This project is a dedicated device. It dials a single phone number when the handset is lifted. There are two different modes of operation.. A slide-switch on the PC board allows the project to operate in automatic or manual mode. If the switch is in “auto dial” mode, a pre-programmed phone number is AUTOMATICALLY dialled when the phone is lifted. If the switch is in “push to dial,” the project dials the number when the phone is lifted and “push to dial” button is pressed. This allows the phone to be used as a normal phone.
TAXI PHONE hasn’t been designed exclusively as a taxi phone, that’s just the name we gave it.
It’s the simplest phone dialing project you can get and since the program is so simple, it’s an ideal place to start.
The concept has so many possibilities. Take the example of a “Free Taxi Phone.” Imagine a phone at the front of a night club or restaurant, to ring a taxi.
As soon as you pick up the phone, it automatically dials a nominated taxi company and by quoting a code number, or sending a DTMF code down the line, the company knows where to pick you up.
Or take the example of a doctor’s surgery, police station, fire station or real estate office.
A cheap wall phone can be mounted outside the premises for after-hour’s use. By lifting the handset the call will be directed to an operator, the owner of the business or a representative.
The same can apply at professional suites, or land-development projects. The proliferation of mobile phones has brought remote communication into the hands of nearly everyone but this project still has its uses.
It’s cheaper and free to the user. A customer would prefer to use the “Company Phone” than his own mobile.
Take the example of a “House for Sale.” By lifting the phone connected to the sale board, you could be in touch with the agent and arranging a meeting before your enthusiasm is diverted to another property.
Or the example of a Panic Phone for a granny, babysitter convenience store. All they need do is lift the receiver and the call goes through. This is already available at some exchanges but the emergency number is only dialed if the exchange does not detect any other number being dialed within 3 seconds. In the Auto mode, our design prevents the phone being used for any other purpose. This can be an advantage in some circumstances, such as a boarding house or as a “hot line.” It prevents the phone being used for any other calls.
The commercial advantage is also enormous. Imagine a phone at an airport or railway station linked to a local hotel or tourist attraction. A small display would highlight the features of the establishment and the free phone could be used to make a booking.
Even booking mini cruises and tours at the departure points could be made, when no-one is in attendance.
It all adds to the efficient running of a business and reduces the cost of attendance during the low periods.
I am sure we can all think of a use for a dedicated phone like this and even one at the front door of your own house would be “magic.”
How many times has a friend dropped by or a delivery been made when you were absent?
Either the goods were left on the door step or brought back three day’s later. How inconvenient. A simple phone call to your mobile could keep you in touch with everything.
The same with business addresses and suites. By providing a phone at the door, the owner will never miss a prospective client.
Not only that. The image and efficiency will rise in the eye of your customers.
So, after seeing the potential of this project, why not put it together and use it yourself or sell it to others.
The only problem is a project like this has to be approved by the authorities for connection to a public telephone system. The cost of approval could be anything up to $5,000 to $10,000 or more and
the delay in approval could be 6 months or more.
Lots of ideas like this have not seen the marketplace, for this very reason. The cost of compliance is enormous. That’s why we have presented the article as an idea, mainly to show what can be done with the ‘508A microcontroller.
Once you build the project, you can modify it to suit almost any requirement.
We have included a number of ideas but it’s certain we haven’t thought of everything.
HOW THE CIRCUIT WORKS
When the handset is “on the hook” (handset down), no current flows in the telephone line and thus the project is not activated.
As soon as the handset is lifted, current flows in the line and the diodes in the bridge rectifier direct the current to flow in the project so that the top rail becomes positive.
The purpose of the bridge is to create a positive and negative rail, no matter which way the project is connected to the line.
The zener diode and the LED form a zener reference voltage of approx 5.6v and this allows the chip to be powered with its correct voltage of 5v to 5.6v
In microprocessor-terms, this voltage rises very gradually to full voltage (partially due to the 100u electrolytic across the rails) when the phone is lifted. The ‘508A has a delayed reset feature inside the chip but there is no guarantee it will see the delayed reset AFTER the full rail voltage. If it does not work, the micro will start anywhere in the program and fail to ring the number.
The answer is to utilise the watchdog timer. This is a completely separate oscillator that counts for 18mS and resets the micro. If this timer is enabled, you must make sure your program resets the
watchdog timer before the 18mS has expired. This is especially difficult to do if you have delay routines or operations such dialing a telephone number, as they run for longer than 18mS. It can be done, but it takes a lot of programming skills.
Fortunately the watchdog timer duration can be increased by feeding it into a divider circuit (it’s called a divider but in fact it actually multiplies the duration).
By setting bits 0, 1 and 2 in the OPTION register, the WDT time-interval can be increased to 128 x 18mS = 2.3 seconds. This is what we have done. The only occasions when the WDT needs to be cleared is before entering a delay routine.
If the micro gets powered up and the program counter starts the micro part-way through a routine where it gets “locked-up”, it could take two seconds or more for the watchdog timer to time-out and reset the micro.
This concept is not foolproof. It is essential the check the program for loops containing CLRWDT instructions, where the micro can enter the loop and never emerge.
If the micro enters the program at an undefined point, it may carry out instructions such RETLW and RETURN with a “junk” RETURN address from the stack and jump to any location in memory.
At the moment the only place the micro can get stuck is at Main5, where a loop has been created to hold it from escaping. The entrance to this loop is only 3 instructions wide and the chance of the micro
getting into it is fairly remote.
Under normal operating conditions the voltage across the micro will start from 0v and the micro will start at the beginning of the program. This will allow us to create a program to dial a pre-programmed number contained in table1.
This table has a rather unusual feature. It is burnt with the initial phone number at the beginning of the table. A further 100 RETLW 0FFh instructions are added to the table for future use. If you need to
change the phone number, the old number (including 0E) is burnt down to 00, i.e: to 00 00, and the next ten locations used for the new number. This allows the number to be changed up to 9 or 10 times. This is a brilliant concept for a chip that is supposed to be a one-time programmable device.
To allow this feature to be used, the chip must not be code-protected.
For more detail: TAXI PHONE