This page describes a simple circuit for strobing an LED or laser diode along with control software.
This is a DropletKitchen project. Original source code of the project is provided under MIT License. The source may contain code under a different license, if this is the case it is stated in the LICENSE file.
If not stated otherwise, the designs and documentation in this repository is Copyright (c) 2016 Martin Fischlechner and DropletKitchen Contributors, and made available under a Creative Commons Attribution 4.0 International (CC BY 4.0) License.
When imaging high-speed events, one has to minimise the exposure time of the camera to avoid blurred images. Depending on the camera model, this is often only possible to a certain extent. The most straightforward way to further reduce exposure time is to strobe the lightsource instead. Here, a simple circuit is shown that allows an Arduino-compatible microcontroller (here a Teensy 3.2) to strobe LEDs and laser diodes. A software frontend based on Pure Data (PD) is provided to facilitate control. You will need a ‘global shutter’ camera, which, in contrast to ‘rolling shutter’ cameras, expose and read out all lines of the imaging sensor at the same time.
This circuit has been published by Wilson et al. (“Performance evaluation of an overdriven LED for high-speed schlieren imaging” S. Wilson, G. Gustafson, D. Lincoln, K. Murari, C. Johansen; J Vis (2015) 18:35–45, DOI 10.1007/s12650-014-0220-7). In a nutshell, it uses two big capacitors to deliver a lot of current to the lightsource in a quick manner. Switching is achieved by a Power MOSFET controlled by a high-speed MOSFET driver. A Schottky diode protects the circuit.
| Schematics Code | Item | number | bought from | part number | approx cost (£) |
|---|---|---|---|---|---|
| C1 | PANASONIC EEUFR1H681L Electrolytic Capacitor, 680 µF, 50 V | 2 | Farnell | 1800687 | 0.7 |
| D1 | INFINEON IDH06G65C5 SCHOTTKY DIODE, 6A | 1 | Farnell | 2443365 | 3.80 |
| LED | eg OPULENT REBEL-STAR-ES-NW200 High Brightness LED | 1 | Farnell | 2110405 | 4 |
| AVX SR301E105MAR Ceramic Capacitor, 1 µF, 100 V | 2 | Farnell | 2332992 | 2.40 | |
| M1 | MOSFET IRFB3206PBF | 1 | Farnell | 1436949 | 3.20 |
| U1 | TEXAS INSTRUMENTS UCC37322P MOSFET Driver | 1 | Farnell | 8463034 | 3.30 |
| DIL socket | 1 | Farnell | 2445620 | 0.07 | |
| Teensy 3.2 microcontroller or other Arduino compatible | 1 | CPC-farnell | SC13539 | ~20 | |
| Stripboard, PCB headers male/female, wires | NA | ||||
| adjustable Lab Power Supply (easiest to use) | NA | ||||
| Cameras tested | must be global-shutter | ||||
| Phantom Miro EX2 High-speed camera | Vision Research | ~ 10000 | |||
| Firefly MV USB2.0 Camera | Point Grey | FMVU-03MTM-CS | ~200 | ||
| Emax SONY 811 700TVL 1/3-Inch CCD Video Camera | www.unmannedtechshop.co.uk | CC7-E5A-55E | 19 |
The circuit can be assembled in a straightforward manner with stripboard (see schematics and image below). Please also use the component datasheets for sample circuits and wiring instructions. The shorter the pulse, the higher voltage an LED or laser diode can digest. It is therefore convenient to drive the circuit with an adjustable lab power supply. It is a good idea to buy more than one LED when exploring the circuit.
Schematics of the circuit
Image of the circuit with a teensy microcontroller connected.
You can find the program here (./files/strobing.ino). It uses the OSC protocol to get its values from a software frontend made in Pure Data (PD). Have a look at the code, it contains a fair amount on documentation and can be changed quite easily. In a nutshell, it retrieves ‘delaytimes’ (in microseconds) from the frontend, and uses an interrupt to pulse the light in desired intervals. As is, simple ‘delayMicroseconds’ is used to switch the lightsource on for a given length of time, therefore minimum exposure time is limited to one microsecond. For smaller exposure times you can use a signal generator or play with ‘random instructions’, eg a little for-loop in the interrupt instead of ‘delayMicroseconds’.
To avoid reprogramming the microcontroller to adjust settings, a simple PD-program is used to control strobing. The version provided here uses PD’s ‘Graph on Parent’ feature to allow for a clean interface and is written to use with PD-extended (https://puredata.info/downloads/pd-extended) or PD-L2ORK (http://l2ork.music.vt.edu/main/make-your-own-l2ork/software/). Download ./files/o.io.slipserial.pd (a little helper for OSC communication), ./files/StrobingPDExtended.pd (the program itself where you can make changes) and ./files/StrobingPlay.pd (the simplified frontend) into the same folder. ‘StrobingPlay’ is configured to quickly adjust the length and number of exposures per image frame at a given framerate. This might not be convenient for every experimental setup but can be adapted towards other needs in a straightforward fashion.
Screenshot of the PD-frontend ‘StrobingPlay’.
Strobing light is the easiest way to enhance imaging of fast events - and allows you to use relatively cheap cameras for microfluidic rigs. If you expose one frame multiple times, a single image will contain movement in time, useful for example to calculate velocities of beads or droplets in a microfluidic channel.
Droplet generator producing sample at 30kHz at a velocity of 700mm/s, imaged with a FireflyMV camera (Point Grey) at 200fps with its minimum exposure time of 30 microseconds. Top: without strobing. Bottom: Strobed once every frame for 1 microsecond.
Droplet generator producing sample at 30kHz at a velocity of 700mm/s, imaged with an inexpensive Sony CCD board camera at 25fps (PAL). Top: without strobing, LED adjusted to maximum light intensity for the camera (regulates the exposure time of the camera down to 10 microseconds automatically). Bottom: With the LED strobed every 10 microseconds for a duration of 1 microsecond. Due to the minimum exposure time of the camera being 10 microseconds already, image improvement by strobing is smaller than with the FireflyMV. Images have been deinterlaced with a small helper program in PD (./files/DeinterlaceWithSave.pd).