I can't get Dave's soft latch switch circuit to work (PNP version). - Page 1
Forum section: electronics / beginners

author: blink13
author_rank: Newbie
timestamp: June 04, 2022, 11:26:00 am
content: Hello! I am pretty new to electronics, after not having any toggle switch with a kit I bought, I figured why not build one myself? I came across Dave's video and tried to build his circuit at the end, I don't have a Mosfet like he was using so I just used a PNP one (BJT - 2N3906) and the two NPNs are simply BJT - 2N3904.However when I built it, the PNP seems to be operating when I think it was mentioned it shouldn't be when the circuit is initially powered. Because of that, the LED is always on. I cannot toggle between on and off. Could my BJT - PNP be the issue?I am not really doing anything complex, just the switch circuit and a 1k-ohm resistor and then a 5mm-LED as a load.1. Dave's Video:2. Falstad Simulator -https://tinyurl.com/2bbcmo86I also tried NI Multisim live to simulate the same circuit in order to make sure that the result I am getting is a correct one. But indeed, they have the same result.

author: Peabody
author_rank: Super Contributor
timestamp: June 04, 2022, 02:17:15 pm
content: One thing Dave mentioned is that his circuit needed a load before it would work. But with the LED, when it tries to turn off, the output doesn't go all the way to ground. You might try it without the LED - just have the 1K resistor directly to ground, at least until you get it working.Even if it powers up in the on state, does the button not work after that to turn it off and on?

author: ledtester
author_rank: Super Contributor
timestamp: June 04, 2022, 02:30:58 pm
content: One problem... in your Falstad circuit it appears the 1K resistor that's on the collector of the left NPN is connected to the output AND the input. In Dave's diagram that resistor (which is 100K) is only connected to the input.Note the blue square dot at the junction.

author: ledtester
author_rank: Super Contributor
timestamp: June 04, 2022, 02:39:39 pm
content: Also leakage current might be at work here. For instance, in this simplified circuit ltspice shows nearly 9mA running through the load:

author: blink13
author_rank: Newbie
timestamp: June 04, 2022, 02:44:51 pm
content: Quote from: Peabody on June 04, 2022, 02:17:15 pmAfter it power on it button does nothing actually, It LED stays lit.

author: blink13
author_rank: Newbie
timestamp: June 04, 2022, 02:46:51 pm
content: Quote from: ledtester on June 04, 2022, 02:39:39 pmOooh, this might be it, because I went back, tried his first circuit, the one with only PNP, at power on Emitter - Collector current is present even though I did not even ground the PNP base, it can't be the NPN since it is assumed that there should be no current/voltage there.

author: blink13
author_rank: Newbie
timestamp: June 05, 2022, 07:45:23 am
content: So no ways to make this work with a BJT PNP? I'll scratch this one then. Thanks for the replies

author: ledtester
author_rank: Super Contributor
timestamp: June 05, 2022, 11:06:39 am
content: Here's an idea, but it uses one button for on and another for off:Soft Latching Power Switch circuit Push On/Push Offhttps://youtu.be/DOqpsOJRQgYIn the video it appears the guy simulates with Multisim as well as demonstrates it on a breadboard.

author: ledtester
author_rank: Super Contributor
timestamp: June 05, 2022, 11:29:28 am
content: Here's another BJT latching power switch idea:https://circuitdigest.com/electronic-circuits/soft-latch-switch-circuitand an ltspice schematic:

author: ledtester
author_rank: Super Contributor
timestamp: June 05, 2022, 01:06:48 pm
content: A little simpler circuit idea from#18: Push on, Push off, Transistor Soft Latch Circuit on a Breadboard -- Steve Morrisonhttps://youtu.be/5_GvxUTos_UAlso see this video for a modification for use with a 5V supply:#22: 5v Soft-Latch Pushbutton Circuit on a Breadboard -- Steve Morrisonhttps://youtu.be/faSuVNClp0k

author: pqass
author_rank: Super Contributor
timestamp: June 05, 2022, 02:53:38 pm
content: Dave's original circuit can work (from 10mA to 1.6A switched current) if you use a darlington and a low value base resistor on the series-pass PNP.I changed the Beta/hFE in the simulation to 20; a more realistic value for a power transistor.Also, I changed the cap to 10uF to slow down the switch hold-to-off timing. Real life may be different.I don't know your max. current requirement but you'd want to raise that base resistor's value as much as possible since it consumes 820mW (if spec'd for 1.6A max. switched current) regardless; more than the power transistor at 256mW. However, a higher value base resistor may not pass enough base current to fully switch on/off the load causing the transistor to operate like a resistor; introducing a significant voltage drop and driving the power dissipation up.Simulation here:https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEAmAzNALAdkggnMmAGxFYKSpILIhWS0CmAtGGAFAAuIWqRKWNHn2TIMUECwkwiADlypIYPKmVheyPkxhYZWYgnwVUGasPoATBgDMAhgFcANhzYAnELJkhUMsR5SjxMHg4NgAlbh9-MRk5KPF6DHo8eKhoBDYAc25eEET6IVzieM5aLD4MIoQMMQq+eggYDFlUHCJ8Qgoy1IQWuBkiZAM8JSIm8DgQS1tHZwBnWgIvSNH6bzF6kA4XOwY2AHd3RKXfGOOofYWaNdpqs8gLqrFrx9yJ+7cX59ulGnrg+4OLx+3Go4EWAJBNGBZBoInWFwE9Dh7lOyPepXKlS6aPG-we2ICfjRmRuNVQNBeTXW5zc1U8yLpXkg1KCIWINEZChqGE8vHpk2s9icTAcDHM4BSMEUJM5slyPNe9CVbAAxhjCnwEF0MHg6qlgg00r1IP1BvgRmMpdKslryrr1QgwL9zgcYXEdGJiW4CuT8jk+eJZGxeuqGQrkQ0aV45chTgGA-QZNBkBc8tHPLLPBDrmH6QF7iGCsgBNk+L6Jcq035rj7Ig0ADqzMCNmDUJreAZEBSDIQttJ4GLFpoDkhlXCN1mNqZCriQCeNrSwLAVGI+SijMD9XR9vSkOBEMAVZAUE0YCdwKeCmbgNgAZTOyJzp1WIFsDlmDHEGAu1wDaYTrhnPGkSxlmuKAWmoGKum4jFiEbiULCcaggmtDQEQP4oXK8Zytm2HYZEAEAA4oKiARQQGkYQlBj4gfmFwUdhWFZmwQA

author: anda3243
author_rank: Contributor
timestamp: June 08, 2022, 01:28:42 am
content: I have made this version of the soft latching circuit.Number 1 is a pushbutton that pulls down to groundNumber 2 is a stay on signal from CPU. As long as the CPU pulls this one high, the current will flow into the system. The CPU can let this pin go low to fully power off,Number 3 is an input pin on the CPU. The function button will pull this one low. The CPU can recognize short press, long press etc, and act accordingle.On the outside of the product it is only one button, it can be used for starting up, and the long and short press can be programmed for different functions.I hope this info can help or inspire. I also would appreciate if you could check and see if something in the design that can be improved.