Latch circuit for start/stop switch on jet ski (controlling speeduino)

Thread Starter

oldcarguy85

Joined Mar 19, 2023
11
Hey all - First, I am not an EE (though I really wish that were my field). i'm a software engineer fighting my way through making a trimmed down version of speeduino (open source engine management based on arduino) for a freestyle jet ski application. My board just does ignition control and a few GPIO. I need to make a latching start/stop circuit with two different button inputs to interface with typical lanyard start/stop switches. I tried using a bipolar mechanical latching relay, but it seems the vibration will make that unreliable. I've pieced together something that seems to work and I tested it out with an IRF540N mosfet and two 2N3904 transistors. it seems to work ok but I have no idea what I'm doing :) - I'll have to find suitable surface mount alternatives when I add this circuit to my board design.

Any advice, feedback, tips, etc is greatly appreciated.

here's a link to the simulation https://tinyurl.com/2hvjp6xx
 

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Thread Starter

oldcarguy85

Joined Mar 19, 2023
11
oh also just to be clear, the LED in that circuit is taking the place of the speeduino - where the LED is would actually be a connection to a LM2940 5V regulator
 

sghioto

Joined Dec 31, 2017
5,430
The N channel will not operate to full saturation as a high side switch. You need to use a P channel mosfet you show in post 1.
Use a IRF9540.
 

Thread Starter

oldcarguy85

Joined Mar 19, 2023
11

Thread Starter

oldcarguy85

Joined Mar 19, 2023
11
So I got my latest board revision and the darn latching circuit is not working. I tested it out before ordering with components on a breadboard and everything was fine. Granted, I switched to similar surface mount components but I'm really stumped. I can never power it off - the MOSFET just always has power running to the gate. One note, I did have to remove D4. For whatever reason, this diode heated up and drew a ton of power on first power up. Removed it and all seems fine with power regulation. Attached is the full diagram. I tested the transistors in the latching circut and all internal diodes seem fine. MOSFET seems fine as well. I have a second board that does the same thing. Really appreciate any advice here, I'm stumped.
 

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