Need help converting this relay latch to solid state.

Thread Starter

Vakito227

Joined Sep 8, 2018
4
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.

Thanks
Peter
Circuit.png
 

crutschow

Joined Mar 14, 2008
23,783
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

upload_2018-9-8_23-33-5.png
 
Last edited:

Thread Starter

Vakito227

Joined Sep 8, 2018
4
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!

Cheers
Circuit2.png
 
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