Classic circuits you should know: the latching switch

Classic circuits you should know: the latching switch
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[@nigeljohnson9820]

Small point and being rather pedantic, but I think best practice would have the unused input connected high, rather than joining the gates inputs together. It really does not matter for this circuit, but for faster circuits it matters, as it increases the load on the driving gate. This is a really nice solution. I assume that if the button is held pressed the circuit toggles on/off in oscillation. This would certainly happen if it were a Schmitt trigger nAnd. Without any hysteresis in the gate, the oscillation might be very fast, holding the output near mid rail at a guess. Bit like the arrangement used to linearly biased the gate when using it as a Pierce oscillator.

[@nigeljohnson9820]

@@andymouse the neat trick is the use of the second gate to hold the oscillating gate in its last state before the switch is opened. It's a simple circuit, but I think a T or D type flip flop is slightly better, as with suitable debounce, it will only switch once on each button push. This circuit is a useful building block that can use up a couple of gates that might be otherwise be wasted.

[@nigeljohnson9820]

@@andymouse i think the idea is to make the period of oscillation very long, so that it is unlikely that the button will be held for a sufficient time to see the oscillation. The video shows the circuit gives a stable operation. It would be interesting to look at the output as it switches, to see if it is oscillates during the transition. It is still a nice idea, worth adding to the tool kit of building blocks.

[@nigeljohnson9820]

@@andymouse in as much as it has two stable states, I think it qualifies as a latch. I do have a problem with connecting the input to an RC. If you look at the specs for these devices, they require the input voltage to transition through the undefined level between high and low very quickly. This is because there is a risk of both the high and low output transistors being on at the same time. This results in a short across the supply rails. There are a number of amateur circuits where the transition time rule is broken and the circuits still work, but professional designers try and avoid such circuits as there is a risk of unexplained failures. There is no guarantee that ICs of the same type made by different manufactures will perform in the same fashion when operated in an unspecified mode. One of the rules for professional circuit designers is to ensure the circuit components are used in a way that fully conforms to their data sheets. The reason is simple, self preservation. A designer cannot complain to the IC manufacture if the product starts to show unexplained failures, when the IC is being used outside its specification. The risk, and cost, of a product recall is then the responsibility of the circuit designer.

[@nigeljohnson9820]

@@andymouse I think the circuit can be made to work with a Schmitt trigger, this solves the transition problem, but the choice of the feed back resistors becomes important to ensure the input voltage crosses the hysteresis input voltage. Usually not designing to the data sheet is by accident, either due to failure to fully read it or because it is ambiguous. A circuit designer needs to be very brave, arrogant or stupid to do it deliberately. Those who think they know better than the IC designers usually come to regret it. That said, it is not uncommon to find errors in the IC designs themselves, forcing IC manufactures to issue work arounds for bugs on the chip.

[@ohsnapfit2096]

When I built this, the button seemed unreliable, I have to press it very very quickly, and still isn't bulletproof. It worked best with an LED and 1.2k resistor. I tried different capacitance values, and it worked best with electrolytic 1uF. I don't really know what I'm doing here, just seeing what sticks, but will try tying the pin 5 and 2 to VCC. Any other thoughts? It is build on a bread board.

[@jonathansera6134]

Dude, this vid is great. I've been trying to crack the code on how to have a single-button latch for so long now. You rock!

[@learnelectronics]

You are welcome!

[@raym9691]

Great video and circuit, I was looking for this one 6 months ago, I ended up using a 555 to flip flop a transistor.. way more parts this circuit would simplify it thanks for sharing!

[@opalprestonshirley1700]

Hope you are doing well in the current craziness. Electronics is one of those hobbies that's great for these current times. Great circuit. I don't have any MOSFET's have order some and setup this circuit. Thanks Paul.

[@uguraltunbilek]

thanks for your good explaned

[@teslakovalaborator]

Thanks for another great video ;) To be honest, this circuit is new to me, because I so far I have used ICs such as 7472,7474, 4013

[@KarlA-vk4kn]

Hi Paul great video and tutorial. I have a 4093 would this be ok to use also? Hope you're keeping well. Stay safe

[@andyjohnson4485]

Stay safe Paul, from hunkered down Southern Spain!!

[@michaelpadovani9566]

Another great circuit you should know!

[@shvideo1]

It is an interesting circuit but hard to understand how it works with the capacitors connected like that and gates tied together. I wish you would explain it a little more.

[@LostInLeiden]

me too

[@FixDaily]

What's the standby power consumption, do you have any idea? I would love to see the Schmitt trigger with feedback PS: No need for a P-channel mosfet, it can be an N-mosfet as well, since it can go high or low. although P-channel mosfet will be more efficient when turned off

[@melplishka5978]

Yes plz , do a Schmidt trigger one bud.

[@zman4150]

Thanks for the vid! I would be interested in seeing an LED bargraph voltmeter using a couple of LM3914's.

[@JahanZeb1976]

*wonderful circuit. I didn't understood where did you connect the mosfet gate in the end?*

[@FixDaily]

Yes it will be at the end, that point can go HIGH and LOW, opening and closing the mosfet as well.

[@mrdimpzz]

Hope ull recover soon from your heart problem. Take care.

[@LostInLeiden]

I built this with nand gates from a 7400 and 5V Vss. It should still work right? But mine doesn't

[@myetis1990]

GREATE JOB FRIEND. I think You have a healthier heart rather than those who post smth stupid to you, because of that they would not post smth stupid if they had not a dirty and unhealthy heart.

[@NeoMK]

Cool, but I dont see the source in the diagram. Did I miss it?

[@JahanZeb1976]

Wonderful question! They are standard pin 14 positive supply and pin 7 negative supply in many digital ICs including 4011 ic.

[@Electronzap]

Yeah, like learningZone said, you power the Integrated circuit and the output can power an LED directly. Sometimes the schematic is drawn to show the op amp/logic gate IC power connections and other times it isn't. So you have to keep that in mind when looking at schematics.

[@baldrickdaisy8982]

Thanks for another great classic. Stay safe! Rick down under :)

[@learnelectronics]

Thanks, you too!

[@richardjagielski8499]

Wouldn't this work equally as well with an N channel or P channel mosfet?

[@melplishka5978]

Can this circuit have bounce ?.

[@keithking1985]

as soon as you pointed out the cap i recognized the circuit but made with bjt's instead of NAND gates also with option of a mosfet controlling a higher load that KASAN TV's channel put up once.. good channel too.. videos called one button push circuit! I think! PAUL that was awesome as usual... : )

[@georgemorris8766]

I would like to see the Schmitt trigger with feedback

[@ohsnapfit2096]

Where is power connected to? Edit: pin 14 on the chip from the data sheet. I was just confused looking at the video.(getting my feet wet, sorry for the noob question)

[@ku4nz]

Another good video, interesting, useful. 73

[@learnelectronics]

Glad you enjoyed it

[@vegtersbaas]

Can you do this circuit using Schmidt triggers such as 74HC14D. Thanx :)

[@learnelectronics]

Yes

[@rolandberendonck3900]

Sorry to ask you Paul, but why would this circuit not work with an N Channel mosfet with a pull-down resistor?

[@adabill295]

Paul, great video but a lot of us are not EE's. It would be helpful to show the voltage supply. Do you need a resister ahead of the LED? Yes i would helpful to use a MOSFET so we can control things of higher voltage. How high a voltage can this control. Maybe you audience is changing and I need to get back to a more basic channel! I have really enjoyed your circuits but I need them more basic for my limited understanding.

[@Allan-mf1he]

Hows this circuit with switch bounce? Did this with a 555 before and there was no switch bounce. Would be nice if you could cover debouncing.

[@jonathansera6134]

looks to me like the two 1uF caps add some debouncing. look up "Schmitt trigger debouncing circuit"; I found that circuit to be a bit more reliable than cap debouncing, but it makes the circuit slightly more complicated because you need one more chip and a diode. Lastly, there is a specific chip made for debouncing which is the MC14490, which works beautifully, but they're harder to come by than schmitt triggers.

[@BrucesWorldofStuff]

Wow that is cool. I have leds under my kitchen cabinets that ia have hooked up to a Dark Circuit (off of the interscreen) that I built with some used parts and a old night light sensor an made my own etched PCB for it... I have the set so that the Leds go on at dusk and off at dawn via a 12v PS and a relay. Everything is in its own little box and everything... :-D Been working great for 3 years but sometimes I want to be able to turn them the Leds on, when it's a cloudy day and it's still to light for the sensor to work the relay. This would be cool to add as a secondary bypass to make that happen... I don't have mofests either but you could use a A1015 or S9012 PNP or 2N3904 NPN transistor and have it turn on / off a secondary relay connected to the Robot right? Anyway this is in my "ToBuild" video saved list to do because I want to update the Led circuit and now that I am stuck in the house for 2 week because of the 19 bug, sothis is a great time to make PCBs and circuits! LOL Stay safe my friend! Remember the 10 Ft rual is a thing to do for now... /:- ( LLAP

[@MalagasOnFire]

Thanks for the video. wash hands, measure temperature, clean , clean and clean .. follow the medical procedures

[@incxxxx]

Where is the input for the latch???

[@billfield8300]

Hey Paul.. great circuit but I didn't see where to attach the positive rail... ??

[@learnelectronics]

Pin 14

[@billfield8300]

@@learnelectronics Thanks Paul. Be safe.

[@gartmorn]

My first family member is in self isolation as his partner is unwell. Hopefully not cv but possible as she works in a restaurant. It doesn't really hit home until it affects you personally! The large supermarkets here are starting to ration certain items as people are panic buying. It's a crazy world! Stay safe my friend!

[@learnelectronics]

Thank you, I am trying. You take care as well.

[@georgechambers3197]

You Kind of lost me on the MOSFET connection? A cool circuit that I could use too. Thanks for the video!

[@FixDaily]

He meant to connect the p-mosfet to the Ouput of the latch circuit. Since it can go Low or High it will open and close the mosfet as well.

[@keithking1985]

i was just going to tell ya what FixDaily did. but he put perfectly.. just connect the MOSFET's Gate to the out put where the LED was. And its drain & source to a separate/higher voltage/current... (but not AC) but you can also control AC with a relay & this too. P.S hope this helps you George!!

[@davidluther3408]

WHERE DO YOU APPLY THIS TYPE OF CIRCUIT?

[@rolandberendonck3900]

To switch on a robot he explained.

[@davidluther3408]

THAT DO NOT TELL ME ANYTHING.

[@talideon]

@@davidluther3408 To toggle something on and off. Like an on/off button, or the caps lock key on a keyboard. It just takes a bit of imagination.

[@pulesjet]

I have a butt load of 4011 stashed away here some place.

[@Chrisamic]

Hunker in my bunker, Hunker in my bunker. Hunker in my bunker, 'cause there's frig all else to do. (sung to the chorus of Good Ship Venus aka friggin' in the riggin'). Yeah I may be showing my age here.

[@bblod4896]

Hunker in the Bunker...Stay healthy.

[@johancornelis3719]

is it me? a switch toggle ON and OFF don't get why all the other stuff? Love your video's btw ....and not an electronical person

[@Chrisamic]

It's because you may want to use a very tiny switch which has a very low profile. These switches are momentary - they are only closed when you are pressing them and as soon as you let go it goes open again. That's not something you want in an on/off switch so they are used in a lot of flat panel applications - control panels in a lot of equipment like computer printers, HVAC, modern TVs etc. Behind each and every one of these flat panel on/off switches is this circuit, or something very much like it that has a latching effect so your equipment stays on when you take your finger off the button. These switches are also extremely reliable because they are nothing more or less than a formed metal dome which flexes up and down. Technically it's not even a moving part in the normal sense. In addition to this, the switch also only needs to switch milliamps at low voltage instead of mains current at many amps. Switching mains voltage can result in surges of many dozens of amps (inrush current) which can severely limit the life expectancy of conventional switches. Have you ever turned on a light switch and heard it go pop? That's a spark between the switch contacts that has just burnt yet another tiny crater in the contacts. In the worst case, the contacts can be welded together so the switch never turns off, ever again. Another reason to use this type of latch circuit even if the device might be able to use a micro-controller to do the same thing (like turning on the robot which has micro that presumably could do this function) is that the inherent capacitance in this circuit provides it's own "debouncing". That's a bit more technical and something I won't go into in this reply, but it's a big problem if you try to control a micro processor with any type of switch with no other circuitry. (Yes there are software work-arounds but they are never perfect).

[@johancornelis3719]

@@Chrisamic Whow what a reply! A latch is an electronic logic circuit that has two inputs and one output. One of the inputs is called the SET input; the other is called the RESET input. Latch circuits can be either active-high or active-low. Had to look this up. Thanks a lot! Sorry for sounding stupid ;-)

[@Chrisamic]

@@johancornelis3719 It was not a stupid question, it was just a question that required an answer. Stupid questions are the ones you don't ask :-)