Converting a POT into LTE/4G

Having an old project in mind on converting an Plain Old Telephone (POT) to work with LTE/4G became more present when I got the Kirk F61 POT, manufactored in 1964. Leaving out the POT line parts ( terminals, capacitors and linecoil ) provide enough internal space to fit interface electronics and a small 2Ah power bank. The bells and bellcoils are reused to get the original sound on ingoing calls. The original bottom metal plate is replaced by a plast glass plate to ease the LTE/4G signals passing in/out and to hold bells, bellcoils, a “simple” 4G/LTE mobile phone and the power bank.The rotary dial mechanism provides a normally-closed switch that opens a number of times depending on the dialled digit. Dialling 1 opens the switch once, dial 2 opens the switch twice a.s.o. Dialling 0 opens the switch 10 times. A mechanical switch needs a time to settle to get a stable switch signal. To achieve a clean electrical signal on each dial pulse, a debouncer circuit with a Schmitt-trigger is made . As the switch is normally closed, the first switch terminal is connected to 5V and the second terminal is connected to the debouncer input. Resistor R4 (1 Mohm) was chosen to get a low bias current and with C2 = 10nF to use a time constant of R*C = 10 msec., enough for a delay to cope with the mechanical switch settling time. A similar circuit is used at the hook switch to detect hook on/off.

DSZ_Printing equipment output

For the microcontroller I use the Arduino Nano with an ATmega328P or ATmega168P microcontroller, because of its small size and it works at 5V supply. A 2Ah power bank with USB connectors for input/output is used to power 5V for the Arduino Nano and the interface electronics. I looked for a suitable mobile phone and found the Nokia 220 4G mobile phone, because it fits into the POT, but still leaving space for the power bank, the bells, the bell coils and the interface electronics.  I used the connector and the microphone from a cheap earphone headset to modify the POT handset. The original POT loudspeaker works nice with the Nokia earphone jack, so no change for the POT loudspeaker. I used the Nokia earphone Right output signal for the POT loudspeaker (red wire). Adjust the Nokia earphone output setting to fit the modified POT handset.!

The bells, bellcoils and the power bank in place on the plastglass bottom plate. An alu bracket was made elevate the bellcoils to leave space for the Eletra EFB2000BK powerbank and the Nokia 220 4G mobile phone. The original Kirk F61 bottom plate screws are reused to fix the plate to the POT housing.

Mock up / experimental version 1:

To use the Nokia 220 4G functions for outgoings calls, at least 12 buttons must be used: HookOn, HookOff and numbers 0..9 . The Nokia 220 4G keypad has 2 nice gold plated contact pads for each button. On the PCB the button groups (1, 4, 7) — (2, 5, 8) — (3, 6, 9) has a common connection to the outer contact pads ( indicated by the red arrows on the picture ). It makes the wiring a bit easier, that is, with only one common wire for each of the 3 group’s outer contact pads, that is, two contact pad wires are saved for each button group.! My idea was to use the CD4066BE analogue switch for each button contact to shorten and thus emulate the human touch of a button. Each switch is controlled by a dedicated microcontroller pin. The CD4066BE switch has an ON resistance of max. 1 Kohm at 5V supply and tests reveal that the Nokia keypad buttons work nice when with 1 KOhm resistor across the button contact pads.! The button-touch emulation board was made on a piece of Veroboard fitting the shape of the original keypad cover. I soldered in wire-wrap wires to connect the keypad contact points to the button-touch board. At the button-touch board schematic, the ONOFF button is the Nokia keypad ON/OFF/HookOn button. The LIFT is the Nokia keypad “start ingoing call” or HookOff button. The Eagle schematic design of the button emulation board version 1: For an ingoing call, a notifier signal must be found inside the telephone. There is no vibrator inside the phone , the ingoing notifier sound is fed to the internal loudspeaker. The internal loudspeaker is not used because the POT handset is connected to the earphone jack. The internal loudspeaker is removed and 2 wires is soldered to the loudspeakers PCB contact points.

So far, the block schematic looks like this:The version 1 of the Interface electronics with the Arduino Nano includes a transformer to emulate the POT 60V AC to activate the bell ring coils. When the CPU detects an ingoing call, it generates a 31 Hz square signal that feed to Q1 and Q2 to push-pull the transformer to provide the ringing voltage to the bell coils:

The bell coils get voltage from the output transfomer. The power bank 5V DC supply to the transformer is controlled by the CPU via T1 and Q3 to set the ringing  pulse/pause relationship. Experiments revealed a synchronizing problem between the C statemachine and the Nokia screen when a call is terminated. In case the POT hook is on ( not lifted ) during the call, the call is closed by emulating a touch of the Nokia ONOFF button. But when the call is terminated by the “remote end” and the ONOFF button is emulated as before, the Nokia screen goes into lock mode, because the call has already been terminated by the “remote end” !. The missing “call terminated” synchronization between the Nokia and the statemachine was solved by adding 2 analogue switches to emulate the touch of the Nokia “Unlock”  and * buttons in order to be sure to get back to an unlocked Nokia screen. At the connector SL6 the 2 control signals for the extra CD4066 switches are available. The basic statemachine behaviour looks like this:

To make the embedded software working, the Nokia settings for disabling the SIM pin and the screen timeout has to be set-up via the Nokia 220 4G menus ( do this before making hardware changes to the Nokia keypad !! ) :

Settings -> Connectivity -> Dual SIM -> SIM settings -> SIM1 -> PIN code request to DISABLED

Settings -> System -> Backlight -> Timeout to 10 seconds

The embedded software is made in C with Atmel Studio v. 7.0.2542 (Microchip Studio) and an AVRISP MKII programmer to flash the CPU directly, that is, the Arduino bootloader is omitted. The embedded C program for the version 1 includes a power down option ( hook lifted and dial 000 ) for the Nokia phone. The embedded software for version 1 can be seen here: Kirk_C_v1

A video of an ingoing call is here: Kirk_F61_call_in

Mock up / experimental version 2:

The button emulation board will use a PCB with SMD parts to: 1. Add a 4-16 decoder for the analogue switch control pins. 2. Add decoupling capacitors. 3. Minimize the number of wires to the Arduino Nano. 4. Improve EMC. The schematic of button emulation board v.2:Just for future options, a button emulation for the Nokia key # , is added. The PCB design for button emulation board version 2 ( shown without the top/button ground planes ):

To leave space for the wires at the top of the PCB, approx. 1.5 mm of the bottom display window (black) frame is cut away The PCB has been filed a bit as well to fit the Nokia plastic frame.!

The images show the Nokia 220 4G keypad break-out wiring and the button emulation board v.2. The version 2 schematic of the Interface electronics with the Arduino Nano looks like this:A video with a call out session can be seen at:

Power consumption considerations:

Button emulation board: When powered off, the CD4066 analogue switches are not in high impedance state that implies a current drain from the Nokia 220 keypad circuit. In close down, the Nokia battery looses full charge during approx. one week. To keep the CD4066 analogue switches in high impedance state, the board need to be powered 24/7 via the power bank. The board max. supply current at 25 deg. C is 9 uA ( 4×0.25 uA from the CD4066s and 8 uA from the 74HC4514).

POT interface board at 25 deg. C without Arduino Nano: The NAND gate chip CD4093 has a max. supply current at 25 deg. C is 1 uA  and the input circuits of IC1A and IC1D consumes approx. 10 uA , i.e. 2x5uA caused by the normally closed switches of POT dial- and hook-switches. The cut-off leakage currents for the 4 ring generator transistors adds up to approx. 10 uA. Making a total current consumption of 21 uA.

Arduino Nano hardware: To keep the current consumption low, the following components of the Arduino Nano are removed ( marked with red squares on the schematic ):

Arduino Nano software considerations:

The X-tal for the CPU clock has been changed from 16 MHz to 4 MHz to lower the CPU power consumption. Furthermore, the DIV8 fuse for the CPU is set to divide the internal clock by a factor 8 to become 4Mhz/8 = 0.5 MHz clock frequency.

The CPU enters a power sleep mode approx 15 min. after the last hook down and wakes up again at hook off or on an ingoing call.

The Nokia phone can be closed down by dialling “040” . The Nokia phone can be started up by the rotary dial of “050”, but allow 30 sec. for the phone to initialise. For the statemachine, an extra state “sleep” was added to separate the Nano power down sleep mode:The embedded software version 2 of the Arduino Nano is here: Kirk_C_v2g.pdf

The power consumption measurements for the button emulation and the POT with Nano:

The power bank reveils an internal leak current of 50 uA.

With the CPU in power down sleep state, the two I/O boards and the Nano uses 0.14 mA , adding up to  0.19 mA including the power bank internal leakage current. With the 2Ah power bank it should reach to 1.2 year of stand by !.

With the CPU in active mode, running the statemachine without in-/outgoing calls, the current consumption is 2.5 mA

On an ingoing call and active bells the current consumption is 95 mA.

Unexpectedly, the power bank is charging the mobile phone via the common power connection when the external 5 V DC power input is removed. To avoid this situation, an external 6 V DC / 0.5A adapter is used but with 3 diodes ( one schottky + one ordinary diode for each power path) to separate the power distribution:

NOTE: The relatively small power bank used does not support “pass through” charging, that is, the output of the power bank for the interface electronics is disconnected during charging. Before connecting the 6V DC adapter for charging, do  close down the Nokia phone by dialling “040”, and leave the hook down. Then connect the 6 V DC adapter and observe the charge status of the power bank and Nokia phone. When the power bank and Nokia phone is fully charged, you can dial “050” to turn on the Nokia phone again.

Made a version 3 of the interface electronics board with the ATmega328P microcontroller to be independent of the  modified Arduino Nano:

The new PCB with the ATmega328P in place in back side of the POT:

And the updated block schematic now looks like this ( the power bank has been upgraded with a 3 Ah 18650 battery ):

A small final version demo video is here:

I have a few PCBs left over for the Nokia 220 4G keypad emulation board, the I/O with Arduino Nano and the I/O board with the ATmega328P. One PCB is for sale at 5 Euro , 2 PCBs for 8 Euro , postage is to be added. In case you are interested, use my email address: snuskebasse at