Suggestions please

Gentlemen, I am trying to build, for hobbyist purposes, a circuit which would meet the following requirements:

-It would drive a luxeon LED 3W continuously, with variable brightness.

-It would also operate the same LED in a single flash mode, with external triggering by a flash slave trigger. Ideally the flash duration should be variable within a range of uSec to a few mSec, but a fixed flash duration of 0.1 - 2mSec would be fine. the idea is to use the LED as a microscope illuminator, and also allow flash photography with the same LED to capture fast moving protists. The LED in flash mode would be triggered by the camera flash via a slave trigger, which I already have. Unfortunately, I lack the expertise to design a suitable circuit - so am asking for help, oh, and it should be simple enough for me to build - I am an old codger from the thermionic era, and can't really cope with PIC programming, which I guess would be a way of doing this. If anyone can suggest a solution, or produce a schematic of such a device, I'd really be very grateful, and am sure many of my fellow microscopists would also be indebted. many thanks, HJ

Reply to
UKoncology
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Reply to
Brendan Gillatt

we will need to know a little more about the LED but lets make a start:

What you want to do is control the current into an LED. Ideally this will not depend on temperature etc.

ASCII art: V+ ! V Big LED ----- U1 ! Vcontrol ----------!+\\ R1 !!- ! >-+-/\\/\\---!!- Q1 ----!-/ ! !!- N-MOSFET ! =3D=3D=3DC1 ! ! ! ! ---------+--/\\/\\-----+ R2 ! \\R3 / \\ ! GND

The power supply connections on U1 are not shown. U1 is nearly any op- amp. It likely is powered by V+ and GND. I will come back to the Vcontrol signal.

R1 in series with the gate of the MOSFET is something like 50 Ohms. MOSFETs have useful gain up into the RF and we want to prevent stray capacitances and inductances from making this an oscillator. A small resistance kills the Q of any tuned circuits the wiring may make.

R3 is the resistor that does some serious work here. The op-amp moves its output up and down until the voltage on R3 is exactly equal to Vcontrol. Ohms opinion then sets the current in the MOSFET's source and thus also its drain and hence the LED.

C1 and R2 are also about preventing oscillations. With a slow op-amp you don't need them. They just ensure that the inverting input of the op-amp gets the news that the output has changed without the delay of going through Q1.

What supply are we running this from?

I suggest that an LM7805 be used to make a local regulated voltage.

For now:

10K +5V ---/\\/\\-- ! \\ 10K /
Reply to
MooseFET

What is the power source? If you're plugging this to a power supply, you can design a simpler circuit than with batteries, I think. (no need to worry about varying voltage over time,etc) I think the 555 solution sounds good. What is the "slave trigger" signal ? I've heard these signals can be at 12-24V on older equipment.

Reply to
a7yvm109gf5d1

Thanks everyone. Very grateful for your help. The power supply can be either batteries or a regulated mains supply. The slave unit is a simple circuit which is open by default, and closes momentarily in response to a bright flash. It is used to short the contacts of a flashgun slave so that it fires synchronously with another master flashgun, the latter triggered by the camera exposure.

Reply to
UKoncology

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