Technical Information

The following is technical information on the Speleonics Michiephones. This information is a condensed version of a talk given at the Electronics Workshop at Muotathal, help after the UIS Conference in Switzerland, August 1997.

The Speleonics Version of the Michiephone

The original Michiephones used the ubiquitous 741c op-amp. While modern versions of this op-amp feature low power dissipation the 741c based Michiephone exhibited significant signal distortion as the input signal on the Michiephone line approached the supply line. The original Michiephones were also susceptible to interference from local radio stations when used in built-up areas. They were also labour intensive to build and complicated mechanically to assemble.

For these reasons I needed to look at a redesign of the entire Michiephone for improved rejection of interference and better manufacturability. I also needed to be careful that the inherent simplicity, performance and ease of use of the original Michiephone was retained.

The Op-amp and Other Discrete Components

The Speleonics version uses a Texas Instruments TLC271CD surface mount op-amp [2]. This op-amp uses the Texas Instrument's silicon gate LinCMOSTM technology and features high input impedance and low power consumption. The chip operates over a wide range of supply voltages, from 3V to 16V. The output voltage range includes the negative rail. Operating temperature range is 0 °C to 70 °C. Other temperature ranges available are from -40 °C to 85 °C (which may be needed in caving areas where sub-zero temperatures occur) and the military range from -55 °C to 125 °C.

The TLC271 op-amp offers a bias select mode which allows the user to select the best combination of power dissipation and AC performance. The bias mode used is determined by whether pin 8 is connected to VDD , the midpoint of the supply rails, or ground. The Speleonics Michiephones use the op-amp in high bias mode as operation in low bias mode resulted in some oscillation in receive. The high bias mode consumes a little more power than the other modes however this is still very small and comparable to the LED current.

Resistors are surface mount, 1206 package, and capacitors are also surface mount in 0805 package except for the 4.7 µF tantalums (C1 & C6) which are in the larger B case. Surface mount for all components was chosen because it is likely that surface mounted components, being smaller and more flush with the PCB, would be more resistant to the high forces developed if the unit was dropped. Assembly of the PCB is also faster which helps to reduce the cost of the units. Assembly is done using a manual pick-and-place machine. A conformal coating is used to provide moisture resistance to the PCB and prevent corrosion.

Radio interference and Bandpass filter implementation

The conventional Michiephone is prone to pickup of some audio frequency interference from local AM radio stations and power line hum. This is usually not a problem in most caving areas in Australia as there are no nearby radio stations and the Michiephones are operated within a cave, however, there were some regions which have caves in the middle of town and in some circumstances one may wish run the Michiephone line for considerable distances along the surface. Because of this I wanted to incorporate a bandpass filter into the design while still maintaining the single chip simplicity and reliability of the original Michiephone.

The filter works when the Michiephone is in receive mode. The high pass filter is a Sallen-and-Key filter [3] implemented by the two 1.0 nF capacitors (C3 & C4) connected to the non-inverting input of the op-amp and the 560 kOhm resistor (R5). The low pass filter is a VCVS filter (voltage-controlled voltage-source) [3] implemented by the two 47 kOhm resistors (R1 & R2), the 47 pF to ground (C2) and 0.1 nF feedback capacitor (C5). These two filters together give a rolloff with 3dbV points around 200 Hz and 50 kHz to the receive circuit. The low pass really just acts to reduce the RF carrier from AM radio stations so there is no advantage in it being lower than 50 kHz. Note that in transmit mode nearly all of the components making up these filters (C4 is the exception) appear after the op-amp output, but before the coupling capacitor C1, and connect the op-amp output to ground. The impedance though of all these components is high compared to what the line sees so they have little effect on the performance of the transmit circuit. Capacitor C4 appears across the potential divider resistor R4 but again has little effect.

Design of the circuit was done using a circuit analysis package and the calculated frequency response was compared to the actual response measured on a Hewlett-Packard Gain/Phase Analyser. The agreement was quite good. Values for the input impedance, both calculated from Microcap circuit modelling software and also measured by a Gain/Phase Analyser, are shown in Table 1.

  100 Hz 1.0 kHz 3.0 kHz 10.0 kHz 100 kHz
Calculated (kOhm) 330 297 263 100 46
Measured (kOhm) 340 290 240 86 40
Table 1: Speleonics Michiephones - Impedance (Real part) versus Frequency

To further improve rejection of unwanted radio the input line signal passes through several turns of fine wire around a small ferrite bead. Whether this provides any improvement though is untested.

Input impedance

There has been considerable discussion in the CREG Journal and on the Speleonics newsgroup (US Speleonics email group, not Speleonics, Australia) on the merits of high impedance versus low impedance single-wire-telephones (SWTs). An excellent analysis of the SWT system can be found in D. Gibson's article in the CREG Journal [4]. The choice of an appropriate input impedance was not clear cut and I settled on a value that was higher than the original Michiephone but not in the class of high impedance. Values for typical designs that have been published and the Speleonics value are shown in the Table 2.

Designer:Input Impedance (Real part in Ohms)
Lovell [5]: 10 Meg
Walraven [6]: 1 Meg
Speleonics: 250 k
Michie [1]: 60 k
Table 2: SWT Designs and their Input Impedance

In the Speleonics design the 2.2 MOhm resistors (R3 & R4) of the potential divider would by themselves result in an impedance around 1 MOhm however the presence of the feedback capacitors in the bandpass filters reduces this impedance to around 250 kOhm.

Operational aspects

The LED colour is red to maintain any dark adaptation in a cave and is a high efficiency type to minimise power consumption. The series resistor (R7) of 22 kOhm was chosen to provide a sufficiently bright intensity in the cave rather than on the surface.

Total current consumption is around 1 mA in transmit and around 3 to 4 mA in receive. The LED accounts for about 0.5 mA of this. Note that the receive current is higher than the transmit current. The louder the person at the other end talks the greater will be the current drain in your Michiephone. A 9 Volt PP3 alkaline battery has around 550 mA.h capacity which means that in typical intermittant operation the battery in the Michiephones will last for hundreds of hours. In cold caves where temperatures may reach near or below freezing alkaline manganese batteries will not provide the output voltage or service life. The use of a 9 Volt, PP3 lithium manganese battery would be recommended in this case as their operating temperature range is from -40 °C to 70 °C, the shelf life is 10 years and the capacity is around double that of an alkaline manganese.

The Michiephone cable with alligator clip is 512 strand (512/0.05 mm), extra flexible, instrument wire with PVC insulation. Strain relief is provided within the diecast box.

Maufacturability and Maintenance

The PCB is held in the diecast box at one end by the On/Off and PTT switches. At the other end of the PCB the speaker is attached and surrounded by closed cell foam.

The unit can be disassembled by unscrewing the nuts from the On/Off and PTT switches which allows the circuit board to be removed from the diecast box and layed to one side. It is still attached to the diecast box by the Michiephone cable and a tie-tag for strain relief but there is enough access for cleaning to be done. If necessary the tie-tag can be cut and the alligator clip unsoldered for complete disassembly. A piece of felt separates the speaker from the speaker grill to protect the speaker from grit and mud ingress.

Conclusions

The Michiephone has now been in use for caving and cave rescue around the world for over twenty years and the fact that even with modern technology the basic design is still used is testimony to the elegance and simplicity of the concept. They have proved to be a valuable contribution in cave rescue and in situations where cavers need communications with the surface. The New South Wales Cave Rescue Squad has found them to be invaluable in exercises and rescues. Speleonics has been fortunate in being able to build on this and provide a commercially manufactured Michiephone to cavers and rescue squads in Australia and overseas.

References

[1] Michie, Neville, "The Incredible Long-range Talking Machine", Journal of the Sydney Speleological Society, 1974, 18(11):297

[2] Texas Instruments data sheet, "TLC271 LinCMOSTM Programmable Low-power Operational Amplifiers", November 1987.

[3] Horowitz and Hill, "The Art of Electronics", Second Edition, 1989

[4] Gibson, D., "Ground Shorts with a Single-wire Telephone", CREG Journal 28, June 1997, pp. 10-13.Page

[5] Lovell, Nigel, "Practical Earth-return Telephone Design", CREG Journal 13, Sept. 1993, pp. 9-12 and CREG Journal 14, Dec. 1993, pp. 3-6.

[6] Walraven, K., "Single-Wire Communications", Elektor Electronics, November 1994