The TV Timer is designed for use in the hospital and hospitality industries, or any situation where access to television channels needs to be controlled. Existing solutions to implement 'pay' television range in complexity from simple coin-operated time switches, which cut the power or aerial connection to the set, to set-top decoder boxes, which selectively decode the channels that the user has paid for. The problem with the simpler systems is that they lack flexibility. A hospital ward, for example, may wish to offer free access to information channels and local television stations, but charge a fee for a 'movie channel'. A scrambler/decoder system can offer this functionality, but can be very costly. The TV Timer represents a compromise between these approaches, offering the following features:

• Access control of 'pay' channels while allowing transparent access to 'free' channels
• Simple operation using custom infrared remote-control
• Enable times from 1 hour to 30 days
• On-screen status display
• Self-contained within television set
• Low cost (under $40US per set)

The TV Timer system requires 'pay' channels to be confined to a particular band (eg. UHF) on the television aerial network. 'Free' channels are, if necessary, remodulated to another band. The TV Timer PCB controls access to the 'pay' channels. Once fitted to a television set, the unit detects when the tuner is receiving the 'pay' band, and blocks the video signal, unless the timer has been enabled. Staff can use a remote-control unit to enable the device for any period up to thirty days, in one-hour increments. A real-time clock, backed with a super-capacitor, can maintain the timer for more than two months without power.

The heart of the TV Timer, and also the remote-control unit, is the Philips(TM) P87LPC764 microcontroller. The microcontroller controls all the functions of the unit in real-time, including a bidirectional infrared interface, a status display through an SGS-Thomson(TM) STV5730A OSD chip, and monitoring the elapsed time kept by a Dallas Semiconductor(TM) DS1302 RTC chip.

The P87LPC764 20-pin microcontroller is a member of the recently introduced 51LPC family. Among the features that the P87LPC764 offers are: Accelerated 80C51 architecture, 4K bytes of code memory, a hardware UART, two analog comparators, I2C port, and on-chip oscillator, power-on and brown-out reset circuits.

Now the bad news. The code memory is only one-time programmable, and a low-cost programmer was not readily available at the time when this project was first conceived.

Having said that, the good news is very good indeed. The chips are inexpensive - around a third of the price of flash chips of similar capacity - and the P87LPC764 has a number of features built-in which require external components on other microcontrollers. In fact, when configured to use the on-chip RC oscillator and reset circuit, connecting power is all that is necessary, leaving 18 pins for I/O. Because it is 8051 compatible, a wealth of development tools - assemblers, compilers, editors, simulators - are available at very low cost. And at 20MHz the CPU runs at the equivalent of a 40MHz 8051, making the P87LPC764 fast, flexible, and cost-efficient.

Now for the really good news. The author has developed a simple low-cost programmer for the P87LPC76X series. The programmer requires only a few dollars worth of components, and attaches to the parallel-port of any Windows 95 PC. The software, circuit and source-code are being made available to the public domain. Hopefully, this will encourage hobbyists and experimenters to make use of these powerful chips.