Simple Circuit for Controlling a Microscope Shutter
I'm a graduate student in a physical biology laboratory at Caltech. The researchers in my group are working on several different experiments to study forces and motion on the scale of cells and molecules, so we need microscopes, especially fluorescence microscopes — microscopes that use a very bright blue or green light source to track the motion of particles that fluoresce in the light. There's a picture of one of our fluorescence microscopes at the right. The experiment that is run on this particular microscope involves following slowly moving particles for a very long time (many hours) and recording the video on a computer with a digital microscope camera. Unfortunately, five hours is more than enough time for the bright light to bleach the fluorescent dye in the particles, just as a bright red shirt left out in the sun gets bleached to pink in a few days. The particles effectively disappeared before we could finish the video.
So what we needed to do is turn the light on only when we are taking a picture, using a shutter. Microscope shutters aren't too expensive, so we bought one. It consists of a small DC motor and a black metal paddle; applying a voltage to the motor moves the paddle over to block the light beam, and reversing the voltage lets the light through again. However, for some unknown reason, the electronics required to control one of these shutters costs around $2,500, and it isn't even compatible with our camera's software; we were looking at paying over $3,500 to get the shutter working in our setup.
Building a shutter control circuit
The solution? Build the shutter controller ourselves. It seemed like a simple enough project, because a DC motor is easy to control. Ideally, we would have been able to write a program on the computer that would open the shutter, take a picture, then close the shutter again. However, the software that controls our camera (like almost all software found in a biology lab) was not designed with any level of programmability in mind. Luckily, the camera, like most microscope cameras, can be put into an externally triggered mode. In the triggered mode, a TTL (5V) input on the back of the camera signals it to record a frame of video. So we needed to send two signals with our circuit: one to open the shutter, and one to trigger the camera. The circuit needed to be controlled by the computer so we could easily adjust timings on the fly.
Here's the circuit we built to control the shutter and camera. It uses the two motor ports of the Micro Motor Controller from Pololu to control the motor and camera, and a USB-Serial Converter as a connection to the computer. Total cost for all of the electronics was about $60, and it required a single afternoon of assembly and experimentation.
The documentation for the USB-serial converter suggests that most people only need the ground (GND), receive (RX), and transmit (TX) lines. It's not at all clear (and this is a perpetual problem with serial port documentation), but "receive" means that the signal travels from the circuit to the computer. In fact, we didn't need the RX line at all, since the motor controller does not need to send any signals back to the computer. The TX line, on the other hand, gets connected directly to the "serial control input" line on the motor controller, and we use both the GND and the +5V lines on the USB-serial converter to power the motor controller.
A little experimentation with some batteries and DC wall adapters showed that 5V was not enough to close or open the shutter reliably, so we couldn't just power the motors from the USB line. Instead, we used a 9V DC wall adapter as the motor power supply, plugging its output directly into pin 1 of the motor controller. Note that an unregulated wall adapter rated for 9V actually starts out at around 13V, when no current is applied, putting it out of the specified range of operation for the motor controller. So if we burn it out, it will be our fault.
The motor controller is designed to control two reversible DC motors; we use one of its ports in the normal way, by connecting pins 8 and 9 on the motor controller (motor 0) to the shutter motor leads. To control the camera, however, we just use pin 6, the motor 1 positive output. When motor 1 is set to drive forward with 100% speed, this line will be at +9V, and at 0% speed, it will be at 0V. A simple NPN transistor circuit, shown in the diagram at right, allows us to convert +9V to the +5V required by the camera. As you can see, we also threw in a bunch of diagnostic LEDs to make the board more colorful and help us figure out what was going on in the circuit.
One note of caution: we were doing this with a relatively inexpensive microscope camera and shutter. But laboratory equipment can be very expensive and fragile, and if you do something wrong (e.g. apply +9V to the camera TTL input) you could instantly destroy it. So make sure you know what you are doing, test everything before you plug it in, and keep in mind that if you aren't getting some educational value out of the process, it is probably not worthwhile to do it yourself!
We downloaded the drivers for the USB-serial converter from the
page, and installed them on the Windows computer that we use to
record video from the camera. The port showed up as COM3 in Windows;
within our Cygwin shell it is
/dev/ttyS2. A simple C program was sufficient to drive
the shutter and camera; to learn how to write programs that use the
serial port in the Cygwin environment,
After the circuit was assembled and the program written, the shutter worked exactly as planned. We could open it or close it in about a tenth of a second, take an exposure with the camera, and close it again. Here are pictures of the shutter opening:
It works! If you are interested in doing something like this in your lab, and you need some advice, please let Paul know.