Miniature, low power, telemetry system that can be used with commercially available strain gage pressure transducers accurately measures pressure with a small implantable pressure cell and transmitter. The system has been used to date only with pressure transducers, but the circuit is equally applicable to any measurement using a strain gage sensor

Fryer, T. B.

English

NASA Technical Reports Server

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December 1966	 Brief 66-10624 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Miniature Telemetry System Accurately Measures Pressure
The problem: 
To design a telemetry system to accurately measure 
pressure with a small implantable pressure cell and 
transmitter. The system must operate with low power 
consumption. 
The solution: 
A miniature, low power, telemetry system that can 
be used with any of a number of commercially avail-
able strain gage pressure transducers. A small, solid 
state, strain gage pressure cell, designed for implanted 
physiological applications, is used with the new cir-
cuitry to provide a complete, implantable pressure 
transducing system. 
How it's done: 
The electronic circuit uses a pulse code modulation 
similar to ones previously used for temperature and 
biopotential monitoring. The subcarrier modulation 
technique allows accurate transmission of the low
output level of the pressure cell from an implanted 
location to a remote radio receiver. The small strain 
gage signal (approx. 15 my for 250 mm of Hg) is 
chopped by means of a solid-state switch (Qi, 02, 03, 
04, 05, 06) and amplified by an ac amplifier Qi and 
Os (gain approximately 5). After amplification the 
signal controls the period of an astable multivibrator 
(09, Qio, Qii, 012) operating at approximately 1 
kHz. The pulse derived from the astable multivibrator 
is applied through C8 to obtain synchronous operation 
of the solid-state switch, thereby causing the period 
of the multivibrator to be controlled alternately by 
the voltage derived from 05 and 06. The difference 
between successive periods then is proportional to 
the bridge unbalance signal and hence the pressure. 
The interval between pulses at bridge balance would 
be identical, but in order to avoid ambiguity the 
bridge is initially unbalanced in such a manner that 
one period remains smaller than the other over the 
(continued overleal) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19660000622 2020-03-11T17:46:08+00:00Z
entire operating pressure range. A typical modulation 
of ± 20 percent of the mean period is obtained for 
a pressure change of 250 mm of Hg. 
The short pulse developed by the astable multivi-
brator (approx. 20 microsecond long) is used to turn 
on the rf oscillator, Qi. Li is used both as a tank 
circuit inductor and as a transmitting antenna. Since 
the information is derived from the time period be-
tween rf pulses, amplitude and frequency changes 
in the rf link do not affect the accuracy. After the 
pulses are received on a commercial FM receiver 
(88-108 MHz) a suitable demodulator is used to 
obtain an analog signal. 
The telemetry system is shown with a protective 
coating of elasticized silicone rubber applied. In this 
condition, the system is ready for implantation. 
Notes: 
1. The system has been used to date only with 
pressure transducers, but the circuit is equally ap-
plicable to any measurement using a strain gage 
sensor. The pressure transducer is commercially 
available. 
2. The transducer used is 6.5 mm in diameter and 
1 mm thick. The lead-in wires terminate on the 
back of the transducer in a package that is 3.5 
mm in diameter by 4.5 mm long.
3. The compensated temperature range of the trans-
ducer is from 77°F to 113°F. The telemetering 
electronics are suitable for temperatures to 150°F. 
4. The battery lifetime of 500 hours is associated 
with a transmission distance of 3 to 5 feet. In-
creased transmission distance will be accompanied 
by increased power consumption with a reduced 
battery life. It is estimated that the battery life 
would be reduced to 125 hours for a transmission 
distance of about 100 feet. 
5. Similar applications are described in Tech Brief 
64-10171 for biopotential monitoring and Tech 
Brief 66-10057 for temperature monitoring. 
6. Inquiries concerning this invention may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B66-10624 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: T. B. Fryer 
(ARC-74) 
Hermetically 
Sealed 
Electronics
Battery 
Antenna	
Pressure 
Transducer 
Miniature Telemetry System 
Ready for Implantation 
Brief 66-10624	 Category 01


Miniature telemetry system accurately measures pressure

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