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Use a momentary / tactile button, as a latching switch - circuit?

ARG

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I've been having pretty good time with OSHPark recently. Typically 3-4 weeks from order to my hand.
Chinese ones on the other hand... Those can take three weeks to three months.
 

rhd

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I've been having pretty good time with OSHPark recently. Typically 3-4 weeks from order to my hand.
Chinese ones on the other hand... Those can take three weeks to three months.
Complete opposite experience for me - if I want it quickly, I use Seedstudio or iTead. If it's small, and I don't mind waiting, Oshpark.
 

Sigurthr

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Ah wow, thanks RHD! If you've got a bunch to spare, sure I'll take one, if you've only ordered a min amount just send the board files over then =) . Got a BOM made up for it btw? I'm rusty on SMD parts (esp. sizing), only ever do SMD work for repairs really and that's mostly dpak fets.

I don't think the hFE ratings are important in this so the DIP npn BJT should do fine. Just make sure the fet you chose is comparable in Vth to the one Dave selected or it won't fully turn on when it latches closed circuit. I'd check it but I'm not on my pc much now since I broke my ankle/foot two weeks ago.
 

rhd

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The FET I chose has these specs:

FET Feature Logic Level Gate, 2.5V Drive
Drain to Source Voltage (Vdss) 30V
Current - Continuous Drain (Id) @ 25°C 4.3A (Ta)
Rds On (Max) @ Id, Vgs 44 mOhm @ 4.3A, 10V
Vgs(th) (Max) @ Id 1.3V @ 250µA
Gate Charge (Qg) @ Vgs 12.2nC @ 4.5V
Input Capacitance (Ciss) @ Vds 1200pF @ 15V
Power - Max 1.4W

It has a really low Vth, so I think it should work fine for single or double cell builds - but hopefully I haven't misunderstood the circuit's operation.
 

Sigurthr

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Without rewatching Dave prattle on for ~20min again I'd say it looks like Bob's yer uncle with that FET (it should do just fine). I do like Dave btw, but he needs a fast forward button, or at least an abridged replay feature, lol.
 

ARG

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Yeah, Dave's videos are great, but they are almost always too long, and they can go off topic here & there. Could easily make em each half the length, but I guess he doesn't have the time to edit them down to that size.
 

rhd

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That's so funny that you guys say that, because this was my first watch of him, and I thought the EXACT same thing. He spent at least 2 minutes just setting the stage!
 

CDBEAM777

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I don't know... you asked me if it will handle the current from your setup, and then proceeded to tell me everything except what that current is ;)
Well...Dunno !!! In the arena of electronics....I am still in the " Kiddie Pool "
:yabbem:

I will do a little research !! Thanx !

Lite'em Up CDBEAM=======>
 

Wolfman29

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Really interesting circuit. What's your application for this, rhd? I assume you want a "click-on, click-off" with a tactile feel and can handle high currents?
 

rhd

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Well...Dunno !!! In the arena of electronics....I am still in the " Kiddie Pool "
:yabbem:

I will do a little research !! Thanx !

Lite'em Up CDBEAM=======>
Excellent - if you tell me your current, I'll tell you whether it will work :) I suspect it will.

This is nit-picking, but can you properly put your signature in the signature field of your profile. If you don't, then it gets picked up every time someone tries to quote you (see above) and then we have to manually delete it.
 

rhd

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Can anyone see a reason not to replace the two transistors with MOSFETs?

My thinking is that there are ICs that incorporate 4x MOSFETS in one package. Using only MOSFETs would allow for a reduction to one IC.
 

Wolfman29

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I'm not sure, but MOSFETs can typically only be in on or off states, whereas BJTs can vary their outputs... so that may play a role.
 

Cyparagon

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You're forgetting that FETs are voltage controlled devices. You'd need to add pulldowns everywhere and recalculate the pullups. Vgs threshold values appropriate for the low voltage found in pointers can be hard to find.

MOSFETs can typically only be in on or off states
That is not true AT ALL.
 
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Sigurthr

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Yep, stick with the BJTs. MOSFETs have their place but low voltage low power switching is not one of them. Cyparagon is spot on; you'd have to completely redesign it to switch to fets.

All transistors (the T in FET) exibit a linear state where the switch is not fully on or off, it's just that mosfets are easier to bias into saturation (fully on) and typical useage applications benefit most from a quick switch from cutoff to saturation with minimal time in the linear region. You can make a sweet and simple audio class-A amplifier by biasing FETs into their linear region and cascading their outputs. Inefficient as using an array of toasters to heat your house, but it works perfectly well.
 

Sigurthr

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Haha, yes quite true as they're purely resistive. I should have said it's as efficient as using an array of toasters is efficacious for heating your house. Seriously though an all FET class-A amp is a space heater, lol.
 




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