progosc

Description: The objective of this project was to create cost-effective electronics that could be assembled by a small, informal PCB manufacturing plant in India, easily repaired, and produced with minimal setup. "Jellybean" (common, generic, low-cost) parts were utilized wherever possible. Although integrating the A/D converter, watchdog, and EEPROM within the microprocessor could have reduced the component count, keeping them external allowed the use of a very simple and basic microprocessor for which cost-effective programmers were readily available. The design aimed to facilitate easy manufacturing with minimal setup challenges. The A/D converter receives signals from a matching pressure sensor PCB, which conditions and amplifies the pressure sensor signal. The selected A/D converter is a straightforward component that uses the +5V rail as the reference and provides an 8-bit parallel output read by a port of the microprocessor, which also serves as the data bus. Although the A/D converter has an interrupt output, it is not utilized in this design. An octal latch controls all relays on the board, with the clock continuously running during normal code execution, also feeding the watchdog circuit. The watchdog's output clears the latch, ensuring that if the code stops functioning correctly, the watchdog will activate and reset all relays to the "off" state. The board incorporates a standard I/O bus known as Picobus, and the circuitry is similar to Picobus for multiplexing the LEDs. However, there are sufficient outputs on the CPU to drive the multiplex select lines directly, allowing for the omission of one 4017 chip. An octal flip-flop and three transistors drive a total of 24 LEDs, including 10 within a single bar graph LED package. The device is operated via 12mm tactile push buttons directly mounted on the PCB, which connect to the microprocessor port pins, utilizing the internal pull-ups of the CPU. The power supply design includes VDR overvoltage protection and two reverse polarity diodes. VDR protection was also implemented on the relay contact outputs to mitigate inductive back EMF issues. A basic serial circuit was integrated into the PCB, designed for low-speed and short-distance operation, utilizing two transistors as drive elements without reaching full RS232 compliant voltage levels. The board layout was intentionally kept simple, open, and robust, adhering to 12 thou x 12 thou design rules, with many tracks made larger than necessary to facilitate production. Most wiring connectors are located along one edge, and the design emphasizes low unit cost and simplicity, employing plain 5mm wiring blocks without plug/unplug capabilities for cost-effectiveness. Ribbon cable connects the reeds, with one end crimped to an IDC10 connector and the other end stripped and soldered permanently to each reed switch. The serial interface is a standard DE9.

The circuit design prioritizes ease of assembly and maintenance, making it suitable for small-scale production environments. The choice of jellybean components not only minimizes costs but also enhances the availability of parts, ensuring that replacements can be sourced easily. The external configuration of critical components like the A/D converter and watchdog circuit aids in simplifying the microprocessor architecture, allowing for a more straightforward programming and debugging process.

The A/D converter's integration with the pressure sensor PCB exemplifies a practical approach to signal conditioning, ensuring that accurate pressure readings are obtained before processing. By utilizing an 8-bit parallel output, the design allows for efficient data handling within the microprocessor, while the decision to forgo the interrupt feature simplifies the overall operation.

The watchdog timer is a crucial safety feature, providing a fail-safe mechanism that resets the relays in the event of a malfunction. This design choice enhances reliability, making the system robust against software glitches. The use of an octal latch for relay control is efficient, enabling simultaneous operation of multiple relays while minimizing the complexity of the control logic.

The LED multiplexing strategy employed in the design allows for effective visual feedback without requiring additional components, optimizing the use of available resources. The tactile push buttons provide a user-friendly interface, and their direct connection to the microprocessor simplifies the input handling process.

The power supply design incorporates protective elements to safeguard against voltage spikes and reverse polarity, ensuring the longevity and reliability of the circuit. The inclusion of VDR protection on the relay outputs is particularly important in inductive applications, where back EMF could otherwise damage sensitive components.

Overall, this project exemplifies a commitment to cost-effective, reliable, and straightforward electronic design, suitable for small-scale manufacturing and easy field maintenance. The thoughtful integration of components and protective measures ensures that the system operates effectively while remaining accessible for repairs and upgrades.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.

This board layout was deliberately kept simple, open and robust. 12 thou x 12 thou design rules were used, but often the tracks were made larger than required just because they could be, and it helps in production. The majority of wiring connectors were kept along one edge. This project has emphasised low unit price and keep-it-simple engineering, so the wiring blocks used were plain 5mm type without plug / unplug capability - cheap but effective.

The reeds were connected using ribbon cable as this is a very cost effective way to do simple field wiring - at one end the IDC10 connector is crimped on, then at the other end the ribbon is stripped back and soldered permanently to each of the reed switches. The serial is a standard DE9.