Friday 7 September 2012

The Good, the Bad and the Ugly

A key consideration if one wants to do electrophysiology on a virtual reality rig is space for all the required equipment. One such thing is the light source. Ideally it only lights up a small area so that the animal won't be stressed or disturbed by the sudden bright light. To get an ideal solution for this I decided to make a frame for LEDs that would allow me to shield stray light and place it just where I needed it using a snake-arm on a magnetic base.

The Good

The light-source, like all other bits of equipment for this project, will be used in a clean facility. That means no loose cables or half-done jobs. However, to drive the LEDs a small circuit was required that would provide the right current and voltage. Further, being able to dim the LEDs might be beneficial. Less light means less disturbance for the animal. And of course, a nice enclosure makes everything look a bit more professional.

Box and frame. No electrical safety test was required as only 24V DC is going into the housing.

The top of the box features an on/off switch and a knob that dims the LEDs. Of course the knob is just a potentiometer underneath. The box is made of plastic, but the quality is good and it comes with anti-slip pads so that it sits on any surface quite firmly. Drilling the holes to fit switch and potentiometer through was even easier than drilling the aluminium frame for the heating mad system.

The second part of the system was the holder for the LEDs. I decided that using 7 "super bright" white LEDs will be sufficient. The description on the seller website said they are so bright that looking into them directly would hurt your eyes. Well, I thought, if one hurts your eyes, seven must burn your brain. And falling short of that, still enough to illuminate a work surface.

These LEDs are designed to emit light at a 20 degree angle, accompanied by some stray light, suiting my application quite well. While using a convex lens would have been the more elegant solution to focus the light I opted for the cheap and easy way of just shielding any unwanted light. With so many lumens at my disposal, shielding some won't be a problem, or so I thought.

The Bad 

Unfortunately, it turned out that the "super bright" white LEDs are not as bright after all. While it is true that they are unpleasant to look into, it's nowhere near bright enough to light up an area to look at it under high magnification. Even when 7 of them are focusing on point only 4cm away from the LEDs it didn't produce enough illumination. While in the picture below it certainly looks like the LEDs almost burn a whole into the table, it really wasn't that bright. Add the fact that they can't be placed at the ideal angle to shine onto the surface of the brain and it becomes rather useless.

~4cm distance from LEDs. While the light is nicely concentrated in one spot it didn't produce enough illumination, the picture is quite misleading.

One problem with using a simple potentiometer to alter the voltage on the LED for dimming is that LED have an exponential voltage response. That means that low voltages for large parts produce almost the same (low) amount of light, and only the last few hundred mV approaching the LEDs max. voltage rating produce a huge change in response. However, LEDs are very sensitive to applying too much voltage, which means if you turn the pot too far you will fry the LEDs. That is also the reason why proper LED drivers are a bit more sophisticated.

Luckily, in the process of soldering up the LEDs a friend from the workshop showed me some real LEDs. These are industry-grade, not some hobby-project components. They hurt your eyes even when standing on the other side of the room. Exactly the kind of thing I needed. On top of that a real LED driver (which can be bought for <£20.-) will be used instead of just a potentiometer, allowing dimming the LEDs linearly.

The Ugly

Approaching the halfway point of my PhD at an alarming pace I decided that time is becoming a currency ever-increasing in value. It makes a lot of sense, therefore, to save time where possible, even at the risk that one of my contraptions won't last for the next 50 years. While some soldering couldn't be circumvented, I decided to use a mini-breadboard and some jumper-wires to assemble the circuitry inside the box. With hindsight, it didn't save an awful lot of time, but at least it made it a lot easier to assemble and take apart again in case there were any mistakes in the circuitry.

This might fall to pieces the first time someone knocks it but until then it will do the job. If I can find a glue-gun I'll just glue it all down.

Instead of using a double-stranded wire I used two single-stranded ones (blue and red) to connect the LEDs. That was purely for the reason that I didn't have a double-stranded one available and I needed to get this up and running.


Arguably it is a viable option to simply buy an off-the shelf lightsource with some gooseneck light-guides. However, designing your own lighting system guarantees that it will fit exactly where you want it and only illuminate the area required. While my current solution is suboptimal due to low light output, the next generation will hopefully do the job.

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