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# Triggered monostable multivibrator

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#### gguerdat

##### New Member
Hello,
I am attempting to design a circuit to produce a 5V logic 5.0ms pulse when an input lead goes from +12v to floating (think motorcycle points ignition). Continuing with the automotive theme this circuit will have to operate over a voltage range of 10-14v and heat range of 10F - 150F.
I took a few circuits classes around 20 years ago so I know just enough to be dangerous but am willing and excited to learn/relearn things.
My current thinking is a power regulator circuit to create a stable 5v environment then some sort of monostable multivibrator?
I have been poking around and found CD4047BE which is an RLC based multivibrator but I am not sure if this is reliable/accurate enough or if I would need something that uses a crystal? (are these more accurate?)
Lastly I could use some help with how to deal with a +12V -> floating trigger line. I would guess a pull down resistor? But what to do about the line when its +12v?
Thanks for any help!
-G

I believe the points typically are closed to ground and then open at the time the spark is occur. To trigger the monostable use a resistor from the points to +12V. This will give a positive going pulse when the points open.

I think a 555 can operate directly off the 12V (although you should add an RC filter and zener to suppress noise spikes).

I think a 555 can operate directly off the 12V (although you should add an RC filter and zener to suppress noise spikes).

Thanks for the quick response Carl. You are correct about the operation of the points I will use them that way.
I actually need the pulse to be 5.0 Volts and precise even under changing conditions (temperature and voltage moving up and down) to trigger a subsequent circuit.

Would there be an efficient way to drop a 12V 555 output down to 5V? (5V Diode to ground?) Or would it make more sense to run a 5.0V 555 and 5.0V through the points?

Thanks again,
-G

...
I think a 555 can operate directly off the 12V (although you should add an RC filter and zener to suppress noise spikes).

I have measured the voltage across the points with an oscilloscope. I saw voltage excursions as high as +-200V as the points open. To use this signal for a tach pick-up, you have to do some serious clamping/filtering. The sensing circuit also has to maintain a high-input impedance not to effect the spark.

Google around for homebrew tachs. They solved the problem of interfacing to the distributor points...

I have measured the voltage across the points with an oscilloscope. I saw voltage excursions as high as +-200V as the points open. To use this signal for a tach pick-up, you have to do some serious clamping/filtering. The sensing circuit also has to maintain a high-input impedance not to effect the spark.

Google around for homebrew tachs. They solved the problem of interfacing to the distributor points...

Here is sim of the situation. I created a crude model of the coil/distributor. I adjusted the coil inductance, resistance, and the damping resistor to create a signal similar to what I remember observing on the o'scope.

I use a 100K resistor at the distributor connected to the rest of the circuit through ~5ft of shielded cable. This does two things, first it prevents radiating the ugly points signal into other wiring (or your radio), and the capacitance of the cable acts as a low-pass filter.

The rest of the circuit filters/clamps the voltage to switch the NPN transistor. The duty cycle of the transistor output reflects the dwell of the points...

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• TachPickup.png
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Thanks Mike,
Yikes! I'm guessing that spike is flyback from the coil?
With this project I am actually trying to replace the coils on my motorcycle with an updated system:
Megasquirt Sequencer Coils

Using these LS2 coils the points would only need to trigger a precise 5v 5.0ms pulse to control the dwell. So if a 555 can provide a precision 5.0ms pulse across changing voltages/temperatures then I'm good.
At this point I am curious if it makes more sense in terms of reliability/precision to regulate the whole circuit and voltage across the points to 5.0V or run it at 12V and drop the output from the 555 to 5v?
-G

I'm not that familiar with the charging system on motorbikes, but I would be inclined to regulate the nominal 12V battery voltage to about 9V. 5V is a little low for 555 circuits; they seem to behave better at ~9 to 12V. The circuit I posted above is for a standard points/capacitor ignition system. It could be used to trigger a 555. You could use a much simpler circuit to interface the 5V pulse you showed to a trigger a 555 powered off 9V.

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maybe off in left field but why use a 555?
why not just use a D flip flop and have a hall effect switch or photo detector circuit connected in the distributor (get rid of the poimnts) and just clock the D flip flop
every other revolution equals one spark.

maybe off in left field but why use a 555?
why not just use a D flip flop and have a hall effect switch or photo detector circuit connected in the distributor (get rid of the poimnts) and just clock the D flip flop
every other revolution equals one spark.
But which ever other revolution. It has to be the correct one.

the distributor goes at 1/2 the rpms of the motor anyway. am sure there are electronic distributors made or a retrofit kit for your bike.
just tap off the output to clock the D flip flop and on to your coils.

the distributor goes at 1/2 the rpms of the motor anyway. am sure there are electronic distributors made or a retrofit kit for your bike.
just tap off the output to clock the D flip flop and on to your coils.
You need one spark every revolution of the distributor. And if the points are on the crankshaft you still need one spark every revolution (one wasted spark) since you don't know which revolution is the compression stroke.

the points are on the crank so both plugs fire every revolution?
is this a two stroke or four stroke engine?
not sure but do bikes have an electronic control module that controls spark advance etc?

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