I am trying to create an alarm system for a sliding gate for my college project. The idea is that when the gate is closed the circuit is broken but when the gate is opened the circuit is closed and after 45 seconds the alarm goes off.

So far I have worked out that I can use a normally closed reed switch so when the gate is closed the circuit breaks but when it opens the reed switch finishes the circuit. I can't think of a reason this wouldn't work but I have never been great at electronics so i might be wrong.

Assuming that works the problem I'm having is that with this set up the alarm will go off instantly and i want a 45 second delay. Any help/advice would be greatly appreciated.

 

Welcome to AAC.
A 555-based timer circuit should just about cope with a 45 second delay, if you can tolerate an error of a few %. Google "555 delay timer".

 

it is fairly trivial to obtain 45-second delay: an rc network, a 555 timer, a counter, a mcu, a plc, etc.

what actually works for you depends on your skills and parts / tool availability, among others.

 

What is the "college project"? If it must include circuit design, we can guide you through something depending on your skill set and what parts you have access to. Note - since we don't get the grade, you must do the work. If the task is to just get something done, you can buy a timer module on ebay for $4,.

ak