When small single-chip transmitter and receiver modules became publicly available, it was an invitation to design a skin temperature telemetry system for those parents. I used the very robust modules from Linx Technologies, with 418 MHz chosen over 315 MHz because of the slightly shorter antenna. Essentially, if the transmitted child skin temperature gets too hot (fever peak) or too low (malfunction), an alarm is triggered at the receiver. The version whose receiver is integrated with an alarm clock was patented in the USA under number 5,844,862, which shows variants and other complementary information.
The first task is building a receiver that plugs, as an example chosen here, into the parallel port of a computer. This receiver is extremely easy to build, and can be assembled on a breadboard. Later, it can be made to all fit within the housing of a DB-25 plug, except for the short antenna. If the antenna is a rigid wire don't forget to terminate it by a loop at least an inch diameter, to protect your eyes; the only important factor is that the overall length about 6.25 inches.
Receiver schematics, Parallel Port version
The next thing is writing a "shake-down" program that does two things:
(1) It sets all eight pins of the parallel port high (Power from Parallel Ports). These are summed together through the eight diodes, of Germanium type, chosen for their low voltage drop. Not all computers provide the same oOomph on their parallel ports. After the diodes, some provide 4.6 Volts, others only 3.3 Volts, not sufficient to power the Linx 418 receiver module. In this case, a separate battery should be used to provide the power.
(2) It reads in the status of the receiver bit, takes note of the time at which it transitions high-to-low and low-to-high, calculates the duration of any detected pulses, and sorts them into bins, to create a Pulse Width Spectrum, to later display in graphical form.
Histogram of pulse durations on 418 MHz
After logging more than 10,000 pulses, the results shows that nearly half of the pulses are in the 1 ms bin, and that overall the 418 MHz airwaves are quite active below 10 ms. These are probably bit-streams from garage door openers. The unexpected finding are those periodic spikes at 11, 27, 43 ms, etc., of origin unknown. Nevertheless, valleys of low activity in between are clearly seen; a pulse duration of 20 ms was therefore selected for the transmitter. The final program will later follow a single logic loop:
Synoptic flow-chart of final program with 2 alarms
Based solely on the time of occurrence of the low-to-high and high-to-low transitions, the duration and the period of the pulses are analyzed. There is a first alarm if the time elapsed between pulses is too long. This happens if interference prevents the recognition of any pulses, or if the transmitter has failed. There is a second alarm if the recognized 20 ms pulses occur too frequently (fever) or too sparingly (sensor too far from skin).
Transmitter seen from the top, antenna in top right corner; obviously a hand-made first SMT project!
Transmitter seen from the skin side, with the thermistor in the center
The transmitter uses two cascaded "LMC555CN" timers. The first one controls the period of the pulse, through a 100 microFarad tantalum capacitor and a 1 MegOhm thermistor (large time constant). The second one controls the duration of the pulse, set by its own "RC" combination (short time constant) to obtain 20 ms.
Transmitter schematics, with all the "negative" pins of TXM creating a ground plane under the chip
Rough topological layout of transmitter; don't forget jumpers J1 and J2
Only draw the copper traces after all the components have been collected. The negative trace circles all around to provide a ground-plane; there are two jumpers J1 and J2 needed to complete the circuit.
Hand-drawn Printed Circuit Board
The relatively huge battery can provide months of continuous operation.
For the prototype, a toy inflatable balloon was filled with hot tap water, and it was left to cool off for 2 hours, while recording the period of pulses from a transmitter strapped against it, and measuring the actual "skin" temperature with a certified thermometer. A factory process would need only 3 measurements in temperature-controlled enclosures to do the same.
Transmission period over cooling time, used for calibration
With the "555", thermistor resistance is proportional to pulse period. The relationship between temperature and resistance of the thermistor is all but linear ( 1/T_K = a + b Log(R) + c Log(R)^3) ). Make a 2nd-order approximation valid only in the relatively small span of temperatures to be monitored, for example between 94 and 108 deg F, with 3 measurement points (T_F = a + b PER + c PER^2), and again, compare the telemetered and real temperatures:
Telemetry temperature vs. thermometer temperature
The small deviations of the telemetry data were probably due to spike or data-loss noise, well within the tolerances. The time response also needs to be evaluated. The thermal properties of the transmitter were measured by applying it from an ambient temperature of 70 deg F to a 110 deg F water balloon, then, after stabilization, doing the opposite:
Time response to a step strimulus
Temperature equalization is much faster with physical contact than in open air (as expected), and its time constant is less than the period of the measurement pulses. For a commercial product, further testing would be necessary, for example to study the effects of decreasing battery voltage.
Overnight raw data, 19-month old girl
Because of the "spike noise", the data is filtered here using a Median Filter, at a cost of losing a couple of minutes of warning time, which is acceptable in the dynamic of a fever:
Median-filtered temperature data
At 9 PM there was a diaper change, a temperature drop of 10 deg F. At 10 PM there was a totally unexpected fast-onset fever peak reaching more than 105 deg F (more than 106 deg F of core temperature), which triggered the alarm, set at 103 deg F. At 5 AM the diaper belt probably gaped open somewhat, and temperature dropped accordingly (there was no low-temperature alarm set for that trial); it could have masked another temperatre peak. Finally 7:30 AM was the wake-up time. Twice overnight, 4 consecutive pulses were missed by the receiver, and could not be ignored by the data filtering.
One day I got a frantic phone call from a mother who had tracked me down from the Patent Office records, past even an address change. Her son had repeated ear infections and high temperatures and she was desperate to find where she could buy the device. I still feel bad having had to tell her that it is not for sale anywhere.
But this is where you come in, to improve the system and develop it further. Be the first to make an USB version!
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