LED Array Test Box Project

This simple project is a little test box that includes 6 LEDs that can be safely driven from the GPIO ports of any MCU.   The device is very useful for quick testing of prototypes - a few lines of code can drive LEDs on any spare pin of the prototype to provide a visual indication of what it is doing.   I tend to set up these indicators and leave them largely unchanged, and use the serial monitor for detailed debugging information.   It works like a very simple logic analyzer. 

The box has connections for 5v and Gnd, and an input pin for the six LEDs.  The inputs are buffered so that the load on each input is about 1.6mA each: that is well within the 20mA per-pin maximum of the Arduino UNO, and also well within 200mA total for all pins.


DM/SN7407 Hex buffer, or similar (eg, 7417)
6 x LEDs with dropping resistors to suit.  This project uses bright white LEDs with 120 ohm resistors. Standard red or green LEDs typically use 220 ohm resistors.
Prototype board approx. 4cm x 6cm.  Many styles would work - this project uses this one.
Headers: 2 x 3-pin, 1 x 6-pin.
3D printed case.

DM/SN7407 Integrated Circuit

7407 ChipThe 7407 medium-scale integrated circuit is a hex buffer/driver with open collector outputs.   Hex simply means there are six separate buffers/drivers within the chip.  Open collector means that the output is a transistor switch - on or off - that switches the supply from some external source to ground.   In this case the external voltage source is actually the same 5v supply that powers the chip, but in other applications it could be a supply of up to about 30v.  The maximum current for each switched circuit is 40mA - well within the 20mA typically required for a LED.  These chips are easily recovered from obsolete equipment - they are large enough to solder with a typical hobby soldering iron, but small enough to fit into a suitable device case. Note that there are several variations of this chip, such as 'LS' and 'S', and also 5407 which is a lower voltage military version: any of these variants is suitable for this project.

Note that the notch at the end of the chip defines the pin numbering.  With the notch at the left-hand end, looking from above, pin numbers start at the lower left-hand corner and proceed anti-clockwise.  The lettering is usually aligned with the notch at the left, but do not rely on it.


LED Array SchematicPins 7 and 14 of the 7407 are connected to the ground and +5v 3-pin headers respectively.  The 6-pin input header is connected to the input pins of the 7407, while the output pins are connected via resistors to the LEDs and then to ground.   When the logic level at the input of the 7407 is high then the corresponding output goes high and connects the LED to ground.


LED Array LayoutThe parts are laid out on the prototyping board in a form that keeps the wiring as simple as possible.  In this example I have placed the LEDs across the middle, with their resistors running to a buss at the top.  The +5v and Gnd headers are off to one side, and the input header is aligned below the LEDs.  The board can then be wired point-to-point on the underside.  The hookup wire came from an old parallel printer cable.  This wire is easily tinned and soldered, the covering is very heat resistant, it is quite flexible, and the multitude of colours means that the wire can indicate the function of the connection, reducing the risk of mistakes. Note that I printed a standoff for the row of LEDs - this helped to keep them properly aligned for height and position.


The case is a simple two-part assembly, with a base that the prototyping board sits down into, and which then screws through countersunk holes from the back into standoffs in the lid.  The lid has cutouts for the LEDs and the headers. The screws self-thread into the holes in the lid - a solution that is adequate for such a small case. The screws were slightly too big for the holes in the prototyping board: the corners could be cut away completely because the board is held securely between the base and the lid.


LED Array Usage The completed unit, with the lid screwed onto the base and connected to a UNO..  There is small marking molded into the lid to identify the +5v and Gnd headers, but it doesn't show in the images.   To use the unit, +5v and Gnd connections are plugged in the respective headers, and then output pins from the MCU are connected to the input header.  When the output is high the LED is lit.   The +5v and Gnd headers provide a convenient point for taking off connections to other items being tested. 

If the +5v supply does not come from the Arduino then it is critical that the GND from the test unit is connected to the Gnd on the Arduino.

Note that this circuit does not separate the +5V supply for the 7407 IC and the positive supply to the open collector drivers.  These drivers can take up to 30v.  If using a device like this with a higher driver voltage then  the 7407 Vcc supply must be provided separately at 5v.   Of course, whatever the drivers are driving must be configured correctly for the higher voltage.