# Best Momentary Latching Pushbutton Switch Circuit

Joined Feb 27, 2010
249
took a bit of searching, but i came to the conclusion that this switch, as found here posted by bill, seems best

before, i had tried a switch that was all transistors and resistors... but that was a clusterfrick and it didnt work anyway.

this circuit, ive tried three times now. what happens is i push the button, i get a brief dim flash of the LED light, then its out. if i push the button too soon after that, nothing happens. the capacitor needs to recharge for about half a second before the LED will flash again if i push the button.

is the reason im not getting desired results because im using a 3.7v battery? if so, what do i modify to make this work?

this is for a homebuilt 3w headlamp. once i get it working, ill have the current LED boosted by a transistor, so when the main 3w LED cuts out from low voltage, this small led stays on for 'emergency' lighting.

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#### SgtWookie

Joined Jul 17, 2007
22,221
You tried to link to an image, but it didn't "take".
Try posting the link for the thread and indicate which post# it is attached to or shown in.

[eta]
Ah, ok - this one:

[eta]
The circuit diagram clearly shows a 9v supply being used.
A standard bjt (transistorized) 555 timer won't work properly below around 5v; you would have to use a CMOS 555 timer.
R1 and R2 would have to be decreased to around 300 Ohms.

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Joined Feb 27, 2010
249

#### SgtWookie

Joined Jul 17, 2007
22,221
Yes, the TLC555 is a CMOS 555 timer. It can't source (10mA) or sink (100mA) as much current as a bjt 555 (+/-200mA), but it will work on lower voltages.

[eta]
Come to think of it, that won't work either. The threshold and trigger voltages won't let it work, and you can't get at both of them externally.

You're going to have to use the original circuit with a 9v to 16v supply.

Joined Feb 27, 2010
249
then i cant use this circuit. its for a headlamp that runs off one 3.7v lithium cell.

i just put a 9v supply on it too, and it didnt work. the LED is always on, and when i push the button i get a slight brighter flash of the LED and then it just stays on.

well i spent all day on this, trying to figure out a solution. why its so damn hard to figure out how to make a pushbutton switch a latching one is beyond me....

edit- this is the other circuit i tried. it too didnt work, but again i was using 3.7v supply, and i could easily have screwed it up too. this circuit is actually a little complex when you go to actually make it in a small form factor

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Joined Feb 27, 2010
249
lookie what i found-

this is made by dave johnson submitted to discovercircuits.com

assuming i can find all the parts, this looks easy enough to build and its 1.65-5v

looks like mouser has the dual-schmitt trigger for dirt cheap- http://www.mouser.com/Search/ProductDetail.aspx?R=74LVC2G14GV%2c125virtualkey66800000virtualkey771-LVC2G14GV125

so i got 25 of those coming. everything else that i need i have. this is by far the easiest circuit ive seen yet... i didnt realize at first that is only one logic chip in there.

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#### Wendy

Joined Mar 24, 2008
22,436
A CMOS version of my schematic would work at 3.7V. The cap charges to the output voltage, and lasts long enough to trip the TLC555. I can see the standard 555 getting into trouble with 4.5VDC, since its output doesn't go rail to rail. This is explained in chapter 4 of LEDs, 555s, Flashers, and Light Chasers. If you need a drive circuit for the TLC555 let me know, that is the one aspect of those chips that is a real PITA.

Joined Feb 27, 2010
249
honestly im really liking this dual-schmitt trigger circuit. just add 2 resistors, a cap and a mosfet and youre done. i ended up ordering the exact parts listed in the diagram, because after comparing datasheets for hours the ones listed really are the best fit for what i want. i was tired of spending the 1200% ratshack tax too.

basically, this circuit with those parts is good for 1v min -12v max signal on the trigger, a 20v spike protection overall, 2A+ constant sink current, and switching as fast as you can push. you can buy all the parts shown at mouser for about $1. of course, there is shipping to, so i bought enough parts to make 15 of these for$17 shipped

Joined Feb 27, 2010
249
well i ran into a snag now that i got my parts in.

these chips are freakin SMALL! im not sure how to even solder these.... i have some solder braid, do i just pool up solder across all the pins and then use the braid to wick the excess back?

i think that will work if i can find a way to get the wires lined up... yikes this is going to be harder then i thought!

#### declancole

Joined Jan 27, 2015
3
Have you successfully built this circuit with the SN74LVC2G14?
Did it function as intended?
Many thanks

Joined Jul 18, 2013
23,141
I used this flip-flop relay based version before.
Max.

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#### declancole

Joined Jan 27, 2015
3
I used this flip-flop relay based version before.
Max.
Thanks Max, looks good.
However, it is a bit over-kill for my design - I am switching a 3.7 Lithium supply to a SIM900 GSM modem.