Power-up and triggered delay (with 555/6 ?)

[slapshooter]

I'm trying to put together a simple circuit to detect the state of an oil pressure switch on a petrol engine with magneto ignition stimulus ... note - there is NO local DC supply available - so I have to derive it from the negative pulse used to drive the coil that I subsequently have to mute!
Intention is to use a diode & cap to keep the circuit alive, a zener to regulate the operating voltage and something!, FET or other - to implement the mute function at the lowest power & budget possible - to keep the whole thing as small as possible. Concept schematic attached with functional principles...

I'm not an analog person, and with this in the negative VDC domain - and very little experience with 555/556 design - but I'm thinking it may be a good direction to explore - andy ideas - TIA

**broken link removed**

Looking at this again - becomes obvious that the controlled 'output' contact will either -
- need to be DPST - to open-circuit the oil switch during start - but to allow that switch to becoem active once it opens (engine running)
- use the oil switch as an input to the logic - to detarmine the run state in conjunciton with the -Vcoil presence.

Simple problem - tricky to visualise !

 

[bychon]

First, I don't believe a magneto can drive a spark plug with 100 volts. Please explain.
Second, I have no idea how to mute an ignition coil, or why you would want to. Please explain.
Third, using a $2 battery might solve this much easier.

 

[slapshooter]

The magneto drives the primary of the coil with the --100V pulses - the secondary EHT to the plug is not discussed in this circuit requirement

Muting the coil primary to ground is how the engine manufacturer stops this 'perpetual motion machine' !
We are simply wiring parallel to their SPST switch to stop the engine when the oil switch goes to ground - but it's also closed when the engine is stopped - so can never get the primary pulses going to the coil in order to start ! - hence the holdoff and other requirement

The $2 battery or a supercap are certainly in there - just searching for ideas that real EEs might suggest

thanks for the thoughts

 

[bychon]

I see we are having trouble with the definitions of words. This word, "muting" is what we call "grounding" or, "shorting". This becomes much easier now that we have solved the language problem.

The real goal is to use the oil switch to kill the engine. Pressure switch is closed to ground while starting. After engine is running long enough to get oil pressure, if switch closes, activate something to short the primary of the coil to ground, and keep it grounded for at least 5 seconds, even while the power from the magneto is failing. True?

I think even simpler than a 555 timer chip.

 

[slapshooter]

TRUE !
thanks - I used muting in the original sense... (not audio !) - glad you picked up on it

 

[bychon]

More details please.
Is the oil switch grounded through its threads so that it can never be completely isolated electrically? or does it have 2 wires?
Can you measure the current pulses through the ign coil primary so I can calculate how much power I can parasite off it without killing the spark?
How many cylinders?
What's your idle RPM?

 

[bychon]

I assume the magneto voltage can never be allowed on the oil switch because 1) the switch would not survive the 100 volts and 2) any spark in the sump might ignite fumes and blow the oil pan off.

 

[slapshooter]

Yes - one side of the oil switch is threaded/grounded to chassis 0V
option (2) is probably a good safe proposal - although the switch is engineered for the application - it is better to play safe.
Measured 800mA available on the --100V with a Fluke true RMS meter - way more than enough (which I sort of don't know how to estimate the true 'continuous' draw avaialble).
We should be looking in the very low mA unless a relay is involved, which in turn would suck the joules out of a cap pretty fast !

 

[bychon]

This is taking way too long. Hours of design time to avoid buying a battery every 3 years.

Anybody else want to try this?