Temperature to Digital Converters (Continued)
Temperature-to-Frequency
Converter for Remote Sensing
If a frequency proportional to temperature is needed, then
the LM34 can be used in conjunction with an LM131
voltage-to-frequency converter to perform the desired con-
version from temperature to frequency. A relatively simple
circuit which performs over a +3˚F to +300˚F temperature
range is shown in Figure 16. The output frequency of this
circuit can be found from the equation:
where resistor R
S
is used to adjust the gain of the LM131. If
R
S
is set to approximately 14.2 kΩ, the output frequency will
have a gain of 10 Hz/˚F. Isolation from high common mode
levels is provided by channeling the frequency through a
photoisolator. This circuit is also useful for sending tempera-
ture information across long transmission lines where it can
be decoded at the receiving station.
LED Display for Easy Temperature
Reading
It is often beneficial to use an array of LED’s for displaying
temperature. This application may be handled by combining
the LM34 with an LM3914 dot/display driver. The tempera-
ture may then be displayed as either a bar of illuminated
LED’s or as a single LED by simply flipping a switch. A wide
range of temperatures may be displayed at once by cascad-
ing several LM3914’s as shown in Figure 17.
Without going into how the LM3914 drivers function inter-
nally, the values for V
A
,V
B
, and V
C
can be determined as
follows:
V
A
is the voltage appearing at the output pin of the LM34. It
consists of two components, 0.085V and (40 mV/˚F) (T
A
).
The first term is due to the LM34’s bias current (approxi-
mately 70 µA) flowing through the 1 kΩ resistor in series with
R
A
. The second term is a result of the multiplication of the
LM34’s output by the resistive string composed of R
1
,R
2
,
and R
A
, where R
A
is set for a gain factor of 4 (i.e.; 40 mV/˚F).
V
B
represents the highest temperature to be displayed and is
given by the equation V
B
= 0.085V + (40 mV/˚F) (T
HIGH
). For
the circuit in Figure 17 ,V
B
= 0.085V + (40 mV/˚F) (86˚F) =
3.525V.
V
C
represents the lowest temperature to be displayed minus
1˚F. That is, V
C
= 0.085V + (T
LOW
−1˚F) (40 mV/˚F) which in
this case becomes V
C
= 0.085V + (67˚F −1˚F) (40 mV/˚F) =
2.725V.
With a few external parts, the circuit can change from dot to
bar mode or flash a bar of LED’s when the temperature
sensed reaches a selected limit (see LM3914 data sheet).
Indoor/Outdoor Thermometer
An indoor/outdoor thermometer capable of displaying tem-
peratures all the way down to −50˚F is shown in Figure 18.
Bar-Graph Temperature Display (Dot Mode)
00905117
•
= 1% or 2% film resistor
— Trim R
B
for V
B
= 3.525V
— Trim R
C
for V
C
= 2.725V
— Trim R
A
for V
A
= 0.085V + 40 mV/˚F x T
Ambient
— Example: V
A
= 3.285V at 80˚F
FIGURE 17.
AN-460
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