Help for a project for engine start detection

Hello guys!
I would like to build a project for engine start detection with NE555 timer. It is possible?
The circuit have to control a relay, which is normally open, to close it. When the control circuit is powered by car battery the voltage is 12 to 12.7v.
After engine starts the voltage becomes above 13.8v. After this I would like to switch on the relay, and if voltage becomes below 13.8 v to switch off the relay.
Anybody can help me with this schematics?

Thanks in advance...

Do you have to use the NE555? This is a timer. Do you need a "time function"?
It looks to me you only need to detect >13.8V or <13.8V.

Do you have any information on the relay?

No, I do not need a "time function". I would like to use this circuit to control the DRL (Day Running Lights) on my car. When I switch the key on position Power ON - the lights go on without running engine. I would like to catch this signal and to turn on the lights after engine starts.
The part number of this relay is 7700414484.

Usually that is done with an oil pressure switch.

I would simply detect the system bus voltage. With key-on, engine not running, bus voltage will be less than 13V. During cranking, voltage will sag even lower, to ~11V. After start, as the alternator comes on line and the engine is running even at idle, the voltage will be over 13.00V almost immediately.

Try it with a DVM connected to one of the Accessory circuits coming from the fuse panel. The Accessory circuits are already interrupted during cranking while the key is in the start position (if you still have a key?)

ClydeCrashKop said:

Usually that is done with an oil pressure switch.

Click to expand...

It is not an option, because sometimes the lamp for oil pressure goes off without working engine.
Thank you!

MikeMl said:

I would simply detect the system bus voltage. With key-on, engine not running, bus voltage will be less than 13V. During cranking, voltage will sag even lower, to ~11V. After start, as the alternator comes on line and the engine is running even at idle, the voltage will be over 13.00V almost immediately.

Try it with a DVM connected to one of the Accessory circuits coming from the fuse panel. The Accessory circuits are already interrupted during cranking while the key is in the start position (if you still have a key?)

Click to expand...

I know all that Mike, but I need a schematics that will detect the difference (voltage before and after) and will switch the relay.

What relay? Coil resistance?

MikeMl said:

What relay? Coil resistance?

Click to expand...

about 85 Ohms.
I have a Renault part number 77 00 414 484

Here is a circuit that has a precise cutin of 13.05V, and a cutout of 12.05V, assuming you use the specified 1% resistors. R7 controls both cutin and cutout. R1 controls the hysteresis.



If you feed it from one of the key-switched accessory circuits, it will begin with the relay off, and the relay will not turn on until the alternator comes on line and the battery voltage comes up... With the hysteresis, it shouldn't ever go off, even if waiting at a stop light with the engine idling and the AC running...

MikeMl said:

Here is a circuit that has a precise cutin of 13.05V, and a cutout of 12.05V, assuming you use the specified 1% resistors. R7 controls both cutin and cutout. R1 controls the hysteresis.

View attachment 91676

If you feed it from one of the key-switched accessory circuits, it will begin with the relay off, and the relay will not turn on until the alternator comes on line and the battery voltage comes up... With the hysteresis, it shouldn't ever go off, even if waiting at a stop light with the engine idling and the AC running...

Click to expand...

Thank you very much Mike!
I appreciate the time you've spent for my project! Thank you again!
The day after tomorrow I'm going to buy all necessary elements and try your schematics.
Have a nice evening.

MikeMl said:

Here is a circuit that has a precise cutin of 13.05V, and a cutout of 12.05V, assuming you use the specified 1% resistors. R7 controls both cutin and cutout. R1 controls the hysteresis.

View attachment 91676

If you feed it from one of the key-switched accessory circuits, it will begin with the relay off, and the relay will not turn on until the alternator comes on line and the battery voltage comes up... With the hysteresis, it shouldn't ever go off, even if waiting at a stop light with the engine idling and the AC running...

Click to expand...

Hi Mike!
I would like to thank you again! I've realized this circuit and it's working on my car from couple of months - perfect!!!
I've tried two more circuits, but only this working as needed!!! Congratulations!
I use this circuit to turn on the DRL on my car, after engine starts. Before I use this, when I switch ignition on the headlights are on. When I crank the engine headlights turn off, and when engine starts they are on again. This is not good for a halogen lamps.
After installing this module the headlights are on only after engine starts. Next that I'm planing to do is to add a circuit for soft and delayed about 20-60 seconds (adjustable) start of headlights.
I'm attaching a photo with a module mounted in car. I've put the board together with the relay socket in a little box.