8-Channel Low-Side Driver Board - Part 1#

Today I decided to start work on another building block board: an 8-channel low-side driver board.

The idea behind this board is fairly simple. I want a reusable module that can switch external DC loads from logic-level control signals, without having to rebuild the same MOSFET driver circuit every time I need to control a relay, solenoid, small motor, lamp, or other moderate-current load.

This board is intended to sit between a controller and the load. The controller provides logic-level signals, and the driver board handles the actual current switching.

What the Board Is For#

The board is designed to drive loads such as:

  • relays
  • solenoids
  • small DC motors
  • lamps
  • buzzers
  • LED strips or indicator loads
  • other small DC devices

The current design target is around 1 A per channel, although the exact practical limit will depend on the MOSFET choice, PCB layout, connector rating, thermal performance, and how many channels are active at the same time.

This is not intended to be a large motor controller or a high-power industrial output module. It is meant to be a practical prototyping and small automation board for moderate DC loads.

Low-Side Switching#

The board uses low-side switching.

That means the load is connected between the positive supply rail and the driver output. The MOSFET then switches the low side of the load to ground.

When the MOSFET turns on, current flows through the load and into ground through the MOSFET. When the MOSFET turns off, the circuit is opened and the load turns off.

This is a common and useful arrangement for switching DC loads, especially where the load voltage may be higher than the logic voltage used by the controller.

For example, a microcontroller running at 3.3 V can control a 12 V or 24 V relay coil, provided the MOSFET, protection components, and board layout are designed appropriately.

Control Inputs#

The board is controlled by eight logic-level inputs.

The intention is for the inputs to work with both 3.3 V and 5 V logic systems. To help with that, I am using a 74HCT541 buffer between the input header and the MOSFET gates.

The 74HCT541 is useful here because it can accept 3.3 V logic-level inputs while being powered from 5 V, and then provide 5 V output drive to the MOSFET gates.

That gives a wider choice of MOSFETs than trying to drive everything directly from 3.3 V GPIO pins.

The basic signal path is:

Controller GPIO
    ↓
74HCT541 buffer
    ↓
MOSFET gate
    ↓
External load switched to ground

Using a buffer also helps avoid placing the full gate-drive burden directly on the controlling microcontroller or expansion device.

MOSFET Output Stage#

Each output channel uses an N-channel MOSFET as the switching element.

The MOSFET is placed on the low side of the load. The source connects to ground, the drain connects to the output terminal, and the load connects between the output terminal and the positive supply rail.

When the gate is driven high by the 74HCT541, the MOSFET turns on and completes the circuit.

Each channel is intended to have its own MOSFET, so the board provides eight independent switched outputs.

At this stage, I am designing around relatively small DC loads rather than high-current motor control. The target is roughly 1 A per channel, which should cover a useful range of relays, solenoids, small motors, and general automation loads.

Why Not Just Use a Driver IC?#

There are existing driver ICs that can handle low-side switching, and in some cases they are absolutely the right choice.

However, for this board I wanted something built from simple, understandable parts:

  • a logic buffer
  • discrete MOSFETs
  • input protection and conditioning
  • output terminals
  • flyback protection for inductive loads

Using discrete MOSFETs also gives more flexibility around current rating, voltage rating, thermal behaviour, and layout.

It also makes the board more educational. One of the goals of these building block boards is not just to create useful modules, but also to document the design decisions behind them.

Inductive Loads#

Because this board is intended to drive relays, solenoids, and small motors, inductive load protection matters.

When an inductive load is switched off, it can generate a voltage spike as the magnetic field collapses. If that spike is not controlled, it can damage the MOSFET or cause unreliable behaviour elsewhere in the circuit.

For that reason, each output channel needs suitable flyback protection.

For simple relay and solenoid loads, this would typically mean a diode path across the load or from the switched output to the positive supply rail, depending on how the board is arranged.

This is one of the areas I need to be careful with, because the board is intended to be general purpose. I want it to be useful without making assumptions that are too narrow.

Power and Output Connections#

The board is intended to expose the switched outputs through screw terminals.

For practical use, screw terminals make sense here. Loads such as solenoids, relays, motors, and lamps are unlikely to be connected using small pin headers in a real installation.

The board will need:

  • a positive load supply input
  • a load supply ground / return
  • eight switched low-side outputs
  • logic input header
  • logic supply input for the buffer
  • common ground reference between logic and load side

Because this is not an isolated board, the control electronics and load supply need to share a common ground reference. That is important for the MOSFET gate drive to work correctly.

Protection Considerations#

There are a few protection details worth building into the board from the start.

At minimum, I am considering:

  • flyback protection for inductive loads
  • gate resistors for each MOSFET
  • gate pull-down resistors so the outputs stay off during reset or disconnect
  • input resistors or protection on the logic inputs
  • sensible power trace widths
  • clear labelling of supply polarity and output channels

Depending on the final design, I may also include supply-side protection such as a fuse, polyfuse, or TVS diode.

The challenge is balancing protection, cost, board size, and general-purpose usefulness.

Design Intent#

This board fits into the same general direction as the other Protoworks building block boards.

The goal is to create small, practical, reusable circuit modules that can be used during prototyping, testing, and early product development.

Rather than designing the same support circuitry repeatedly, I want a small catalogue of boards that solve common problems:

  • power input
  • voltage regulation
  • RS-485 communication
  • digital I/O expansion
  • low-side load switching

This 8-channel low-side driver board is another piece of that toolkit.

It should be useful for simple automation systems, test rigs, lighting control, small actuators, and embedded projects where a microcontroller needs to control real-world loads.

Current Status#

At this stage, the board is still in the design phase.

The basic architecture is:

8 logic inputs
    ↓
74HCT541 buffer
    ↓
8 N-channel MOSFETs
    ↓
8 low-side switched outputs (with optional flyback diode protection)

The next steps are to finalise the MOSFET choice, check the thermal behaviour, decide on the flyback protection arrangement, and complete the PCB layout.

As with the previous boards, the intention is to keep the design practical rather than overcomplicated.

It does not need to be a perfect industrial output module on the first revision. It just needs to be useful, understandable, testable, and honest about its limits.

That is generally where these building block boards are most valuable.

If you are looking for reusable open hardware modules to speed up your bench testing, or need support designing custom breakout circuits, check out our catalog at Protoworks (NI).