MOSFET use

[alec_t]

the RST is linked to the 12 V input, but not to the timer circuit. Is that how it works?

Yes, if you're talking about the circuit I posted. The 555 in that is configured as a monostable circuit so needs an external trigger at pin 2. RST is held at 12V to prevent the output pulse being interrupted.

 

[malc9141]

Thanks. Yes it is that one - I should have recopied your schematic. Its brilliantly simple - tho of course we had to spend a lot of time learning what was needed, building the original current generator, and being misled because of an exhaust scavenging problem>smoke>which made us think we were running rich. Then analysing what current etc we were producing.

 

[malc9141]

Can I ask a further question?

In your circuit post #42 (which works beautifully), you show a square wave (falling) input on your diagram.
I have an RS 307913 light sensor picking up a reflection on the flywheel. On getting the reflection, the circuit closes to Ground with a voltage drop to the 555. On short term tests, it all works.
But I presume input is not truly a square wave and therefore have concerns about long term regularity. It might miss from time to time.

So I thought, I need to put in a NAND gate (a la original Hartley circuit). But then I thought: "If that system (it was a ic4039) can convert an analogue (? correct term) pulse to a digital one, then surely so can the 555 that is central to your circuit.

So my question is: should I rely on the 555 to recognise the input from the light sensor, or should I put in a wave-squaring sub-circuit?

 

[alec_t]

Providing your input pulse voltage drops below about 1/3 the 555 supply voltage the 555 should trigger reliably. The input pulse shape isn't critical: even a sinusoidal waveform would do! So the light sensor should be fine as is.

 

[malc9141]

Thanks again. Clear!!!

 

[malc9141]

Hello Alec
I have a problem. I am not certain whats wrong but I seem to destroy the 555. Its OK with a trial torch battery and sometimes with the Power Pack (trial at 10 v).
But should I put a bigger R in at the supply to the LM7012? I have 1K R at the input to the LM7012. Is the ~12 v out giving too high a current for the 555?
Thanks
Malc

 

[audioguru]

Are you talking about the LM7812?? You do not have an LM7012.
The minimum input for the LM7812 is +14.5V.
Why do you have 1k ohms at the input of the LM7812??
The maximum supply voltage for a 555 is +18V so it is fine with your +12V. If its output is shorted then it will get hot and burn out but its output is not shorted in your photo.

 

[malc9141]

Sorry, yes, LM7812.
My problem is I'm an Mech Eng not an electronics wiz. I struggle.
The circuit (Alec's post 42) seems fine & worked with a 9 v battery test run but seemed to fail when put it with 15 V power pack. I "thought" it safe to have the 1k ohms at the input in case 15 v would drive too much current. Well, it must be something else.

I have made a simple 555 astable (?got that right) at 20 Hz, and I use a Saleae Logic device with my Mac. I can no longer get an output signal on the Mac, nor the "light** flickering". I can't find a fault with the board so I blamed the 555. Luckily, I have just met a local Univ guy who can test it on a CRO, & apply a timed pulse.
Thanks for the reply. Its helpful.
Will report Monday.
** I have a Yellow LED and a 470 ohm R.

 

[malc9141]

Just adding, with the LM7812, I have three capacitors as shown in schematic. Why should they be close to the LM7812, (which they are) ? Is 10 mm away from each other, therefore max 20 from the LM7812, OK ?

 

[audioguru]

I do not know why your parts are very far apart. Electronic ICs switch very quickly and if important capacitors are too far away then the wires connecting them to the IC have inductance (the opposite of capacitance) and cause the IC to oscillate at a very high frequency. You show an input 0.1uF (100nF or 104) capacitor and an electrolytic output capacitor but the 0.1uF output capacitor for C2 seems to be missing.
The wrong type of capacitor can also be inductive (like an electrolytic type) which is cancelled with C2.

The 7812 voltage regulator does not need a resistor in series with its input because your 555 circuit uses a low current. The 1k input resistor reduced the voltage to the circuit to only a couple of volts where it will not work.

 

[malc9141]

The C2 is there OK. Its just the other side of the 7812. I'm happy with them.

But you reckon I should:

Move the electrolytic cap closer (easy)
Remove the 1000R completely.

 

[alec_t]

The circuit (Alec's post 42) seems fine & worked with a 9 v battery test run but seemed to fail when put it with 15 V power pack

It won't work with a 1k resistor in series with the input! Remove it. Current is controlled by the 5.6 Ohm resistors R6,R8.
What is the current rating of the power pack (which I assume puts out 15V DC?)?

 

[malc9141]

Its OK, Alec. (I'm not quite that daft).
The "side" lead to the LM7812 had (at first) a 1k resistor. But I found I had already bypassed that anyway - no doubt based on some good advice. (its desoldered now)

The power pack goes up to 22 v but I use 10 - 15 v.

This is what happened there was an oscillator in place of the engine flywheel which made the light flicker and gave a trace with a Soleae Logic device to my Mac. Now it doesn't when I use the Power Pack. So I imagined I had overloaded the circuit (but apparently not). Is there a dry joint? Can't find one but will look again.

 

[malc9141]

Yes its DC - missed the BOLD

 

[alec_t]

I use 10 - 15 v.

The LM7812 needs 15V to regulate properly, but 10V would still be ok for the timer circuit.
Can't say why you get no trace on the SL device. At which circuit point are you measuring? Isn't that device a logic analyser? If so, it's looking for a signal of 5V or less; not 12V.

 

[malc9141]

"LM7812 needs 15 V etc" >>>> Useful to know .
5 v signal for Logic analyzer. Mmmm. Might have a bearing.
It seems to give a perfect signal unpredictably (ie its there one try, not the next) on the separate 555 "signal generator" which gives a sq wave drop at ~ 20 Hz. Could this unpredictably be the voltage being too high?

But the main circuit (exactly your post 42) does zilch now. (So I imagined it must be the 555 blown as I could see no other problem, nor find one. OK its not that we now know).

I used to get the Logic trace off the 555 OUT line Pin 3.

 

[malc9141]

Thanks, all. Its working well now, several little bugs sorted.

We changed R5 from 5k Ω to only 220 Ω and found we could activate Fire in (actually make the injector respond) in just under 80 μs.

An interesting point: the original circuit, at the signal input, had a Schmidt trigger then a NAND gate series (Is this a flip flop?) to sharpen the wave. But it was also a circuit protector. With this new 555 circuit, I was surprised - then not surprised - to see I could hold HOLD on indefinitely. If by chance the flywheel stopped at Fire, it might burn out the injector.

A Signal stopper is easy to introduce, and will be put in.

 

[malc9141]

Hello helpful people

I built the circuit shown Oct 4, 2014 and it worked Ok. I took out R8 when I found my Current source was limited to 4 amps. It was news that 4 A was adequate for opening the valve at the low inductance coil.

The problem has been simply Bench Arrangements. We always seem to miss an observation because we have several controls. A colleague said "it would be easy" (how many times have I heard that as folk disappear never to be seen again) to use a CPU and set Fire at (say) 100 us, adjust Hold between 1 and 3 ms; read out revs, set Fire adjustably relative to Top Dead Centre. I spoke to an academic who said it sounded straightforward but he hadn't time ...

We now are building a Twin Engines prototype and it would Fire both cylinders simultaneously; or one could fire just behind the other if that helped. Are there any comments that might help me go to my former colleague with an intelligent suggestion?

Thank you in anticipation

 

[alec_t]

If you want both cylinders to have the same Fire and Hold timings you could just duplicate the set of parts shown in the dotted box in the schematic below and apply the same control signals as per the present set :

With hindsight I think the value of resistors R4 and R7 would be better if reduced to something in the 47-100 Ohm range. I should have pointed out previously that the Trigger signal to the 555 needs to be shorter than the Hold duration.

 

[malc9141]

Thank you. The R4 reduction was advised also by "my colleague".

Any thoughts on the CPU idea? Its way beyond my understanding but as I said, an academic grasped the idea - if he's to be believed.

Malc