I'm building a circuit that sends power to a photographic flash unit at certain times. Basically the output of a 7555 timer is connected to the gate of a mosfet, which acts like a switch to send 6 volts to the flash unit.
I'm not familiar with using mosfets; how does one pick the best one(s)? Something cheap, easy to find, and a low voltage drop in this type of circuit.
I'm thinking of a logic level mosfet like a IRL 2702-ND (Digi-Key page 641, $0.70 US).
I see nobody has answered, so I'll chime in. Ding!
Did you mean the IRL2703, which is a 30V 60-milliohm (with 4.5V gate drive) TO-220 part? This is pretty respectable for 56 cents, qty 10. Your 7555 circuit is OK, I suppose, but I suggest you add a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.
Normally we'd ask you to characterize the load, but with a nice 60-milliohm switch, I'm not sure if that's necessary! But I am curious about your 6V power source. What's that, a 9V battery? Four AA cells? Usually it's important to analyze your circuit's performance when the battery is dead or nearly dead. Your 7555 and logic-level FET should work down to say 4 volts, although the Ron rises to 80 milliohms or so...
Hmm, "Trail cameras are cameras placed out in the woods to take pictures of wild life, tripped with an IR sensor." Tell us more.
Hi Stephen, Have you checked out any of the forums that discuss trail cams? Lots of knowledgeable people over at
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I'm wondering what will happen with the circuit in this situation... Animal walks by, a picture gets taken (with flash), and the "delay" between pictures is shorter than the 555's 'flash recharge' timer. Seems like the next picture would then have no flash, since the 555 didn't allow another recharge cycle.
Maybe you could incorporate some sort of reset logic to force the flash to recharge immediately after the flash is discharged?
On the MOSFET - couldn't find that part # listed, but look for a one that can handle the Maximum current the flash will require (maybe 2 Amps or so?).
between pictures is shorter than the 555's 'flash recharge' timer. Seems like the next picture would then have no flash,
The camera controller controls the minimum time between pictures.
flash to recharge immediately after the flash is discharged?
While there is a small possibility that a second photo might only get the flash that is built into the camera, unless there is a very simple way to force the recharge, I prefer the K.I.S.S. principle.
a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.
Is there a reason for wanting to slow the FET's rise/fall time?
It's a common Vivitar 2000 Flash. I measured a load of
80 ma after it was charged. I'm guessing 2-3 amps from a cold start up. The flash uses 4 AA's; we might use an external pack of 4 D cells instead.
I expect the circuit to reduce power consumption by a factor of 10.
The IRL2703 was my best guess after learning what I could about these parts and looking through the Digi-Key catalog. Mostly I figured something cheap with a very low voltage loss when the flash recycles.
I'm hoping that some experts will second guess me, in case it turns out that I really don't understand as much as I think I do 8-). For example, is there any reason to add an external component(s) to protect the MOSFET, or perhaps there is another part that would be more tolerant of any transients that might occur.
pictures of wild life, tripped with an IR sensor." Tell us more.
My brother wants to build a couple of these things. Apparently there is a following for these devices. A couple of web sites on these:
Here's an idea that should cover both the "K.I.S.S. principle" and the issue of the missed flashes... Use a micropower dual comparator in an 8 pin package (one with a built-in reference). One comparator goes to the CDS cell, the other to a pair of high value resistors forming a voltage divider that monitors the voltage on the flash capacitor. The 2 comparator outputs are connected in such a way that the CDS comparators output holds the gate of the MOSFET low in daytime, at night the flash monitors output drives the MOSFET On or Off as needed to keep the flash topped up (use some positive feedback for hysteresis).
Cost and complexity should be about the same as the 555 based circuit - and you'll probably see even higher power savings than the timed recharge scheme.
Yes, reduces damaging V = L di/dt inductive spikes, reduces the dV/dt in the switched circuit, reduces RFI. Recommended.
Nah, I think you're set to go. The MOSFET is avalanche rated in case of a short spike. Go for it (adding in the relevant timing refinements suggested by others).
There are hundreds that would suit the application - I like the cheapies from Mouser (about 50 - 70 cents) -
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# 511-TS3V393IN for example - an inexpensive dual in a DIP package with a 'standard' pinout - not the finest comparator by a long shot - but suitable for the job. As far as availability goes - that would be up to the manufacturer - but if you use a 'standard' pinout, you should be OK for quite a while. I'd be more worried about the 555 supply drying up, the way things are heading with micro-controllers and all.
the
That's what the high value voltage divider is for - scales down the
300+ volts to a manageable level, with only minimal current bled off the cap.
The TS3V393 was a nice comparator, operating on 20uA or less, down to 2.7V. Some parts may be in the distributor pipeline, but sadly it was discontinued by ST last year. ST's TS393 and TI's TLC393 are similar parts in steady production; either would be a better choice.
I'm not following the part about "one with a built-in reference". Didn't see anything about this on the datasheets for those parts. I assume that this would eliminate the need for an external voltage reference.
Is there a reason for not using a TLC3702 with a push-pull output to eliminate the two pullup resistors needed with the other parts?
I've put a second schematic using a comparator at:
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based on this thread.
I've included a LM4040 2.5V voltage referance in the circuit, this choice was pretty much guesswork. Is there another choice with less power draw?
By trick, I was thinking of avoiding putting minus 300 volts on the comparator when the batteries are removed. I've included a Schottky diode on the new drawing that would probably do the job.
Ohhh - I dropped that and went with a more general purpose comparator when you mentioned wanting a part that would be around for a long time ('course the example I picked is already obsolete!) Many comparators have a built-in reference hooked up internally to an input on one or both comparators. Maxim makes some really fine parts - but I hear that availability can be a real problem with them (not to mention all those tiny surface mount packages Maxim is so fond of). I'll poke around in some data sheets and see if I can find something that looks good.
Not that I can think of - didn't realize the '393 was not a push/pull type.
Most of the stuff I toss together is just hobby level. I usually just use a pair of 1% resistors as a divider for the reference (on circuits with a voltage regulator).
That's a great point! Glad you mentioned it, as I'm working on building a similar rig for my own cameras.
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