In the past few months we switched all our microscopy illumination to LEDs. LEDs have many advantages: 1) they are cheap ( few dollars a for high powered LED ); 2) they come in pretty colors; 3) they can switch very fast. This post will explore how you can control and switch LEDs fast. Specifically, we will build a setup that allows us to trigger a camera and then send a short (microsecond) LED pulse to illuminate the CCD camera chip. This is important for applications for fast moving objects in microfluidic channels. We also use this technique to measure the speed of droplets in channels. In this case we send a trigger signal and then two LED pulses during the same exposure time. That results in a double exposure image. The distance between the two objects divided by the time difference is the speed of the droplet. The trigger timing can be achieved by a microcontroller or a FPGA (I will write about this later).
But now I would like to describe our TTL driven MOSFET switch. I was looking around on the web for solution but I could not find much detail on how to design such a switch. Most designs on the web did not work for me or did not perform well in terms of switching speeds. So here is my solution. The basic idea is to use a MOSFET that is designed for low voltage gate switching (IRF520). The other is to limit the supply voltage to around TTL level by a resistor that is in series with the LED (has to be 10W or so). In this case we are using a high powered LED that can take 700mA. In this configuration, we achieve 700mA at 5.6V supply voltage. The rest is simple. I included a pulldown resistor of 10k and a small resistor between the TTL in and gate. It turns out that 300 Ohm is best for fastest switching times.
I also experimented with LED power supplies that control the current (Buckpack). Unfortunately, these power supplies produce pretty high frequency oscillations that interfere with high speed switching. We simply use cheap variable voltage power supplies for driving our LEDs. This has the advantage that we can regulate the intensity of the LEDs.
Here is our oscilloscopes output for a 10µs TTL pulse. The circuit switches the LED on at about a microsecond after the pulse arrives and turns it off about a microsecond after the pulse ends. I have no clue where these oscillations at the end come from. The LED visibly blinks down to about 1µs pulse width.
The whole circuit, including the LED, can be build for about 15 Dollars. This is much much less than what companies charge for LED illumination. Some of the cheapest LED solutions are on the order of 300-2000 Dollars. In order to replace your standard microscopy illumination (usually a halogen lamp) with an LED you have to place the LED exactly at the same position as the halogen bulb. In some cases we added a small x-y stage to optimize the placement. Thorlabs sells collimation adaptors for most microscopes. In our case we use a collimator for Zeiss Axioscope (COP4-A) and a coupler (SM2T2). The inside diameter of the coupler is a little less than 5cms and we simply mounted our LED in the middle of a 5cm circular heat sink and wedged it into the coupler. The coupler screws into the collimation adaptor and allows us to optimize the distance between LED and the lens.
This is how the final assembly for our Zeiss microscope looks like.