How can a LM2917-8 be used to create an "Engine Running Only" circuit?

Thread Starter

mterickson

Joined Dec 17, 2019
13
Hello all,

I am trying to create a circuit that will only be "hot" when the engine is running.

I believe the LM2917-8 will work for my purposes but I am having a difficult time getting it to work. I've tried using the schematics in the datasheet. Specifically Figure 18 "Speed Switch," Figure 19 "Zener Regulated Frequency to Voltage Converter" & Figure 32 "Over-Speed Latch Circuit Diagram."

http://www.ti.com/lit/ds/symlink/lm2917-n.pdf

I calculated that Pin 2 should use .22uF CAP and Pin 3 should be 68k w/1uF CAP. That combination didn't work so I reverted back to the data sheet and used the numbers from Figure 19 which are .02uF for C1, 100k for R1 & C2 at 1uF.

Figure 18 didn't work at all.

Figure 19 did work for me, but the chip got really hot, and I was using the 470 resistor on Pin 6. It was also drawing quite a bit of current (over 200mA). I wasn't able to test if frequency going to 0Hz shut this circuit down since I shut the circuit down before anything started to melt.

Figure 32 would only work if I removed the 5k resistor between 7 & 8 and replaced it with a jumper wire. When I did that I was able to get a Vo to activate an LED (using this for test purposes) but the only way to shut off the circuit was by removing Vcc. I need the output to go to 0v when frequency drops below 40Hz, if that is even possible.

Here's what I have to work with:

12v constant power supplied by Body Control Module

"Clean Tach Out" signal from PCM - 4 pulses per engine revolution. I measured 40Hz @ 600 RPM (Idle) to 200Hz @ 3000 RPM. Voltage ranged from 6.69v to 7.14v and duty cycle was at 50%. The higher the RPM the lower the voltage. I do not have a scope and was using a Multimeter to measure the output. This is a diesel pickup so it's a "simulated" tach signal that uses the alternator to determine RPM.

The output from the LM2917 will be used to send power via transistor (preferred) or relay, to a delay-on circuit which triggers a relay to send 12v to the high idle circuit in the Body Control Module. Yes, I could use a key-on circuit and eliminate the need for the F-V circuit but the high idle will not activate with my factory remote start system if I go that route. Plus all the wires I need are right under the steering column with easy access and I've always wanted to learn more about IC's.

I presume it's the resistor/cap combos that are not correct and/or the digital pulse signal that I am trying to use. I don't understand what I need to adjust or how my adjustments affect the circuit. This is the first time I've played around with IC's and I am trying to learn. I would rather avoid guessing and burning up components as well.

Any help is greatly appreciated
 

Alec_t

Joined Sep 17, 2013
10,907
Welcome to AAC!
Wouldn't it be simpler just to monitor the alternator output?
The logical function (Alternator output > 0) AND (starter motor off) would tell you the engine is running.
 

Thread Starter

mterickson

Joined Dec 17, 2019
13
@Alec_t That does sound more simple, but would require wires being routed through the firewall, which is a lot easier said than done on these newer pickups, also getting to the alternator inside the engine bay would a fun chore. I assume you're talking about using a micro-controller of some sort to the decipher the inputs and provide the output I am looking for. I know nothing about using those.
 

TeeKay6

Joined Apr 20, 2019
572
Then you should be able to just use a transistor to generate the signal.
Do you also need a design for the delayed-on signal?
Does the output of that delayed signal need to go to a relay, or can it just be 12V?
Don't forget that the input signal is a pulse train, not a pure DC signal.
 

Thread Starter

mterickson

Joined Dec 17, 2019
13
Here's a concept. The actual component values would change depending on your specific need. One thing you have not told us is to what will you connect the Engine Running Signal?
View attachment 194756
I could try that.

The engine running signal is going into a DPDT relay circuit. When the relay is not activated pole 1 passes 12v through to the delay-on circuit which I'll explain below. Pole 2 completes the ground path for the Park Brake circuit (park brake is an enabler for the high idle) and for the base of a transistor which is used to activate and lock the relay on.

The relay is triggered by pressing a momentary open switch. This opens the circuit and stops the 12v to the delay-on circuit. The relay is reset by removing Vcc. This is where the engine running only circuit comes into play.

The reason for all this is so I can start pickup with or without remote start and have high idle activate. When I get in vehicle I press button on dash to place circuit I've built into "standby mode." Disabling the high idle and shutting off park brake indicator on dash. When I shut truck off everything resets for the next start up. Instead of having to remember manually reseting with a switch.

The timer delay circuit consists of RC network in conjunction with a 555 timer that outputs a steady voltage after a minute or so depending on the values I choose for the RC network. This output triggers a relay that sends 12v from the fuse panel to the high idle input on the BCM.

In other words, I've made what could be a very simple circuit using a toggle switch into an overly complicated thing. All because I asked myself if I could do it this way after looking at an intrusion alarm circuit.

Anyway, hope that helps explain everything. I'd post pic of my schematic but it's hand written by a novice and hard to read. The time delay, relay disable circuit is working as expected.
 

Thread Starter

mterickson

Joined Dec 17, 2019
13
Here's a concept. The actual component values would change depending on your specific need. One thing you have not told us is to what will you connect the Engine Running Signal?
View attachment 194756
I tried this. It would light up an LED when I simulated the square wave with a function generator. When I tried to use it to activate the delay-on circuit it would compete the ground for park brake but the delay-on and pulse circuits wouldn't trigger.
 

crutschow

Joined Mar 14, 2008
24,343
I tried this. It would light up an LED when I simulated the square wave with a function generator. When I tried to use it to activate the delay-on circuit it would compete the ground for park brake but the delay-on and pulse circuits wouldn't trigger.
Need to show exactly what that circuit consisted of.
 

Thread Starter

mterickson

Joined Dec 17, 2019
13
Need to show exactly what that circuit consisted of.
I used the schematic TeeKay6 posted above with the same values; I used that to replace the LM2917 portion of my schematic that I posted pictures of above. I will draw everything up later with the function generator included in the schematic and post it along with a picture of my breadboard.

I will add that I tested it again and the park brake circuit would complete to ground. The delay-on circuit did work but the LED I am using to show that it is working barely lit up after the time delay. The pulse circuit wouldn't trigger at all and when I attempted to put the circuit in "standby mode" that wouldn't work either.

I assume I need to adjust values of components in the delay-on & pulse circuits. For all I know those are incorrect too.
 

crutschow

Joined Mar 14, 2008
24,343
Please label all the components (Q1, R1, D1, etc.) so they can be referred to easily.

Why are you using 7V for the function generator?
Isn't the signal a 12V pulse?
 

Thread Starter

mterickson

Joined Dec 17, 2019
13
Please label all the components (Q1, R1, D1, etc.) so they can be referred to easily.

Why are you using 7V for the function generator?
Isn't the signal a 12V pulse?
When I input 12v I only get 7v sometime slightly less out of the function generator. I built the function generator from a kit I got on Amazon. I am in the process of building another one that only outputs Square waves that should give me more control over the amplitude. The one I have been using is supposed to put out Square, sine and triangle waves but with very limited amplitude control. No amplitude control for square waves (other than adjusting input voltage) and I can only adjust sine/triangle from 0-3v. Not a very good kit if you ask me, but I got to practice my soldering skills.

When I tested the circuit in my pickup I was only getting around 7v (6.69-7.14v) from the tach out lead as well.

I will work on adding labels this evening and post an updated pic.
 
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