Hello again. Thank you all for the help. SO, I have this Critical Information System circuit and I have the project in DipTrace and have the first 2 boards back from Bay Area Circuits. I installed everything and am getting a problem I didn't have before. I am hoping someone can take a look at this circuit and tell me why the following is happening...

The BLUE LED that is connected to Pin 1 of the LM393 is the focus. The plan is when the temperature reaches the threshold set by the Potentiometer at Pin 2 of the LM393, the BLUE LED Lights up, and the Piezo Buzzer sounds off. That part works, but can anyone guess as to why the Blue LED lights dimly even when it is not even close to the threshold, and then gets progressively brighter until it reaches the threshold? The Piezo works as expected and the Blue LED does get its brightest when the Piezo is sounding. What would be a good fix for this problem? Thank you again!

Update: Turns out I have the same problem with my original circuit. It only rears its ugly head when the alternator is charging. Turn off the engine, and everything works as its supposed to. Ideas?

 

You could try moving the connection.

Currently you are connected to the base of 2N4403. Try moving that end of the LED to the collector leg of the transistor. You are seeing the low current feeding the base and it is enough to create a "glow". The collector leg should stay low until the base current gets big enough to turn the transistor on.

 

Thanks Kermit.. That works great. I found an even better idea (maybe, I think) in just tying the negative side of that LED to the negative side of the piezo. Now they are tied together and what one sees, the other sees and only when the relay engages. Does that sound okay or is there something I am missing?

 

If it just a visual cue that the circuit has become active it will work fine that way

 

Thanks for the help. This is an awesome resource! I checked the entire circuit using my test 6 cylinder engine and associated electrical system and it's all working great now with one exception that may or may not be an issue at all...

The Alternator has a terminal 1R (on 10SI and 12SI Alts) that is for the 'idiot light'. I use that to make the rest of my circuit know if the engine
is on or not. Works great, but the one minor thing that bugs me is the Alts charging light behaves like this: It's bright before the engine is turned on which is normal, then on startup it will not dim until it gets to the RPM to start charging, again normal, but then it stays DIM. The light never goes completely out. With high RPM it gets Dimmer and it's noticeable when the charging circuit isn't working because it's significantly brighter. I would just like to see the light go completely out until it's not charging properly. (The problem in question is just the top two legs of the diagram above.) Minor issue but I thought I better ask before I do another run on circuit boards.

 

I was thinking it would be nice to have an "Engine Running" condition. So a relay would latch only at engine startup. I do not see any way in my circuit (first post above) to do that off hand. What would it take to add a latching condition where a relay would engage at engine startup and disengage on engine off? The idea would be to add an hour meter, or anything that a person would like to add but only when the engine is running.

 

I was thinking it would be nice to have an "Engine Running" condition. So a relay would latch only at engine startup. I do not see any way in my circuit (first post above) to do that off hand. What would it take to add a latching condition where a relay would engage at engine startup and disengage on engine off? The idea would be to add an hour meter, or anything that a person would like to add but only when the engine is running.

There are several ways to do this, one of which some BMWs use to let the Motronic control unit know that the engine is running or very close to running, and that is the use of a spark sensor. On my 25 year old 525i, there is a small sensor coil on one of the spark plug wires. Just letting the control unit know that things are turning and ready for fuel, are some of the things accomplished there. Interestingly, the fuel pump circuits won't energize until the Motronic senses sparks. I believe that is unlike most cars. Can't "easily" check fuel pump circuit operation without some sparks. Of course, that's not your issue.

 

The circuit so far has a relay that shuts down on engine startup due to the alternators warning circuit light bias. I am hoping for some sort of a solution that latches ON for the entire time the engine is running. Since the way to make the engine stop running is to turn the ignition key to OFF, my only problem is figuring out how to LATCH (something) to stay on when the relay shuts down.

 

The circuit so far has a relay that shuts down on engine startup due to the alternators warning circuit light bias. I am hoping for some sort of a solution that latches ON for the entire time the engine is running. Since the way to make the engine stop running is to turn the ignition key to OFF, my only problem is figuring out how to LATCH (something) to stay on when the relay shuts down.

Regarding my earlier post on the spark sensor, you could try something like that and use a missing pulse detector to sense the engine turn off. No spark pulses drops the signal. Even easier, if you are working with electronic fuel injection, you could sense pulses going to the injector(s). Just tap into an injector line/wire and run your sensor line to voltage follower op amp circuit. That could work for you. Some cars even have used the oil pressure sensor to accomplish what you are doing. When the oil pressure drops off they conclude the engine has turned off.

 

Because of the circuit I am working with (a vintage car idiot light system), I have a nice circuit that I could add this latching capability to since I am already using an oil pressure sensor for the project. I need to look into latching relays, or latching circuits. Its a nicety more than anything, giving the user an added advantage of using an hour meter or anything else that needs to sense engine on vs engine off. Since we are talking about 1950s tech, and I already have the circuit shutting a relay down on engine start, I am hoping I have what I need built in already to start the latching. As it is, Alternator Warning Circuit means relay shuts OFF at engine start. I need to turn that OFF into an ON for another relay. Engine off is the ignition switch signal since thats the only way to turn the old girl off!