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post #1 of 12 (permalink) Old May 29th, 04, 06:01 PM Thread Starter
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I'm trying to build a switch to turn on with my number one fuel injector. I used a P Channel mosfet which turns on with a negative signal (as the ECU controls the fuel injectors by giving them all power and then grounding the one it wants to pulse). I connected a 10k resistor between the source and the gate so when the injector grounds the switch is on and when there is no ground the switch is off.

Anyway, the pulse is too fast. Is there a way to turn the switch on with every other ground cycle? I'm not sure how to ignore some ground signals and switch with others. Ultimately, I'd like to be able to turn on the switch every 2 - 6 injector pulses.

Any ideas would be appreciated!!

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post #2 of 12 (permalink) Old May 29th, 04, 06:15 PM
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I would think the only way to slow the PRF would be to change the RC network or count it down.
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post #3 of 12 (permalink) Old May 29th, 04, 06:35 PM Thread Starter
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I'm not really following you....


How about rigging up a 555 timer ic. When the mosfet turns on it will feed the 555 circuit and charge a capacitor. Eventually the capacitor will discharge and the timer will switch?


After thinking about it, I think it will work. The 555 timer ic in monostable mode will take the input signal, then do it's thing regardless of how many more times it get an input pulse.....

[ 05-29-2004, 09:40 PM: Message edited by: paulm ]

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post #4 of 12 (permalink) Old May 30th, 04, 07:57 AM
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The injector pulse shouldn't be too short for a MOSFET. I suspect something else is going on. But I think your idea of using a PMOS transistor is a good idea. Just be sure to insert a bit of resistance between the gate and source to help protect it from voltage spikes off the injector coil. In fact it would be a good idea to have a zener diode installed at the gate to protect it from large spikes (but the resistor is a must). You'll have to look at the break down voltage spec on the gate. If it is 20 volts, for example, a 15 volt zener on the gate to ground with a 10K ohm resistance to the ground side of the injector should work well. Any chance the FET popped? Have you looked at the injector pulse width with an O-scope to see what it looks like? It should be plenty slow for what you are trying to do. Perhaps some other portion of your circuit has a slow response time.

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post #5 of 12 (permalink) Old May 30th, 04, 10:19 AM Thread Starter
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Dave, you're right the mosfet can easily keep up with the injector. See I'm injecting water via an electric solenoid, so I want to cut back the water. Both the solenoid and mosfet can keep pace with the injector, I just want to cut back the water ratio. Oh, I am using a 10k resistor between the source and gate.

I have changed my mind about the 555 timer ic. Although it will work it will not keep a direct ratio between the water/fuel. I think that a 4017 decade counter will work better. It takes in a pulse and has ten outputs, so the first pulse turns on the zero output, second pulse turns on the one output, etc. So what I'll do is the first four outputs will go nowhere, and number five and six will power the solenoid and the seventh pulse will be wired to the reset which will cause the count to start over. This way I can cut back the water pulse down as far as 1/10 of the fuel pulse! [img]graemlins/thumbsup.gif[/img]

Here's a little tutorial on it:

http://www.doctronics.co.uk/4017.htm

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post #6 of 12 (permalink) Old May 30th, 04, 08:45 PM
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You'll find lots of great little counter / timer circuits to do the division for you. Just browes through texas Instruments' or national electronics web sites in the TTL logic sections. Digikey is also a good place to window-shop. I'd also recommend looking at one-shot ICs since they can streach or shrink pulses based on a resistance and capacitance value. Also, the 74HC138 is a 1 of 8 decoder that will take 3 or 4 bits from a binary counter and activate one of 8 outputs. The best but not necessarily most feasable for you is a PIC microchip that can be programmed to do about anything you want and only use a hand full of parts. The problem with discreate logic is you can end up with 3 or 4 14-16 pin ICs in no time.

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post #7 of 12 (permalink) Old May 31st, 04, 04:01 AM
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dnult is right, a PIC microcontroller is definitely the way to go on this one. You can pretty much eliminate all the input conditioning circuitry (except for a resistor divider to get the signal voltage down to less than 6v) and program the inputs and outputs to do anything you want. Don't know if you have the ability to work with these devices but they are great for quick little control projects like this and it can be done with one 8 pin IC!

I can also think of a pure analog way to do this with an op-amp circuit called an "integrator". Let me know if you want to know more.

Paul D.
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post #8 of 12 (permalink) Old May 31st, 04, 07:55 AM Thread Starter
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Thanks for the input guys!!!

I am pretty slow with this electronics stuff. I enjoy working with it, but my learning curve is long as I never took any courses in this stuff. I always end up doing the trial and error method which sometimes causes capacitors or mosfets to explode!! That'll get your attention!

Anyway, I'll look into the parts that you suggested.

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post #9 of 12 (permalink) Old Jun 1st, 04, 03:11 AM
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Keep at it!! I've been doing it for 25 years now and I still "let the smoke out" of a circuit occasionally (Maybe 'regularly' is a better word??).

Remember, circuits always work as designed but, not necessarily as intended!!

Paul D.

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post #10 of 12 (permalink) Old Jun 1st, 04, 04:37 AM
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I think you need to recalibrate the Ohmamatic transducer circuit breaker assembly to include a Faraday cross vented rootcanalerator for better cooling of the micro resistance linkage.

What the H##L are you guys talking about? [img]graemlins/clonk.gif[/img]
Nebermind, it must be a 'need to know' thing.
good luck, with whatever you are doing. Its fun to watch and have no idea.

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post #11 of 12 (permalink) Old Jun 1st, 04, 05:40 AM Thread Starter
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Thanks for the encouragement!! It is really fun building circuits especially when it has something to do with my car!


Jim, I'm building a circuit to control an electronic solenoid (valve) to inject water into the engine. I am following one of the fuel injectors, so I keep a direct ratio between the fuel and water. This is to allow me to cut back the fuel which will increase my gas mileage. The sad thing is that our engines only completely use about 15% of the fuel dumped into the engine, the rest is pretty much wasted. I'm just trying to up that percentage a bit! [img]graemlins/thumbsup.gif[/img]

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post #12 of 12 (permalink) Old Jun 1st, 04, 06:05 AM
 
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Well, at least I understood Jim.


Inject water? Huh?


Rick.
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