In CREG Journals 13 and 14, Nigel Lovell published a circuit for an Earth Return Telephone. My telephone is based on his original design, with some additional features.
The telephone has two modes; talk mode and standby/alarm mode. When not in use, the telephone will power down to standby/alarm mode after 25 seconds. Pressing the call button on another phone will activate a loud piezo alarm. The alarm will not sound in talk mode.
The circuit uses a cheap, extremely sensitive jitter switch to detect when the telephone is being handled. When the jitter switch operates, the phone switches to speech mode and the alarm is disconnected. If no pulses from the jitter switch are detected for 25 seconds, the telephone switches to standby/alarm mode, powering off the timer and main speech circuit.
The Circuit diagram and layout are available for download, see below.
A jitter switch feeds pulses into the 'set' input of a flip-flop (two gates of a 4011), and also pulses the reset pin of a 4060 timer. To reduce current, the 4060 is powered from the output of the flip-flop, and therefore switches off completely in standby mode.
A dual micro power op-amp OP296 provides a V/2 reference (labelled 0V in the diagram), and serves as the input signal buffer. An analogue switch IC (DG403) switches the incoming signal between the alarm circuit (in standby mode) and the LM358 speech circuit (in speech mode). The analogue switch also disconnects the V/2 reference from the speech circuit during standby mode. The DG403 and OP296 remain powered in both modes.
The speech circuit is almost identical to Nigel Lovell's original design, and is reproduced with his kind permission. It is based around an LM358 dual op-amp, with a push-to-talk DPDT momentary switch. The speaker doubles as the microphone. For full details of the speech circuit, please refer to Nigel's original articles in CREG journals 13 and 14.
One small change I made was to replace C6 with a physically smaller 2µ2 polarised tantalum, as the voltage across this capacitor is never reversed (unlike C5).
The call detector consists of a pair of transistors, one switching on during the negative, the other switching during the positive swings of a signal of at least 1V peak to peak. This provides some immunity to false triggering due to background interference. The call detector only receives a signal in standby mode.
The schottky diodes are optional, if these are swapped for 1N4148's then the minimum signal needed to activate the call alarm rises to 1.4V peak to peak.
The piezo buzzer is a DC type with built in drive circuit.
The call alarm sender is a simple 7555 timer oscillating at 1500Hz, and is completely disconnected from the circuit when not in use via the DPDT call switch.
A 4060 and a 4011 make a simple standby timer switch for the telephones. The 4060 is powered from the output of the flip-flop and therefore consumes no power in standby. Care is taken to avoid possible latchup (R28 and R17). R16 guards against damage to Q3 due to the extremely low ESR of the 10µF tantalum (C14 and C1).
You'll notice that the flip-flop and timer don't include any circuitry to give a predictable reset state at switch-on. I found this unnecessary because the jitter switch provides a stream of pulses. C7/R30 introduces a slight delay to the standby pulse to avoid a possible race condition.
When the unit is switched on, the LED indicator turns on for about two seconds.
One half of U2 is used as a V/2 voltage reference, with R20/C2 compensating for the drop in gain of U2 at high frequencies.
The other half of U2 forms an input buffer protected by diodes D5 and D8. These diodes will affect performance due to their capacitance, especially with high line or earth impedances. It's probably OK to remove them, but I wanted the circuit to be pretty much bomb proof. You might want to try high speed/low capacitance diodes for better performance.
The DG403 analogue switch is an amazing little device, having negligible current, and able to handle rail-to-rail signals with extremely low distortion. This is used to switch the input signal between the call alarm detector and speech circuit.
The remainder of the speech circuit has been covered in great detail in Nigel's original article (CREG 13 and 14).
The enclosure had to be one size larger than used in Nigel's design, and used a 5004 Bimbox 120x65x40mm with sealing gasket. The mic/speaker, battery and piezo sounder are fixed to the lid, and circuit board bolted to the base.
Two DPDT momentary push button switches are used for alarm call and talk, a latching SPDT serves as the on/off. Sealing hoods give protection against mud and water.
Two 4mm panel sockets provide connections for line and earth. Underground, an earth connection isn't needed, but if you're topside sitting in the car, you might want to make a proper earth connection with a metal peg.
The LED is positioned near the power switch, and a ground connection is taken from the case to the -ve terminal of the battery.
A small section of bicycle inner tube stretched over the buzzer and earpiece gives a reasonably good seal against the lid. Cling film completes the seal, but does muffle the sound a little, especially from the buzzer.
The circuit board and all exposed signal connections were given two coats of insulating varnish to guard against damp and condensation.
The telephones are being used in a Surrey mine. We have a cable running to one of the digs, about 250' from the entrance. The sound is loud with excellent clarity.
In initial testing, the call alarm failed to work. I found that the telephones would switch to standby, then immediately switch themselves back on. This was fixed by adding C12/R22 and using screened cable to connect the buzzer. C12 keeps the darlington pair switched on as the alarm signal passes though the 'dead' zone (-0.5V to +0.5V), giving a continuous tone and preventing the buzzer being switched on and off at 3kHz. The buzzer's inbuilt drive circuitry appears to inject some surprisingly strong HF nasties into its supply leads. Cut the leads short, and connect to the circuit board using lightweight screened cable (RS 367-246 or similar).
In use, there was no detectable breakthrough from the timer oscillator, and the call alarm could be clearly heard 50 feet down the passage.
In theory, the telephones can remain in standby mode for over 7 months using a 500mAh alkaline 9V PP3 battery (MN1604).
A speakerphone IC plus separate microphone and speaker could allow full duplex operation. These chips have built-in automatic gain control for both the microphone and speaker. This would also make the telephone easier to use by losing the push-to-talk button.
It should be possible to increase the input impedance to 22 or 33M by changing R3, but this hasn't been tested.
For cave or mine rescue use, a small modification is highly recommended.
In rescue, it is extremely important to know that the telephone is switched on. Graham Christian of SECRO (South East Cave Rescue Organisation), said he was using a similar telephone during a practice, and the phone was accidentally kicked, switching it off. But because the power indicator only flashed briefly at switch on/switch off, this wasn't discovered for some time.
To avoid this, the power on indicator must be continuously lit and fixed to the front panel where it can be seen.
One of the new extreme brightness LEDs will give adequate indication run as low as 50µA. Replace R31 with a resistor of value 130K and connect to the +ve supply rail bypassing Q2. Remove Q2, C4, R32 and D5. This modification increases the current in standby mode to 140µA.
For the surface you'll probably want an amplified base station. This should be very simple to add using an amplifier of your choice, and should include a volume control (it's handy to be able turn the volume off completely for private conversations).
All capacitors below are Siemens B32529 range (Electrovalue at Egham were cheaper than Farnell for most of these).C3, C9, C10, C12 - 100n
(most high gain, low power transistors will do)IC1 - 4011 Quad NAND gate
I use the excellent Eagle for Linux (http://www.cadsoft.de/) for schematic capture and layout (also available for Windows).
Here is the board and schematic in Eagle version 3.55 format (the board is small enough to load in the freeware version). I've also made the circuit schematic available in PDF format.
A double sided board size 57x95mm was used, with the upper side carrying GND and V+, and the bottom as the signal layer.