Need to build a warning system for a circuit unintentionally left on

[jeef84]

Hi Guys,

As this is my first post and I'm not an electronics professional, please let me know if there's any information I'm missing.

I'm having a problem at my workplace with cars that are put into neutral to be moved. There are two conditions that I'm battling with here. A lot of times, the gearshift is left in the neutral position, which drains the battery because it keeps some of the instrument cluster functions going. The other condition, which is similar, occurs when the key is left in the ACC position in the ignition switch. The end effect is essentially the same.

My idea is to design a circuit that will take an input from both switches. If EITHER switch is in the CLOSED position, then a warning light/sound will illuminate/play. When BOTH are open, the light will go out. I also want to include a reset timer to disable the circuit in the event that either of the switches are intentionally left in the closed position.

I started with an NTE7432 and a 555 timer, but I'm not too sure how to proceed from here. I've included a simple diagram I came up with to help explain.

NOTES:
-The U1A/U1B components are actually part of one 7432. The program I was using wouldn't allow me to put one 4-input chip on the screen.
-The pictured circuit doesn't include the reset timer portion yet. I haven't really gotten around to figuring out how to include it.
-The rocket ship is representative of the light/buzzer. I've got pretty limited options with this program, it seems.

**broken link removed**

Thanks in advance!

Jeff

 

[MikeMl]

It seems to me that you have it backwards. Since occasionally having the car in neutral, or occasionally having the key-switch in the ACC position is normal, you want to defer the alarm unless the switches are left in said positions longer than XXX seconds.

By the time you condition the two 12 V switch closures into logic signals, and then buffer the gate outputs to drive a relay, and power the gates, etc, it is getting way too complicated. Why not use a couple of diodes to create the required OR function...

 

[jeef84]

I see your point. So something more along the lines of this maybe?

**broken link removed**

Can I use the 555 in monostable mode to create a delay timer?

 

[jeef84]

After some searching around I dug this up:

Timer Circuits With 4060B - Electronics

Does that seem feasible?