
...
relatively powerful microcon-
troller, you can use several free I/O pins
BA BB BC BD
C
or an unused input with an ADC to ef-
MICROCONTROLLER
RD
RA RB RC
fect an easy keyboard connection. But, if
the microcontroller in your system has ...
too few free I/O pins and no on-chip
ADC, you can be in trouble. However, if
your system doesn t require a high-per-
This circuit provides an inexpensive and easy way to read a small keyboard using only one I/O line
formance keyboard, you can solve the
of a microcontroller.
problem by using the circuit in Figure 1.
How does it work? At system initializa-
tion, the I/O connection is an output, set whether it resets to logic 0. the duration of TX with the following ex-
to logic 0; hence, C is discharged. In read- 5. If, after TMAX, the input I/O is still at pression: TX RXCloge(VC/VTH). If RX is
ing the keyboard, the following steps take logic 1, no button has been activated. not negligible with respect to RXMIN (but
place: 6. If within TMAX, the input I/O resets the RINPUT of the microcontroller greatly
1. I/O (output) assumes the state log- to logic 0, the measured time indicates exceeds RX), then
ic 1, VOUT. the activated buttons.
2. VC charges to logic 1 (VOUT) or to a 7. I/O becomes output again and resets ëÅ‚ öÅ‚
VOUT RX
TX H" RX " C " loge ìÅ‚ " ,
voltage that RS and the other resistors de- to logic 0 to discharge C.
VTH (RX + RS)÷Å‚
+
íÅ‚ Å‚Å‚
termines. (You can set the output I/O to Several equations describe the opera-
logic 1 by default. In this case, you can tion of the scheme. First, assume some where VOUT is the voltage at logic 1 on the
omit steps 1 and 2, and the routine be- conditions: VOUT is the voltage of the out- I/O output. From the last equation, a
comes faster. This design uses 0 instead of put I/O at logic 1; VTH is the threshold for condition for RX is:
1 to have an inactive signal on the line logic 0 input to the microcontroller; and
VTH
when the keyboard is not checked.) RX is the value of the parallel combina-
RXMIN > RS " .
VOUT VTH
3. I/O becomes an input. tion of RA, RB, and the other resistors.
4. For a duration TMAX, the microcon- Figure 2 shows the timing diagram for
troller checks the input I/O to see the circuit of Figure 1. You can evaluate Note that, if RA, RB, and the other re-
www.edn.com April 3, 2003 edn 69
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design
ideas
VC = VOUT*
Figure 3
Figure 2
VTH
DURATION TX IF BUTTONS ARE PUSHED
TIME
C IN DURATION TMAX IF NO BUTTON IS PUSHED
CHARGE
C IN DISCHARGE WITH PUSHED BUTTONS
I/O OUTPUT I/O INPUT WITH PUSHED BUTTONS I/O OUTPUT
*IN FACT, IF SOME BUTTONS ARE PUSHED, VC = RX/(RX = RS) . VOUT.
The duration, TX, indicates which buttons or combinations thereof you These waveforms at the I/O pin and table show the returned
pushed. value (duration) with different combinations.
sistors form an R-2R string, RXMIN is example of a small keyboard with four case, the effect of RS is negligible, but you
approximately equal to RA/2. RS limits buttons: To choose RS, IOUTMAX of the mi- should then consider the effects of the in-
the current from the microcontroller crocontroller is 25 mA at VOUT 5V, so put resistance of the microcontroller.
and must have a minimum value of RSMIN 200 . So this design uses The duration between two measure-
VOUTMIN/VOUTMAX. This resistor creates a RS 220 . RA, RB, RC, and RD are 1, 2.2, ments is approximately 2 sec (Listing
delay for charging and discharging C of 3.9, and 8.2 k , respectively. You can se- 1). With one byte, the maximum dura-
approximately 5RSC. The following is an lect values that greatly exceed RS. In this tion, TMAX, is 512 sec (when no button
is pushed). So, time TX with RXMAX (in
other words, RD) must be inferior to
LISTING 1 THE DURATION BETWEEN TWO MEASUREMENTS
TMAX. Assuming that VTH is 1.5V (mini-
mum), the equation for TX becomes
ëÅ‚ 5 8200 öÅ‚
8200 " CMAX " loge ìÅ‚ "
A
íÅ‚ 15 (8200 + 220)÷Å‚
. Å‚Å‚
< 512 µSEC CMAX < 53 nF.
So, at the beginning of each measure-
ment, you must append a delay of
5 220 47 nF 52 sec to charge C.
Figure 3 shows the waveforms at the I/O
pin and the returned values with differ-
ent button combinations. The power
consumption of the circuit, with C 47
nF, VCC 5V, and a keyboard reading
every 30 msec, is approximately 0.04 mW
(practically negligible). You can use this
scheme in all applications that don t re-
quire great accuracy or high speed. You
can download Listing 1 from the Web
version of this Design Idea at www.
edn.com.
Is this the best Design Idea in this
issue? Select at www.edn.com.
70 edn April 3, 2003 www.edn.com
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design
ideas
Get more power with a boosted triode
Dave Cuthbert, Boise, ID
250
ven though 6L6 beam- pressor grid provided by two
0
power tubes have been beam-forming plates, but you
 2.5
200
Earound for 66 years, they  5 can treat the 6L6 beam-pow-
 7.5
are still quite popular for use er tube as a pentode. You can
150
 10
in electric-guitar amplifiers, think of a pentode as an n-
PLATE
CURRENT  12.5
and its cousin, the 6CA7 channel JFET with the fol-
(mA)
 15
100
(EL34) power pentode, is a fa- lowing electrode functions:
 17.5
vorite among audiophiles. Thermionic cathode:
 20
50
The developers of these tubes source of electrons (corre-
 22.5
designed them for  25 sponds to the JFET source);
0
Figure 2
pentode-mode op- 0 100 200 300 400 500 600 700 800 Control grid: controls the
PLATE VOLTAGE (V)
eration, and they deliver max- cathode current; operated at
imum audio power in this a negative potential relative to
The load lines for a pentode show that the plate can draw 150 mA at a
mode. On the other hand, the cathode (corresponds to
plate voltage of only 50V.
many audiophiles prefer tri- the JFET gate);
ode-mode operation and, un- 300 Screen grid: electrostati-
0
til now, had to be content with cally screens the control grid
250
10
a 50% reduction in output from the plate, thereby re-
20
200
power. This reduction means ducing the effect that the
30
that they require larger pow- plate voltage has on the cath-
150
PLATE 40
CURRENT
er supplies and twice as many ode current; operates at a
50
(mA)
100
expensive tubes to obtain positive potential relative to
60
pentode power from a triode 70 the cathode;
50
80
amplifier. Figures 1a, 1b, and Suppressor grid: prevents
0
90
1c show the 6L6 secondary electrons from
Figure 3 -50
connected as a leaving the plate and traveling
0 100 200 300 400 500 600 700 800
pentode, a true triode, and a to the screen grid; operates at
PLATE VOLTAGE (V)
 boosted triode, respectively. the cathode potential; and
A pure triode needs 200V plate voltage to draw 150 mA.
The boosted-triode configu- Plate: collects the elec-
ration allows pentodes to pro- trons (corresponds to the
duce pentodelike power while operating The 6L6 is a beam-power tube and has JFET drain).
in a true-triode mode. To understand the cathode, control-grid, screen-grid, sup- Figure 2 shows the pentode s charac-
operation of the boosted triode, it s use- pressor-grid, and plate electrodes. The teristic curves for control-grid voltages of
ful to review some vacuum-tube theory. suppressor grid is actually a virtual sup- 0 to 25V and a screen-grid voltage of
400V 8 400V 8 400V 8
Figure 1
6L6 6L6 6L6
1 µF 1 µF 1 µF
+ + +
100V
47k 47k 47k
250V
_ _ _
 14V  32V  44V
(c)
(a) (b)
A pentode (a) can deliver much more power than a triode (b), unless you use a boosted-triode configuration (c).
72 edn April 3, 2003 www.edn.com
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design
ideas
250V. Note the idealized load line and
TABLE 1 PENTODE, TRIODE, AND BOOSTED-TRIODE PARAMETERS
that the tube can draw a plate current of
DC plate Grid bias Grid swing Output power
150 mA at a plate voltage of only 50V.
Amplifier current (mA) (V) (V) (W)
High voltage gain, high plate impedance,

Pentode 75 14 22 11
and high output power characterize pen- Triode 75 32 64 6


tode-mode amplification. By connecting Boosted triode 75 44 88 10
the screen grid directly to the plate, you
can operate the tube in triode mode. Low plate cannot draw 150 mA at a plate volt- For the boosted-triode circuit in Fig-
voltage gain and low output impedance age lower than 200V. This fact greatly ure 1c, you simply add a 100V screen-to-
characterize this mode. Figure 3 shows limits amplifier efficiency and power out- plate power supply (Figure 4) to the stan-
how the triode curves differ from the put. However, in spite of the limited out- dard triode-amplifier circuit. This ad-
pentode curves. The curves represent put power, some people still prefer triode dition shifts the triode characteristic
control grid voltages of 0 to 90V. Note mode because they claim it produces a curves 100V to the left (Figure 5). Note
the load line and that, in triode mode, the superior-sounding amplifier. the load line and that the plate can now
draw 150 mA at a plate voltage of only
1.5k
100V, rather than 200V as with the pure-
TRIAD N-48X 1N4007 25W
triode-mode circuit. You can obtain sig-
nificantly higher power with boosted-tri-
+
330 F
120V AC 120V AC 2SC4953 100V DC
160V
ode amplification and still maintain the
1N5378B
characteristics of triode amplification. In
Spice simulations of three single-ended
Figure 4
Class A audio amplifiers using Micro-
Cap-7 evaluation software (www.spec
A 100V screen-grid power supply transforms a normal triode into a boosted triode.
trum-soft.com), the control-grid bias for
a quiescent plate current is 75 mA, and
1.5k 100V DC
the ac grid signal is just short of amplifi-
TRIAD N-48X 1N4007
20W 40 mA
er clipping. The transformer ratios pro-
vide a plate-load impedance of 5 k for
+ (PD=3W)
2SC4953 WITH HEAT SINK
22O F
120V AC 120V AC
OR SIMILAR
250V the pentode and 3 k for both the triode
1N5378B
and the boosted triode. Table 1 details the
parameters.
Figure 5
With a boosted triode, the plate can draw 150 mA with a plate voltage of 100V, versus 200V Is this the best Design Idea in this
for a pure triode. issue? Select at www.edn.com.
Anticipating timer switches
before you push the button
Jean-Bernard Guiot, DCS AG, Allschwil, Switzerland
(Editor s note: This Twilight Zone-wor- in the past. In Figure 1, IC2 is a 555-type T 1.1RC2. In Figure 1, you replace the
thy circuit will be the subject of an up- timer (preferably CMOS) connected as a resistor, R, that normally connects to C2
coming network sitcom, My Big Fat An- monostable one-shot multivibrator. The with the circuit inside the dashed line.
ticipating Timer.) pushbutton switch, S1, triggers IC2. You This circuit comprises a 741 op amp, IC1,
t happens to almost everyone that can replace S1 with a transistor or an op- and three resistors: R1, R2, and R3. You
an apparatus or system should have tocoupler, for example. You can connect could replace the war-horse 741 with a
Ibeen turned off a moment ago. The VOUT to a relay or a transistor, if needed. TL081 if your design needs longer time
device in question could be the car heater, You might need to adjust the values of R4 delays.
the air conditioner, the lights... and R5, depending on the output load Taking into account the usual op-amp
This Design Idea offers a solution to and the characteristics of S1. The inter- assumptions equal voltage on both
the challenge of turning devices on or off val during which VOUT remains high is inputs and zero input current you de-
74 edn April 3, 2003 www.edn.com
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design
ideas
VCC
rive the following expressions:
R2
VO V(R2 R1)/R1, and VO
100
V R3IC, where VO is the op
V0
R1
amp s output voltage, V is the
100k
voltage at the noninverting in- R5 R4
2.2k 2.2k
put, and IC is the current IC1
741
through R3.IC is also the current
+
GND VCC
that charges C2.
R3
Figure 1
Combining the cited
1M V
TRIG DISCH
expressions, you can compute
IC2
555
IC
the value of resistor R that the
VOUT
OUT THRESH
op-amp circuit replaces: R
V/IC R3R1/R2. The timing
RESET CTRL
interval of this timer is thus
S1
C3 C2 C1
T 1.1C2R3R1/R2. Using ap-
10 nF 1 F 0.1 F/
100 F
propriate values, you can ob-
0V
tain long time delays that you
can t attain with the basic 555
This innovative timer turns on approximately 18 minutes before you press switch S1.
circuit. But the real innovation
inherent in this circuit is that its output er supply, adjusting R1 contributes min- shown in Figure 1, the interval T is ap-
turns on at a defined time, T, before you imal EMI and other insidious effects to proximately 18 minutes.
press S1. To adjust interval T, use a po- the op amp s input. C1 is a power-sup-
tentiometer for R1. Because the wiper of ply bypass capacitor, and C3 stabilizes the
Is this the best Design Idea in this
the potentiometer connects to the pow- 555 s control voltage. With the values
issue? Select at www.edn.com.
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