EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit
Why Electrical Engineering Stack Exchange recommends it
Electronics Stack Exchange answerers point people to it when someone needs a simple discrete soft latch or push-button power toggle, and askers cite it as the circuit they are building or debugging.
Recommended by 3 people across 3 different months, 2012 to 2021.
What it is
Dave Jones of EEVblog walks through a soft latching power switch built from a single momentary push button, two NPN transistors and a P-channel MOSFET, so one button toggles circuit power on and off. He explains how the circuit works, covers its caveats and shows a brief two-button variant.
What people said
Recommends trpropst · 2016-10 · Electrical Engineering Stack Exchange · on “Silicon Controlled Switches SCS equivalent circuit with transistors not working”
I believe that the load (your LED / resistor) should be where you have SW2, upstream of Q1. Q1 and Q2 will act as a latching switch, driving the base of each other. Q1 needs to be able to pull the base of Q2 low to latch the switch. This is why the load must be moved. The control will be done where you have SW1. You need to be able to pull that node high (the way you show it now), and low (not implemented in your circuit). Dave Jones actually has a pretty nice discussion on this type of implementation at [this video] .
Recommends MicroservicesOnDDD · 2018-09 · Electrical Engineering Stack Exchange · on “Efficient toggle switch”
Dave of EEVblog did an episode on a soft latch circuit that looks like it does what you want. You have to make the capacitor big enough that the circuit doesn't mistakenly oscillate back and forth if you press the button and end up holding it down a fraction of a second too long. Hope this solves your problem. Here's the link: EEVblog #262 – World’s Simplest Soft Latching Power Switch Circuit [this video] [LINK]
Recommends miksolo · 2020-07 · Electrical Engineering Stack Exchange · on “Something like a flip flop”
Maybe I have't understood the qestion, but: If you are looking for a Simplest Soft Latching Power Switch Circuit, here you can see detailed review of it's schematic: in this video from EEVBLOG [this video] To answer first qestion I can advice you some kind of relay IC: for example ADG736 ADG736 [LINK] or similar. If you now looking to use it with high current, you can use relay connected to mentioned schematic.
Cites Oli Glaser · 2012-07 · Electrical Engineering Stack Exchange · on “How to switch power of microcontroller by button?”
EDIT - on reflection, the circuit below (which I'll leave for reference) is probably best suited for use in circuits without a micro. AS mentioned in the other answers, unless you really can't afford the few uA, it doesn't really make sense not to use the micro to control the power toggle, as it uses less components and can be controlled accurately. The simplest version can be something like a IOC (interrupt on change) input with pull up, with button to ground. The micro has power applied all the time, and controls a P-channel MOSFET (with pullup from gate to source) for the rest of the circuit. When it sleeps it lets the gate float to turn the circuit off. Reference circuit: At first The P-
Cites brainstorm · 2016-07 · Electrical Engineering Stack Exchange · on “Cheapest Raspberrypi (or MCU) power supply with both toggle *and* momentary switches?”
TL;DR/main question: How would one modify this PCB [LINK] to add an extra toggle switch that does not just abruptly cut the power to a load and that still allows enough time for a clean shutdown of, i.e, a Raspberry Pi? That is, two different buttons in the above PCB (one momentary, the other latched). I want it to be all passives, without involving MCUs . Long version, see comments below too: Together with a friend we tried to implement the following schematic we found online after a quick search for "simple raspberry pi on/off latch power supply" [LINK] : [LINK] Then we simulated the above schematic through our circuits.io breadboard [LINK] : [LINK] Despite the simulator lagging a bit when
Cites Nat Klopper · 2016-10 · Electrical Engineering Stack Exchange · on “Soft latching push button circuit using 2 buttons”
I'm building a control circuit for switching the VFD on my lathe. The latching push button circuit needs to use two momentary push buttons one for on one for off. I got a circuit working with a double throw relay however the VFD switch circuit couldn't supply sufficient current to hold it on. My hope is to use transistors as they should draw less current when operating. Dave Jones on the EEVblog did a video on a soft latching push button circuit however his final design uses just one button. He briefly goes over a circuit using two buttons however i can't get this to work. He hasn't out any resistor values on it and I don't understand the circuit well enough to work it out. to summarise: Wha
Cites oliver · 2018-09 · Electrical Engineering Stack Exchange · on “Soft latching power switch”
I have stumbled upon a soft power switch with low part count on EEVBlog, which I found very interesting for my hobbyist projects: [this video] Since it uses a PMOS and I only had a NMOS to try the circuit out, I have mirrored the whole circuit: [LINK] Initially there was no R5 but I found the circuit to be unstable (it sometimes switched to on without pushing the button), so I added R5. The circuit works now, but the expected oscillation with continually pressed button is very "short-pulsed", i.e. it stays off pretty long (1 s) which is okay, but there is only a short on-time (0.1 s). Since keeping the button pressed is a possible usage scenario from the POV of the user, I would like to have a lo
Cites Thinqer77 · 2019-07 · Electrical Engineering Stack Exchange · on “Circuit to turn microcontroller on when reed switch state changes (to monitor door lock state)”
I'd like to build a circuit that provides power to a microcontroller (ESP8266 in this instance) when a reed switch is changed from closed to open AND when changed from open to closed. This is to be used in a circuit that will monitor the state of a deadbolt lock. I'm planning on attaching a magnet to the end of the bolt and putting a reed switch inside the strike box with enamelled wire running out to the ESP8266 that will be discreetly mounted on the wall nearby. The circuit will run on batteries as running power to the lock location is not practical (nor will it have a high enough WAF to get approval). When the lock is closed it will open the reed switch and when it is open it will close t
Cites Emre Mutlu · 2021-05 · Electrical Engineering Stack Exchange · on “Soft latch switch circuit with buck converter load does not work”
I have been experimenting with soft latch switches. There is one that EEV Blog has developed that only uses 2 NPN transistors and a P-Channel MOSFET. The circuit is seen at 14:44 in this video. [this video] I have connected that to a buck converter and I am observing some strange behaviour. You can find my full schematic below. The connection is as follows: USB/12V --> Voltage Selector --> Soft Latch Switch --> Buck Converter [LINK] [LINK] On power up the circuit delivers a 5V to the Vin of the buck converter and it works on regulating it to 3.3V. However when I press the button the voltage at the output of the soft latch drops to ~4.375V and reverts back to 5V when I release the button. I am mon