Extracting the Signal

[dw85745]

Thanks a bunch. No rush.

[pebe]

dw85745,
Sorry for the delay in providing the write-up, due to unforeseen circumstances.
I'll try to get it done in the next few days.

[dw85745]

No problem.

Same issue on my end. Always something of higher priority.
Got board half soldered in, so hopefully balance this weekend.
David

[pebe]

Hi David, here is the promised write-up.

If you look at a datasheet for the 555, it shows the IC is made up from dozens of transistors. Although it will detail the various parameters for each pin, it doesn’t help you to understand how it works. I think this block diagram I found on the ‘net explains it better.

555 Basics.gif (6.85 KiB) Viewed 75156 times

You can see that the heart of the 555 is a flip-flop (F-F). It can be set by a negative pulse from COM1 output and will remain set until it is reset. That is done with a negative pulse from COM2 output, or by taking the reset pin (4) low.

When set, the output pin (3) goes high.
When reset, the output pin (3) goes low, and the discharge transistor is turned on, so that the discharge pin (7) is shorted to the negative supply pin (1).

Note there are three equal value resistors across the supply. With a 12V supply, this provides a 4V tap that is fed to COM1. So whenever the trigger pin (2) goes below 4V the F-F will be set.
Likewise, an 8V tap is fed to COM2 so that whenever the threshold pin (6) goes above 8V the F-F will reset.

My circuit uses the F-F in monostable mode to give a single pulse at the output when pin2 is triggered. Referring to my previous circuit, this is how it works. Note that I now realise that D2 and C2 are not needed and R5 can be wired directly to pin3.

At switch-on the F-F starts up in a random state (ie. either set or reset). If set, C1 will charge up through R4 until the voltage on pin6 gets to 8V. Then the 555 resets and pin7 discharges C1. The circuit now waits for a trigger pulse. Then the F-F will set, the output pin (3) goes high and C1 starts to charge via R4. When there is 8V across C1 the F-F will reset, the output will go low, and pin7 will discharge C1 again ready for the next cycle.

The length of each single pulse is determined by R4 and C1, and the time is equal to 1.1 x C (in microfarads) x R (in megohms) so in this case = 1.1 x .01 x .047 = 517µs The control pin (5) is only normally used to give fine frequency control when the 555 is wired up in astable mode to make an oscillator.

Now for the trigger from the ignition lead. The pickup of a few turns round the HT lead will form a capacitor of only about 10pf to 20pf, so the LH side of R1 will pulse high for a very short time. But the base of the NPN will have inherent capacitance hence the small value for R1 so that the base can rise quickly to conduct before the pulse ends. There will be a negative pulse when the voltage on the HT lead reduces to zero, and D1 protects the NPN from that negative base voltage. Note that in the circuitry from the pickup I am treading on virgin ground because I have no access to an ignition coil etc. on which to carry out tests, so the circuit may need some modification.

[dw85745]

pebe:

Thanks for taking your time to write this up for me.
Have a long weekend (our Memorial Day on Monday), so will have some extra time.

=================================================
Tried the unit today. Unfortunately "No Response" from meter.
I tried the following using the #6 plug wire (easiest to get to):
1) #24 gauge copper wire only (no covering) with 2 1/2 turns around plug wire and one end wire connected to box input.
2) Same as #1, but with plastic covering on. Tried this first.
3) Took a 3/4 inch piece of copper tubing, cut it length-wise, and soldered a #16 solid wire to one of the halfs.
Then put both halfs around the #6 plug wire and held them together using plastic electrical tape such that the two halfs were touching each other. I then connected the soldered wire to the box input.

======================================================
I made my box connections as follows:

1) Spark Plug takeoff per above to box input
2) Box 12V lead to B+
3) Box + Lead for Meter to Meter Positive.
4) ONLY made one lead for 0 volts and Box negative lead to Meter since both are grounds
Connected this single leat to both Battery Negative and Meter Negative.

=======================================
Had switch on 6 cylinder.
Tried both Low and High RPM switch.

[dw85745]

Correction. Copper pipe was 3/8 NOT 3/4.

[pebe]

Sorry about that. Approx what were the engine revs when you were testing?

[dw85745]

Sorry about that. Approx what were the engine revs when you were testing?

No apologizes necessary. Appreciate your efforts.
Was running at idle, so guessing 500-600 RPM.

[dw85745]

RE: "Note that I now realise that D2 and C2 are not needed and R5 can be wired directly to pin3."

1) Should I go ahead and do this and see what happens.
2) Re: Idle RPM may be from 500-750 instead of 500-600. (again guessing)
3) Will put a DVM (don't have a scope) and check some voltages.
My guess is the voltage drop across R1 is most important since that is the input trigger.
4) IF you will provide me some check points areas of interest will test for voltages.

David

[pebe]

Hello David,

I have Googled for more information about spark polarities, and it appears my assumption that the plug is provided with a positive pulse was wrong.

Both these links imply that electrons prefer to leave the hot centre electrode of the plug, so they are driven with a negative voltage pulse.

http://mgaguru.com/mgtech/ignition/ig104.htm

and

https://en.wikipedia.org/wiki/Spark_plug (half way down the page)

That being so, the inverting transistor feeding the ‘trigger’ pin of the 555 is not required, so I am attaching a new circuit.

Ignition Pulse generator 2.GIF (4.66 KiB) Viewed 75023 times

R1/R2 sets the voltage level on Pin2 at 40% of Vdd (pin2 triggers at or below 33.3% of Vdd), so a negative pulse of -0.84V (assuming Vdd = 12V) should trigger the 555.

R3 limits the current going into D1 and D2 that are there to prevent Pin2 going outside the supply rails.

The value of R1 and R2 in parallel determines the input impedance that the pickup 'sees' and should be high enough to get the maximum pulse amplitude, but low enough to prevent external pickup on the lead. I hope the values I picked will be OK.

Ideally, the leads between pickup and chassis and 555 should be as short as possible and wired with twin low capacity wire. Don’t use screened cable – its capacity is too high.

At switch-on, Pin2 should be at 40% of Vdd. To prove whether it is the pickup or the tach that is malfunctioning, temporarily disconnect R5 and put a multimeter set to 10VDC between Pin3 and Pin1. It should read 0V because the bistable is reset.

Change the meter to its lowest AC range and run the engine at 600RPM. Measure at Pin3. The 555 should be outputting 516µs (amplitude 10.5V) pulses (set), and resetting for the rest of the 100ms, so the mean voltage should be somewhere around 10.5 / 0.1 x 516µs ≈ 5mVAC.

Can you give that a try?

[dw85745]

pebe:

Thanks
Give a week to redo the circuit
FWIW tried to make distance between components as short as possible and
reduce traces by using the component wiring as a trace whereever possible.

Re:

wired with twin low capacity wire. Don’t use screened cable

Not familar with either. Will check local supply house.
What I did for pickup was strip the plastic off a 24 gauge solid wire and then wrapped around ignition wire.
I then used some multi-stranded (believe 18 gauge) with alligator clips on each end to connect the
wire wrap to the test box. These multi-stranded with alligator clips on each end are common here and
normally used as jumpers or wire extenders for VOM, DVMs. Bought mine years ago, but saw Amazon sells them.

[pebe]

quote: "wired with twin low capacity wire. Don’t use screened cable"

Typically the figure-of eight section twin wire used for wiring up bells, or the thicker stuff used as loudspeaker leads.

[dw85745]

Hi pebe:

1)

ypically the figure-of eight section twin wire used for wiring up bells, or the thicker stuff used as loudspeaker leads.

I assume the above quote is the low capacity wire?

2)
Down to doing the last common ground wire.
The new schematic shows a vehicle chassis ground, 0 volts, and the meter black ground.
a) Why the additional ground (chassis) since I consider the 0 volts to be chassis ground (battery negative)?
b) If needed, do both grounds (chassis and 0 volts) need to come off either end of the ground circuit as shown?
c) If b is true, -- I assume -- I can connect (chassis ground, 0 volts, and the black meter connection to a single ground point (battery negative)?

Thanks
David

[dw85745]

pebe:

Sad to report, I could NOT get the meter to register with the last design.
Unfortuantely don't have a scope to see how the input is picking up.
I did put a neon bulb on the outside of the spark plug wire and it flickers, so obviously the magnetic field is
getting past the rubber plug wire covering.

I tried both a single wrap and two wraps around the wire with a solid core 24 gauge wire.
One end was free the other end connected to a jumper (18 inches in length) to bridge the gap between the
plug wire wrap and the input wire terminal on the board.
So NOT sure what is going on.

One GUESS -- "my logic" -- is whether having the chassis ground connection near the input, would cause a lot of the signal to follow the path through R2 (10K) and be lost?

[dw85745]

pebe:

Don't know if your still with me.

Went back and read your post with the new circuit.
Had forgotten about comments on how to use multimeter to test as focus was on getting board made and assembled correctly.

What I did to test was:
1) Desoldered and lifted leg of R5 (100 ohm) going to pin3.
2) Used approx 1 1/2 wraps of solid copper wire (no plastic around wire) around #6 spark plug wire with a jumper
wire approx 18 inches long to connect to board input lead.
3) Connected up board chassis ground wire to B negative
4) Connected up board 12v input wire to B+
5) Left both board tach wires unconnected.

Then using multimeter to go from pin3 (+) to pin1 (-) in AC mode

1) With vehicle NOT running measured: .0033
2) With vehicle running at idle (RPM's unknown but guessing between 500-750) I measured:
.0315 (highest) to .03295 (lowest). Meter seemed to fluctuate within this range.