Relay and Isolated Input Base Board

This PCB is part one of a stackable series, which will comprise an Automation Controller, controllable via the ESP32-s or ESP-12E SOC modules. `This project is considered the next step in my quest to create the ultimate Home Automation controller.

With this PCB, I attempt to solve a few problems, as well as incorporate some of my already well functioning circuits into a stackable base-board. In my previous project, (Make your own ESP12-E based IoT Controller) I managed to create ( in my opinion at least ) a close-to-very-easy-to-use device. While this may be true, the ESP-12E based controller still had quite a few shortcomings.

-It can only be powered from a DC power source, making it necessary to have external AC-to-DC conversion components.

-It has only two built-in relays

-It has no optically Isolated inputs, thus making it necessary to use external circuitry to protect the inputs from voltages bigger than 3v.

-It has only another 4 spare GPIO on the PCF8574 port expander

-Needs more power ports ( 3v, 5v and GND ), as well as more access to the I2C Bus.

-An external level converter is required to connect devices requiring 5v to the I2C Bus or other GPIO pins.

Of all these, certainly the most painful, to me at least, is definitely the lack of support for powering the device directly from AC mains power.

This project is thus my attempt to correct that. Another big concern is board size. I want to stay in a form factor that is relatively as small as possible, to make it easy to install into already available commercial electrical containers and boxes.

It was thus decided to go for a series of stackable cards, with each adding functionality to the completed device, while not necessarily needing every single one of them in order to have a functional device.

So, let us start with a detailed explanation of the PCB.

Powering the device from AC power.

This part seemed to me to be the most problematic at first, as designing a custom SMPS is definitely not something that I am very comfortable with. I have thus decided to use a commercially available PCB mounted AC/DC module, delivering 5v at 2A to take care of that issue. The HLK-10M05 is quite reliable and already proven in the industry. It is also relatively cheap and easily obtainable.

Having the possibility to power the device completely from DC power is also not an ability that I would like to lose. I have thus also included a DC power jack as an alternative power source. This will power a 5v LDO regulator, as well as the required 3.3v LDO regulator for the ESP SOC. This 5v regulator is protected with a protection diode to prevent 5v power from flowing into it when the SMPS is in use.

I have also noted in the ESP-12E version, that the regulators got a little hotter than I would like when powering the device from 15v DC.

I have thus added some large via-stitched copper heatsinks below the LDO regulators, as well as the SMPS module to help to dissipate some

of the generated heat. In the newest prototype, this seemed to work quite well.

Quite a few filtering and decoupling capacitors were added to the power supply circuit to decrease any ripple to an absolute minimum.

The +3v, +5v and ground lines are fed to the stacked boards through a 2x4 way header in the right-hand corner of the PCB

The usual anti-tracking cut-out slots were also added to all areas where mains power will be present. These areas were also excluded from the ground planes on the PCB.

Optically Isolated Digital Inputs (x4)

In the top left-hand corner of the PCB is an 8-way screw terminal, connected to the Optically Isolation Circuits. These signals will be sent up to the controller board via a 2x2 pin header in the same left-hand corner. The circuitry is based on a previous design of myself, based on the EL357N, and capable of accepting a signal of 5.5v to 32.0v DC. Please note that this signal is completely galvanically isolated from the board. This means that you must provide a separate ground for each individual input as well. This ground should not be common to the PCB, as doing that will defeat the purpose of isolating the input in the first place. Four status LEDs are provided for port status and debugging purposes.

The Relay control circuitry

The relay control circuit is once again exactly the same, optically isolated relay driver circuit as was used in the ESP-12E IoT controller PCB. This circuit performs well and I did not see any need to change it. The only change was the addition of an additional two relays to the PCB.

I think it is worth noting that these relays are MAINS POWER FREE, i.e DRY Contacts. This, in my opinion, provides greater flexibility to the end-user, by allowing him/her to switch AC or DC with a specific relay. The components for each relay driver is placed close together, in order to make repair and debugging in the event of a failure easier. The relay circuits are ACTIVE LOW, and the control signals are received from a 2x2 pin header roughly in the centre, right-hand side of the PCB.

Standalone Use, or using the PCB with another microcontroller.

It is also possible to use this PCB with another microcontroller, for example, an STM32 of Arduino, providing that you take in mind that the optically Isolated Inputs will provide a 3v signal. A level converter can be used to take them up to 5v. The relay drivers are also designed for 3v and need to be driven low to energise the relay. A total of 4 ground pins and two each 3v and 5v are broken out onto headers.

Mechanical details

The PCB dimensions are +/- 100mm wide by 97mm high.

6 mounting holes of 3.2mm diameter are provided for mounting

All AC capable connectors should be 5.08mm pitch screw-type terminals.

Optically Isolated Inputs are 2.54mm screw-type terminals.

All other headers are standard 2.54mm male pin headers.

The device should be fused at 1A, with a slow-blow type fuse. ( Operating current is 700mA at peak )

Code

This PCB required no coding and contains no programmable parts.