+l o
Q)
+l
Q)
Cl
T
,
8TO 7
U,
,
R' 6
2
C,
,
Figure 3-6 Tt.'lIIpen/mre fTal1smirrer circllif
C3
'I
f - i +R 1 R 2
7
"
C2 2
C1 R3 R'
I>--j +
8 n~1 If:: i- L1 '- 3
~ 1000 oms
'~ ~ts
Figure 3-7 Tell/permuTe recei\ler cirClIif
the amplitude of the incoming exceeds that of the reference voltage provided by R2 and R3. lhc output of tbe comparator changes stale until the input signal
falls below the reference voltage.
The output [rom the comparator is connected to a monoslable multivibrator (a one-shot) in the Uv1331 .
C,
•
~ " '" II~
J>: ~ , 0
~
I.---.---.--- -- -_. --- --- - ----
-~ +9VDC
8
U 1
R"
S
* C'
1
DIGITAL MULTI·M ETER
+
OR
DIGITAL PANEL M
CS R 7 ETER
iRiaiRi RiR ial
• o
VOLTS 0+
--
-Each time the comparator switches. the one-shot trig-gers and closes a current switch.1bis allows the output filler capacitor at C5 10 charge for a lime determined by the time constant of R6 and C4. Resislor R7 fun c-lions as a bleeder resistor that continually discharges the charge on capacitor C5.lhereby causing the volt
-66 Elec tronic Sensors fo r the E v il Genius
age on C5 at allY given instant to correspond to the
<I\'erage charge available £rom Ihe currelll source.
Therefore. the \'ohage across C5 is directly p ropor-tionailo the frequency of the incoming signal.
Theouipul from the Li\1331 should be applied to a high-impedance voltmeter or digital panel meter.
Potentiometer R5 allows vou 10 calibrate the circuit fora particular OUlput voltage at a specific input sig-nal frequency. The
LM331
hus a linear frequency response from I 10 10 KHz. The remotc data logger recei\,e-and·display circuit can be powered [rom a standard 9-\'011 transistor radio battery.Constructing Ihe remote data loggcr is prell)'
mnightforward and can be done in less than
2
hours.The sending oscillator circuit is constructed on a piece of experiments proto-board from Dalak (part
=IU;II). Consider using an IC socket for the Li\'1555.
In the C\'cnt of a circuit failure. it is a simple matter to replace an IC when a socket is used. ICs must be
installed correctly in order for the circuit to operate correctly. ICs usually have two differelll types of identifiers.. On some 1Cs. a rectangular CUlOUl is
placed on the top of the Ie. This cutout is just 10 the right of pin I. Other les have a small indented circle next 10 pin I on the Ie. All components except for the thermistor arc mounted on the small circuit board. The transfonner at Ll is a 600 to 600 ohm miniature matching unit. Because the transfomlcr is a 1:1 matching transformer with the slime winding on
both the primary and the secondary. you can install the transfomler without regard to polarity. However note the polarity of the capacitor at C3.TIle sending oscillator circuit is powered from a 9-volt transistor rndio battc'1'. and power is supplied through the on -off switch aI S I.
'lbesending or oscilhHor circuit was housed in a small met<ll chassis box.1lle transformer was
mounted to the circuit board along with all the
components.TIle circuit board was mounted on Ih _ inch standoff and mounled to the bottom of the cha s-sis box. The 9·\,0!t·bauery holder was secured to the
bottom of the circuit board. An RCA jack. an I/s-inch mono earphone jack. and S I are mounted on the lOp of the chassis box. The RCA jack is installed for the thcmlistor connection. while the 1/I;-inch phone jack is used for the audio output connection.
Once the you have fmished constructing the oscillator sending unit. you will need to carefully
inspect the underside of the circuit board to make sllre that there are no cold solder joints and no
unwanted solder bridges between solder pads. Also look for unwanted stray component leads that may have stuck to the bottom of the circuit board.
Next. you will want to prepare a shielded sensor cable with an RCA plug at one end and the thermis·
tor at the other end. Depending upon your app lica-tion. you may wish to house the thermistor in a
discarded metal pen casing. with provisions for the shielded cable to exit the top of the pen assembly.
FirS!. prepare the themlistor by spraying the theml is-tor and the leads with Crylon or a similar plastic
coatin~ to insulate the thermistor and the first 1/~ inch of the thermistor's leads. 'Jbe themlistor leads arc
usually very small-gauge wire. so care must be taken when so ldering the shielded cable to the thermistor.
Heatshrink tubing should be put over the connection or junction between the thennjstor and the shielded cable. and some sort of strain relief should be pro-vided. You will also want to make or purchase a 3· to 5·foot cable with a I/I;·inch male plug on both ends to connect the oscillator audio output to the input of a recorder or transmitter. Place the on·o([ switch in the off position and install the baueI)'.
Construction of the receive/display circuit is a bit more complicated than the oscillator/sending unit.
l lle receive/display section call be built o n a proto-board such as the one used in the sending unit. or you can make your own dedicated printed circuit board.
All components including the transformer at Ll are mounted on the circuit board. None of the com po-nents are critical. however. particular attention
should be paid to lhe orientation of the capacitor at C5 and the Ie at Ul. An IC socket for U 1 is highly recomillended. lCs are usually identified in one of two ways. Some ICs have a small indented circle
placed on the top left side of the IC near pill I. Other ICs h,we a small rectangular cutout on the top of the IC just to the left of pin 1. When installing Ihe Ie. pay particular attention (0 the orientation to avoid
destroying the IC when power is supplied.
Calibration potentiometer R5 is a PC board tri m-pOl. \Vhen installing the transfomler at L 1. be sure to
Chapter Three Heat Detection 67
I
idcnlify from the packaging the two windings. which are different Erom each other. One winding is an S-ohm winding and the other winding is lK ohms.. Be sure to orient the transformer correctly.
Once you ha\'e completed the receive/display board. you will need to inspect thc underside of the circuit board. First check for cold solder joints. and
then look over the circuit foil side to ensure that there are no unwanted solder bridges between foil traces. Finally look for stray or unwanted component leads. which mav ha\'e stuck to the underside of the PC board.
The receive/display board is finally installed in a small metal chassis box. which measures 41h X 6 X 2 inches. The circuit board is mounted atop I/~-inch
plastic standoffs. A 'Is-inch mini phone jack and the power switch SI is installed on the lOp front side of the chassis box. Our prototype circuit utilizes a small
temlinHI strip on the chassis to allow connection to an external digital ll1ultimeter. Note that you could also use a recording multimeter \\ith RS-232 output.
if desired. Alternatively you could moulli an inexpen-sive LCD panel voltmeter on the front panel of the receiver/display unit. if desired. If you usc an internal panel meter. you will need 10 make a two-resistor
voltage di"ide,r to drop down the voltage to the panel meter. which is generally 2 volts maximum. A 9-volt
transistor radio ballery holder is mounted on the bot-tom of the chassis box, Once you have completed the circuit. turn the power switch to the off position
before installing the 9-volt baltery. Finally. you will need to make a cable to connect the receiver/display to the audio source that will be driving the input.
Your cable will ha\'e a 'Is-inch mini phone plug at one end and a connector of your choice on the opposite end, depending upon your application and input source.
Before using the analog data recording system. it is necessary to adjust the sensor circuil so thaI the frequency of the signal it generate-s falls within the proper range. Likewise. it's necessary 10 make sure the frequency-to-voltage com'erler is properly
adjusted.
If you plan to take down the decoded \'oltages by hand. it's imporlant to make sure each data sample is recorded for at leastS seconds. This will allow the voltmeter readout to sellie long enough for you to
record the reading.
If you plan on using the analog data logging system with a tape recorder rather tlum a transmiller and
recei\'er setup. ~'ou will need to create a calibration graph for your particular recorder for best rcsulls. In order to calibrate your recorder. you will need (0 store a knO\m sequence of audio frequency tones sp,lced uniformly across rhe desired signal range.
Connect an audio siunall!enerator
- -
direct Iv to the . inpul of your tape recorder. and record 10- to 15-second tone bursts at 100 Hz intervals betwecn 100Hz and 1.000 Hz. 'm e inpul \'olume to the tape recorder should be adjusted to lh of the range to assure th:1I the input 10 the tape recorder is not o\'er-driven or distorted, Stop the recordcr and disconnect
the sib'llal generator. Connect the receive/display uni" s audio input connection to the output of your tape recorder. 'Iller! connect the receiver/display unit's outpUI connection to a digital voltmeter. Tum on the tape reorder and playback the recorded tape.
As the tape is playing. take notes of the \'oitae.c read-ings for each of the tones recorded on the tape, and then finally make a plot on a piece of paper.At l/~
inch intervals along the bottom of the graph make notations of frequency. Each ·'/i-inch marking repre-sents 0.1. 2, 3.4. 5 ... Hz Illultiplied by 100 Hz. So the firsl marking is I KHz and so on. The \'ertkal scale on the graph represents by voltage. starting with 0.1. 2. 3. -J. Clnd 5 volts lit ~/ii intervals \'ertically. Once you have a calibration graph. you are ready to begin testing your analog data logging system.
Connect the themlistor to Ihe oscillator sending unit. Then connect the output of the oscillntor send-ing unit to either a tape recorder or to the input of a
transmitter. and then apply power to the circuit. If you are rccording your readings. you will ha\'e to play them back into the recei\'cr/display unit when you return to the playback location. If you are using an
RF receiver in real time instelld of a tape recorder.
you can connect the input of the receive/display unit
68 Electronic Sensors for the E vi l Genius
to audio output of the RF receiver. Now connect the output of the receiver/display unit to a digital multi -meter or voltmeter and read tbe incoming signal
from the oscillator sending unit. You should begin seeing a voltage reading on the meter display. Have a
friend go over to the oscillator sending unit and place the thennistor in a bowl of crushed ice to calibrate the sensor. You should now see the voltage reading begin to change to a different valuc. Note the reading
\Iohen the thermistor is placed in the crushed ice: you hove now calibrated your analog. data logging system. and it is ready for field dala gathering.