Need help converting this relay latch to solid state.

Thread Starter


Joined Sep 8, 2018
Hi all, I'm thinking of adding an extra security feature to my 80's Volvo to prevent thieves from starting and stealing it. Basically I'm wanting to add a secret button you have to press once you turn the key to provide power to the ignition system, like a basic immobiliser. If you don't press the button first, the engine will just crank and not fire.

The easy thing to do would be to just have a physically latching button but after a while I'd just become lazy and leave it latched down defeating the purpose of it.

I've drawn up a plan for the circuit using relays but relays can be bulky, expensive and are prone to shock and vibration in a moving car. Can anybody adapt my plans into a solid state circuit?

I believe the ignition system is a few amps but I'd like it to handle 10A just to be safe.



Joined Mar 14, 2008
Here's the LTspice simulation of a solid-state latch circuit using a BJT and a P-MOSFET that should do what you want.
The P-MOSFET can be just about any with a ≥20V voltage rating and a ≤10mΩ on-resistance (to avoid needing a heatsink for the MOSFET @ 10A).
The ignition switch in series with the 12V that applies power to the circuit is not shown.
(V2 is just for simulation of the push-button).

P.S. Is it rude to ask why you think anyone would want to steal an 80's Volvo? :D

Last edited:

Thread Starter


Joined Sep 8, 2018
Hi there thanks for the design, I've drawn it up myself hopefully there's no errors!

Well, I suppose it's not the most desirable car to thieves but you'd be a fool to underestimate the yobs around my area! Currently the only security feature on the car is the key!