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555-based ignition coil, assistance requested

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Theodore

Wed Jul 22 2009, 01:25AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Still got nothing.

I went to Radio Shack (only place open around this time and place), and picked up all the components for the 555-tester. They don't carry breadboards any more (WTH?), so when I got home I took almost everything off of it, rebuilt it via the specifications on that webpage I linked to previously, and tested the 555.

Pin 3 was constantly "high", so only 1 LED lit up and was always on. Ok, 555 is bad.

Popped a new 555 in there, and sure enough, I got oscillating LEDs. Excellent.

Rebuilt the Ignition coil circuit, as previously noted except with a 10k resistor from gate to ground and for the timer circuit itself used the schematic on the aforementioned webpage.

Aaaand, I got nothing.

Just checked with my multimeter, now there is no current at all going to the Ignition coil.

Going to try switching back to the schematic that was included in the datasheet for astable operation. Wish me luck!

EDIT: Thanks for the heads up on the easy way to check a 555's output. I think just for S&Gs I will put together a 555 timer tester to just have around though.

Sincerely,
Theodore

Theodore

Thu Jul 30 2009, 12:40AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

I don't know what else to try.

The 555 is working fine, checked via test circuit (twice). The mosfet checks out, using a multimeter. All the components check out, also by multimeter.

But, the coil is drawing no current. I am showing +12V on both poles of the ignition coil.

Used the 555 timer calculator here to redo the timing, as I had (incorrectly) set up an excel spreadsheet for that purpose and so had the wrong values.

Using a 1uF cap and R1=0 ohms and R2=90 ohms gives an 8khz frequency at 50% duty cycle, which should be about right for this coil.

I even tried just switching the connections on the Ignition coils poles, for the hell of it. Nothing of course.

I cannot for the life of me figure out how to get this thing to work. I am using the most basic, standard 555 setup, a premade coil with a known resonance frequency, a factory made power supply with plenty of juice, and I still can't get a damn spark out of this thing.

Does anybody have any suggestions or help? Should I post another pic of my setup?

I am really out of ideas here.

Proud Mary

Thu Jul 30 2009, 12:49AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Theodore wrote ...

Does anybody have any suggestions or help? Should I post another pic of my setup?

I am really out of ideas here.

Put up the circuit diagram, son, and then we'll have something to talk about.

Theodore

Thu Jul 30 2009, 01:04AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Alright, this is the updated version with the values on the line drawing.

EDIT: Aw crap, knew I should have taken a sec to double check the drawing. On pin 5, the other side of the 1nF cap is connected to GROUND, not to +12V. My bad.

Proud Mary

Thu Jul 30 2009, 01:36AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Is the MOSFET being properly switched on?

I'm guessing that you have very little test equipment, so try this:

Connect a resistor of 200R or so in series with a pair of headphones and connect this between pin 3 and earth. You should be able to hear a high pitched whistle if the 555 is oscillating at 8kHz.

All being well, put an old loudspeaker in the drain circuit of the MOSFET in place of the ignition coil. There should be a very loud high pitched whistle.

If you can hear a whistle at pin 3 on the 555, but nothing from the speaker in the drain circuit, you know that something is wrong with the MOSFET circuitry, or something has failed somewhere between the 555 and the ignition coil.

What is the part number of the MOSFET, lad? It may be that the 555 is insufficient to drive it without an intermediate buffer stage, but we'll get on to that once you get back with the results of these first simple tests.

Theodore

Thu Jul 30 2009, 02:23AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Ok, looked up the pinout on a headphone jack.

Hooked it up thusly: 12V ground -> 100Ohm -> ground on headphone jack --through headphones--> right channel on headphone jack -> pin 3 of 555.

I got no whining, checked the connections by tapping the pinout of 3 on the right channel and got a tiny bit of barely audible static sounds (consistent with plugging and unplugging headphones on a walkman), so I suppose the conclusion is that pin 3 of the 555 isn't outputing. However, I just put the 555 in a test circuit and it seemed to work fine.

I have triple checked the connections and placement of components on the bread board, so there must be an inherent flaw in my schematic, but the schematic isnt very much different from the test schematic which works perfectly!

Differences are resistor values, and the fact that the 555 was driving 2 LED lights alternatively, instead of a fet.
(Tester Schematic here, close to top of page:

)
R1=68K and R2=39K in the test (roughly about 9 hZ), and I am using R1=0 and R2=90 in mine.

Other than that they are identical.

Suggestions?

Thanks very much,
Thoedore

klugesmith

Thu Jul 30 2009, 03:04AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I guess the formula told you to do it, but ... you can't set R1 (between pin 7, Discharge, and pin 8, VCC) to zero ohms!
Circuit is guaranteed not to oscillate & will probably rapidly destroy the 555.

The 555 needs to be able to pull pin 7 nearly to GND, so R1 must be at least 100 ohms (I'd recommend at least 1000 ohms unless you look up & understand the pin 7 sink current specification).
Then if you want a duty cycle near 50%, without invoking diodes and stuff (or driving the timing RC network from pin 3),
you must choose R2 much bigger than R1.
Then if you are aiming for 8 kHz, the timing capacitor must be much, much smaller than 1 microfarad. I think the formulas are pretty valid even down to well below 100 picofarads.

Good luck. Since old guys like Harry and me might have trouble hearing 8 kHz through headphones, we might confirm oscillation by measure the voltages on pin 7 or pin 3 -- should be around 1/2 of the supply voltage.
Rich

[edit] p.s. You should generally put a decoupling capacitor (say, 0.1 uF) close to the 555 power supply pins -- the 555 is notorious for inducing glitches in local power supply network when it switches. A much bigger consideration here is power supply glitches when the MOSFET and ignition coil switch -- I think you need to do some learning about physical layout considerations above & beyond the schematic diagram.

Theodore

Thu Jul 30 2009, 03:40AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

Changed to:
Cap = 1nF
R1 = 100 Ohm
R2 = 9k Ohm

Still no output on pin 3. Checked via headphones and ignition coil.

Keep these suggestions coming guys, I really appreciate the help.

Thanks,
Theodore

EDIT: Hmm, I should have read your post more thoroughly, I used 100 Ohm instead of a 1000. Also, didnt see your PS till just now.)

klugesmith

Thu Jul 30 2009, 03:59AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I remember being at that stage, trying to debug cookbook circuits without understanding how the parts are supposed to work.
Men had not yet walked on the moon!
How about: (assuming you have at least a voltmeter)

Temporarily disconnect pin 3 from the mosfet, in case the latter is miswired or damaged & clamping pin 3.
Change R1, R2, and C1 to values similar to your blinking-LED tester. (no need to disconnect existing cap,
just connect a bigger cap in parallel).
Check pin 3 for very low frequency oscillation using voltmeter, resistor + LED, or clicking in headphone.

If successful, progressively change timing components to increase the frequency.
If not, check that the 555 is not getting hot, and that it still works in the tester. (it's socketed, right?)
Then measure and report to us the voltage on every pin (with respect to GND).

Theodore

Thu Jul 30 2009, 04:11AM

Registered Member #1506 Joined: Mon May 26 2008, 06:19PM
Location:
Posts: 18

I do have a multimeter, mentioned it above.

As for physical layout vs electrical connection, I definitely don't have the knowledge or experience yet to even try to deduce the differences.

Hmm, this looks like a very good test methodology. Unfortunately I wont be able to get to it for a couple of days now, but as soon as I do I will try these things and let you know the results! ^_^

Thanks again for the help,
Theodore

EDIT: Gah, way too tired, I just realized that the cap/R1/R2 combo I used just now will produce ~80khZ, not 8khZ. >_<. Guess I will increase R1/R2 by a power of 10 and like I said, will try again in a few days when I have free time again and hopefully a clearer head. Thanks everyone for the suggestions and help.

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