Generic High Power 8-Channel Digital Output Board

Design

The design provides 8-channel digital control at 12V dc (though it would work with higher or lower voltages). Each of these outputs can be used with Pulse width Modulation (PWM) to enable power control to lights, motors, etc. (to control speed, brightness, etc.). The primary application we have in mind is control of 12V LED lighting within our smart home. We are also using it to control fans, alarm sounders, security strobe, Wi-Fi access points, etc.

RPi B header board
Each board is connected using a 'standard' 10-way PCB header. This is the same header and pin connections as used on our optically isolated input board.

Typically these connect to a header board we designed for the model B Raspberry Pi. On later versions of the Raspberry Pi we use our new version. We also connect these to Arduino UNO processors and a Netiom Ethernet I/O board (as we adopted the latter devices pin-out for the 10-way header).

Each GPIO pin configured as output on a Raspberry Pi provide 3.3V. On an Arduino the pins are 5V output. These are connected to an ILQ74 quad opto-isolator via an 820Ω resistor to limit the current. This provides complete electrical isolation for the chosen processor.

The output stage of the ILQ74 then provides a 12V dc feed to the base of the power transistor via a 1KΩ resistor to limit the base current. Typically we are using a TIP120 power transistor in TO-220 package as these work well and are available very cheaply in volume.

Each load is connected via two screw terminals. The first is connected to +12V dc and the second is pulled to ground via the power transistor.

Prototypes

Prototype output board
This is an early prototype digital output board on Veroboard we created for automation in our shed. This is 5-channel board.

We also built an 8-channel prototype board.

Manufacturing

Digital output PCBs
These are the V1.0 PCBs that have been professionally manufactured by Ragworm.

This is one that has been completed with all parts soldered to it:

Completed digital output board

Typically this board is used to control devices requiring less than 1A at 12V with the onboard TIP120 power transistors. The board has been designed so that high current wiring can also be used directly between the power source, load and power transistors. This supports larger TO-3 packaged transistors (such as a 2N3055) mounted on heatsinks, to enable loads up to 50W per channel if required.

We deliberately haven't gone with surface mount components because they are fiddly to solder and we have tried to use common components across a wide range of projects. The PCB could be smaller and slightly cheaper but the main limiting factor on its size are the screw terminals and the power transistors.

Summary

Put simply, a couple of these boards could control all of the lighting we require in the upstairs of our current home. All of this from bathroom ceiling lighting, to desk lighting, wardrobe lighting, etc. is acheived with 12V LED lights and bulbs.

These boards cost approximately £20 each in parts or just £2.50 per channel. Obviously you have the cost of a Raspberry Pi or Arduino to include but the latest Raspberry Pi can support three boards like this (24 channels). That works out at about £3.20 per channel including the Raspberry Pi!

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