Friday, May 29, 2015

LED lights deadbug style

This will be a brief article about building some LED lights to illuminate small things. The objective is to create some lights that can be oriented as needed to iluminate small objets to work on.

Schematic of the circuit

The basic circuit is a constant current driver using a NPN transistor. The 6k8 resistor, the 1N4148 diode and the adjustable resistor set the base voltage at the transistor between about 0.6V and 1.2V. That sets the votage at Re between 0 and 0.6V. The current at the transistor collecto will be between 0 and 0.6V/Re.
For instance, selecting a 15Ohm resistor we can get currents on the LED up to 27mA.

To power the circuit we will use a 2x6 100 mill spaced connector (J1). This connector works in a pseudocoaxial way with ground at the outer pins and +5V at the center. Having two rows enables us to chain several independent LED drive circuits.

The LED is also connected to the circuit using a 3 pin pseudocoaxial connector (J2) with +5V on the outer pin and the drive current on the center.

The circuit is mounted on a scrap piece of two sided copper PCB. The connections are done directly on the copper using deadbug style.

Deadbug circuit

We want the LEDs to be able to be positioned in the position we want. To do that a SMD LED is mounted on a rigid wire soldered to the current pin at  the center of a 3 pin male connector. A second thin wire provides the +5V supply on one of the outer connector pins.

LED connected to a rigid wire
In order to have two independent LED lights I have build two twin circuits.

Twin LED drivers

Next figure shows the detail of the supply J1 connector.

Supply connector
This connector enables us to chain both boards

Chain power to the twin drivers

Now we can put the LEDs on the driver boards

Twin lights
We can also use different LED colors, although this is less usefull.

Green and White lights

That's all for now. This was only a little project to iluminate small spaces. In fact, with the size of the SMD LEDs, we can practically illuminate anything regardless of the space we have to route the lights.