Circuit Diagram

The brief with this project was to produce cost effective electronics that could be assembled by a small, informal PCB manufacturing plant based in India, repaired easily, and producible with the minimum of setup.

We used "jellybean" (common/generic/low cost) parts for this project whereever possible. While it would have reduced component count to place the A/D converter, watchdog, and EEPROM within the microprocessor, by keeping them external we could use a microprocessor that was very simple and basic, and for which very cost effective programmers were easily available. This project was all about making the board easy to manufacture with the least amount of setup and angst.

The A/D receives the signal from the matching psensor pcb which conditions and amplifies the pressure sensor signal. The A/D converter we chose is a very simple part using the +5V rail as the A/D reference and providing an 8 bit parallel output which is read onto a port of the microprocessor which is coincidentally also the data bus. While the A/D converter provides an interrupt output, it is not used.

A simple octal latch runs all the relays on the board, with the clock to the latch (continuously running when code is executing correctly) also feeding the watchdog circuit, and the output of the watchdog circuit also clearing the latch. In this fashion, should the code stop operating correclty, the watchdog will fire and clear the latch, clearing all the relays to the "off" state.

We have a standard I/O bus that you can learn more about called Picobus. This board uses circuitry that is very similar to Picobus to multiplex the LEDs it drives, however there are sufficient outputs on the CPU to run the multiplex select lines directly, so one 4017 chip is omitted. Using an octal flip flop and 3 transistors, a total of 24 LEDs are driven, 10 of them within one bar graph LED package.

The switches to operate the device are 12mm tactile push buttons assembled directly onto the PCB, and directly input to the microprocessor port pins taking advantage of the internal pullups within the CPU itself.

The power supply is simple, with VDR overvoltage protection and two reverse polarity diodes. VDR protection was added to the relay contact outputs to prevent inductive back emf problems. A very simple serial circuit was integrated onto the PCB - it was not intended for high speed or long distance operation, it just uses two transistors as the drive elements and does not swing to full RS232 compliant voltage levels.