What's wrong with this circuit?

[FoundYourVapes]

Ok, I just felt this title would bring more attention.
Hi, I'm new here. To quickly introduce myself I have been into vaping for 6 months, so not a newbie, but I only vaped regulated boxes to this point. Now I want to try to play with a mech mod. Before I go further, yes, I know ohm's law and battery safety. So I want to try mech mods, but after seeing the tutorial by FindMyVapes, I just have to do my own mod. I want it to be very safe and it to let me know what it's doing, so I came up with this circuit.

SW1 - master switch
SW2 - fire button
Q1 - MOSFET
Q2 - PNP Transistor for low battery signal
LED1 - status LED, lit when the mod is on
LED2 - low battery LED, lit when battery is low
LED3 - coil active LED, lit when the mod is firing
D1 - 3.3V zener diode
R1 - the coil
R2, R3, R4 - LED resistors
R5, R6 - pull up, pull down resistors
R7 - pull down resistor

My main concern is, will the MOSFET fully switch on on say the 3.3V? Or will I need another transistor for switching it due to the voltage drop on zener diode? Also, what values would you give for the resistors? My thoughts:
R2, R3 - 100 ohm (white LEDs)
R4 - 100 ohm (red LED, only on under 3.3V)
R5 - 8.2k ohm
R6 - 10k ohm
R7 - 22k ohm

Safety always first, I don't want this stuff blowing up in my face, also I don't want to do all this job for something not to work correctly. Thanks for any advice and suggestions to changes, if you think you can make the circuit even more compact.

Edit: As user suprtrkr noticed, the Zener's breakdown voltage may still partially open Q1. also R5 and R6 are basically a voltage divider and the current will switch the Q2, which is undesirable.

Edit: I've decided to use a different, much simpler design, using only a simple voltage indicator

 

[mcclintock]

I don't think you'll get a good turn-on of the MOSFET due to the zener and even if you did, as the battery drained you would end up with that problem. You don't want to partially turn on the MOSFET where there is voltage across it and still substantial current, that would probably burn it out due to the power dissipated. The voltage OK LED would also have several tenths of a volt between the fully off and on states. Look into voltage OK "reset" ICs or put the zener output through an amp chip such as CD4049 (with some hysteresis) or 74HC14 (provides hysteresis); feed both the LED and the MOSFET gate from the same detect signal. However even these circuits could oscillate as the battery gets pulled down by the load of the coil and the battery voltage rises in low volt protection mode. Really need a latching circuit, such as a flip-flop with an RC reset circuit at power-on, set until power-off on any low voltage detect.

 

[FoundYourVapes]

I don't think you'll get a good turn-on of the MOSFET due to the zener and even if you did, as the battery drained you would end up with that problem. You don't want to partially turn on the MOSFET where there is voltage across it and still substantial current, that would probably burn it out due to the power dissipated. The voltage OK LED would also have several tenths of a volt between the fully off and on states. Look into voltage OK "reset" ICs or put the zener output through an amp chip such as CD4049 (with some hysteresis) or 74HC14 (provides hysteresis); feed both the LED and the MOSFET gate from the same detect signal. However even these circuits could oscillate as the battery gets pulled down by the load of the coil and the battery voltage rises in low volt protection mode. Really need a latching circuit, such as a flip-flop with an RC reset circuit at power-on, set until power-off on any low voltage detect.

As grateful as I am for your advice, I'm afraid it would be too much space or money wasted if I tried to put a IC in there. It's a pen mech, so really not a lot of space in there. Anyway, thanks for your advice

Edit: I figured out that with some thin wire I might be able to do it. I just need some circuits. Or I will give it a shot in the morning.

 

[mcclintock]

You could leave the low volt circuit for the LED but get the MOSFET out of that zener circuit, just decide yourself when to not use it. Instead of some fancy infallable circuit!

 

[Bunnykiller]

also it looks like the LED while coil on (3)is in paralell with main coil... that should mess with your total ohms when making coils to spec.

try looking for the free download version of ExpressPCB... it does circuit analysis.

 

[FoundYourVapes]

also it looks like the LED while coil on (3)is in paralell with main coil... that should mess with your total ohms when making coils to spec.

try looking for the free download version of ExpressPCB... it does circuit analysis.

I'd think it shouldn't really matter, as there is enough loss on contacts but yeah, it'd be better to put that parallel to R7 after SW2. Anyway, I was thinking about it, and I think I will just keep things simple with option to extend in the future. It will save me a couple of bucks, well not really, but a little bit of work.

Soo, I didn't post this reply, but I think I have a new design. just a mosfet with a switch and the simplest zener diode voltage indicator. probably 3,3 and 3,6V lights, but I'm wondering if I need 3,9V, this will make it look kinda messy IMO, but would be helpful in knowing the batteries are basically freshly charged. I might experiment with the placement so it looks clean though.

Edit: Damn I forgot using a zener diode in series with an LED and a resistor will cause it to breakdown at different voltage that specified.