Multi-source Gate Voltage Cascading MOSFET's Circuit - Help

Hello, I’m not an electronics engineer and I’m looking for assistance to determine the correct MOSFET’s and layout for a latching circuit that incorporates voltage signals from three separate voltage sources. See the attached drawing.
Basic operation of the circuit:
SW1 in source 1 will pull down the gate on T1 to zero and turning T1 ON.
SW2 in source 3 closed will provide the voltage for the gate in T3.
When relay 1 is in the position shown, the 13.3V pullup voltage turns T2 ON.
T2, when turned on, allows the 12V to energize the gate in T3.
T3 will then turn ON and effectively latch the source 1 circuit (the pseudo SW1)
The ‘latch’ will disengage when either the 13.3V pullup in source 2 gets grounded (through relay 1) or SW2 in source 3 opens.


I do not believe that the circuit as drawn can be made to function as desired. (Though as hand-drawn schematics go, this is probably one of the more legible I’ve seen here in a while.)

Your T1 would need to be of P-type to function in the manner mentioned, e.g. turning “on” in response to a decrease in voltage applied to the gate, relative to the circuit’s ground potential. (source terminal being the one connected to SW2 in the drawing).

The problem here is that as drawn, there will be at least 9V applied between T1’s gate and source whenever SW2 is closed; this being well above the turn-on threshold of pretty much every FET in inventory, changes in state of SW1 will not be able to cause T1 to turn “off”; it will simply switch between “very much on” and “even more on”.

There are tools such as LTspice available at no cost which can help a person figure out whether a given circuit idea is feasible without having to physically build and try it. Such tools all come with limitations and learning curves, but the investment is quite worthwhile for those with interest in the field.

If you’d care to rephrase the matter in terms of desired end behaviors, it may be possible to offer more specific guidance toward a solution.

Sure, I’ll give a rephrase a try.

The end result is that I need to latch the SW1 circuit after SW1 is first closed (it’s a manual switch). The latch is only active when SW2 and Relay 1 are closed.
The ‘startup’ sequence of these interconnected circuits is as follows:
SW1, Relay 1 and SW2 are all open
SW2 is closed first
SW1 is closed second (at this point, the latch - pseudo SW1 would close as soon as SW1 is closed)
Relay 1 is ‘closed’ third (the latch would open at this point)

The purpose of the latch (pseudo SW1) is to keep the source 1 circuit closed any time relay 1 opens after the initial ‘start up’ sequence has been executed.

Does that help?

I’m not an electronics engineer and I’m looking for a circuit that could do the above and be built from discreet components (not a programmable microcontroller).

This doesn’t need to specifically be done with MOSFETs. I just drew it with MOSFETs because they seem to be a ‘voltage activated switch’ rather than ‘current activated’. I’m open to all kinds of ideas to achieve the desired function described above. Just not using a microcontroller.

If you can change the relay contact arrangements you can use the extra contacts to get what you want (relay logic). If you can’t change the existing relays, you may be able to add more relays to do it.

I don’t have time this week to work it all the way out for you but I’m pretty confident it would work.

Thanks Paul.

Some more insights that would shed light on the flexibility that I might have.

The 3V in the source one circuit is coming from a pre-programmed, firmware protected, third party microcontroller (I don’t have the ability to change any programing here). SW1, and the subsequent latch, provide ‘feedback’ to this microcontroller.
The source 2 circuit is in another third party device that I really don’t have the option of doing something with. The Relay 1 is SPST and there’s nothing really I could modify there (including tapping the coil circuit)
The most flexibility I have is with source 3 circuit. I could pretty much modify that one to whatever would be necessary.

OK. I revisited my functional requirements and re-jigged my drawing. The following description is in relation to the new drawing below.

SW3 is the governing switch. Nothing happens with the relay latch until the SW3 switch is closed. After SW3 switch is closed, the relay latch needs to ‘make’ at the initial closing of the SW1 switch and only open after that on two scenarios: 1) The SW2 switch is closed or 2) The SW3 switch opens.

I can spec out the relay (which draws 130mA) but have no idea what transistors or resistor sizes to specify. Any help would be appreciated.