Momentary on for red-dot-finder

[Garbz (Chris)]

Got a message from Allan recently who has the same problem I have, I got through CR2032 batteries about once every 2-3 weeks because I forget to turn the damn red-dot-finder off. So... electronics time:

The circuit is very simple needing only a few components. A pushbutton, a diode to prevent back charging, a capacitor and resistor store a charge and bleed that charge, and a MOSFET to act as a switch.

The entire circuit is powered by a 3V battery and the outputs of the circuit can go directly into the battery terminals of the red-dot-finder. Then it's just a case of leaving the red-dot-finder on and when you want to use it tap the pushbutton. It stays on for 60 seconds and then goes off.


Good things:
- Never forget power again.
- No battery use while off (MOSFETs leakage current measured in nanoAmpers)
- Cheap
- Potential for some hardcore miniaturisation (will work on that if the circuit works in practice).
- No modification to red-dot-finder needed as this circuit can connect directly to the battery terminals.

Bad things:
- More electronics.
- MOSFET choice limited due to requiring a threshold voltage less than 2.7V.

Will try to source some parts and get this built this weekend.

[peter_4059 (Peter)]

Chris,

I reckon this will be popular if you can make it small enough.

Peter

[RB (Andrew)]

Nice one Chris.

[Regulus (Trevor)]

Very cool solution.

[allan gould]

Clever boy !!!!!
I am waiting with baited breath.
Allan

[rcheshire (Rowland)]

Have a couple of these I don't need. Happy to post them. Not sure if they fit the bill?

[Garbz (Chris)]

And here I was just thinking that I need to spend a fortune on shipping because Jaycar have a craptacular selection of MOSFETs. If you could send some of those up for some prototyping that would be great Rowland. I'll pm you.

Vgs(th) looks suitable though in terms of miniturisation there are better options out there

[Garbz (Chris)]

Can I get a few people who are interested in this to take a photo of their red-dot finder, specifically around the battery compartment?

I've got a few ways I can make this device and if I'm going to make it tiny and get some circuit boards made up then it would be nice if this thing is not incompatible with any models out there.

I say this because I may be able to make this small enough to fit basically in the battery housing sitting proud a few mm and replacing the cover with a button.

[Astro_Bot]

There's a close-up photo of mine in this thread: Dimming a Red Dot Finder

The battery sits between the projector and screen in this model, so height will be strictly limited.

BTW, I love the idea.

[Garbz (Chris)]

Originally Posted by Astro_Bot

The battery sits between the projector and screen in this model, so height will be strictly limited.:

Well crap.

[peter_4059 (Peter)]

mine is the same.

[Garbz (Chris)]

Rowland got your care package today. Will prototype this hopefully in the next day or two :-)

[rcheshire (Rowland)]

Excellent Chris. Nice to see them put to good use.

[Garbz (Chris)]

Ok first edition of the circuit board, not much larger than the holder for the CR2032 battery itself. Has a button and an output that connects to the battery terminals of the red dot finder.

[Astro_Bot]

Here's a thought: you could use a CR2016 instead of the CR2032 - same diameter, same voltage, just thinner. It might make the difference in fitting into tight spaces.

[Garbz (Chris)]

Limiting factor is the button cell battery holder. The same ones are often used for CR2016 and CR2032. Finding an extra thin one is difficult.

For my own version of this I may go with soldering a button cell directly on the circuit board, but this is probably not useful to anyone who isn't good with electronics as they are hard to remove when flat.

[Garbz (Chris)]

Well Rowland remember how you sent me some MOSFETs along with the wisdom of checking which part you ordered.

Well I think the diodes I ordered are a bit smaller than expected...

[rcheshire (Rowland)]

That is tiny. Smaller than the mosfet driver - how small can they get.

Just a thought - how about a pressure sensitive resistor in place of the pushbutton. A bit smaller than the battery diameter and very thin, except for those long leads. It's less than the battery diameter and could sit outside the battery cover? Don't know? Maybe?

Finger pressure...

http://learn.adafruit.com/force-sens...r-fsr/overview
https://www.sparkfun.com/products/9375

[Garbz (Chris)]

One way or the other this won't fit inside the battery cover. Not if it has to make space for the battery, associated electronics and connect to the terminals inside too.

If I were making a custom case, using a different battery or something this thing would be about 7mm x 7mm sq.

If people were willing to cut open their red dot finder then this would suit that case quite nicely.

[Garbz (Chris)]

Prototype is done. And it works!!!! Who'd have thought right? I also managed to get that tiny component on a perfboard.

A few notes if anyone is looking at this to build their own circuit:
Remember Vth_on is critical for timing in this circuit. I ended up with a MOSFET which had a threshold voltage of 1.4V. With the resistor at 10meg the resulting circuit took closer to 15min to turn off the red-dot finder. I replaced the 10meg resistor with a 1meg resistor and I happily hit almost 60 seconds of light exactly before the laser turned itself off.

Also I've decided against a fancy case. My first iteration of the circuit ended up being smaller than the brightness control of the red-dot finder, so I'm doing what any enterprising hacker would and just building the circuit directly into the red-dot finder.


Now the obvious question to all of you:
If I build like 20 of these would you prefer a stand alone circuit with battery holder that has simply two connections which can be connected to the battery terminals in the red dot finder (i.e no opening the finder, no voiding warranties if you care for that sort of stuff). This would look like the earlier rendering I published above.

OR

Would you prefer a cricuit as tiny as humanly possible integrated in the back of the pushbutton which could then be fitted to the finder with careful application of a hacksaw and some glue? Oh and there's 4 wires to solder with this method. This would look like the rendering below.