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 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.
The filter works when the Michiephone is in receive mode. The high pass filter is a Sallen-and-Key filter  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)  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|
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.
|Designer:||Input Impedance (Real part in Ohms)|
|Lovell :||10 Meg|
|Walraven :||1 Meg|
|Michie :||60 k|
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.
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.
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.
 Texas Instruments data sheet, "TLC271 LinCMOSTM Programmable Low-power Operational Amplifiers", November 1987.
 Horowitz and Hill, "The Art of Electronics", Second Edition, 1989
 Gibson, D., "Ground Shorts with a Single-wire Telephone", CREG Journal 28, June 1997, pp. 10-13.Page
 Lovell, Nigel, "Practical Earth-return Telephone Design", CREG Journal 13, Sept. 1993, pp. 9-12 and CREG Journal 14, Dec. 1993, pp. 3-6.
 Walraven, K., "Single-Wire Communications", Elektor Electronics, November 1994