Self accelerating reed switch magnet spinner.

[TinselKoala]

A request for more help:

The 4017/555 system fires a very short pulse to the blue LED just at the moment of coil turn-on. It would be _awesome_ to have another strobe LED -- say a red or green one -- that fires just when the coil turns _off_. 

This then would allow full control and visualization of the dwell angle, or conduction angle as MH refers to it, without needing a scope ever.

But I can't quite figure out how to get a pulse at the turnoff time. My brain is fried at this point, creative juice is low. So I'm asking for some help from the audience. How can I get a pulse at the turn-off time that I can use as a strobe signal?


I have a hunch that the LM339N quad comparator chip might be very useful in this application. 

[conradelektro]

A request for more help:

The 4017/555 system fires a very short pulse to the blue LED just at the moment of coil turn-on. It would be _awesome_ to have another strobe LED -- say a red or green one -- that fires just when the coil turns _off_. 

How can I get a pulse at the turn-off time that I can use as a strobe signal?

Just an idea:

You take a second "4017/555 system" (a copy of the one you have in place), but you put a NOT gate (4069 hex inverter, just one inverter) in front of it.

You connect the input to the NOT gate at the same point in the circuit where the first "4017/555 system" is connected (there is an arrow in the circuit with the text "TO 4017 PIN 14").

So, the first "4017/555 system" fires (or better said "clocks") whenever the coil (IRFP360) is switched on, and the second "4017/555 system" (with the NOT in front) fires (or better said "clocks") whenever the coil (IRFP360) is switched off.

Greetings, Conrad

[TinselKoala]

Hmmm...thanks, that would work I think. I'll see if I can rig it up. I have another 4017 chip, I think. And to save space a 556 dual timer could be used.

Can you think of a way to use the second op-amp in the TL082 instead of the NOT gate?

[TinselKoala]

Haven't figured it out yet?   

Simply use the second op-amp but with the inputs reversed. The JFET input stages can be paralleled with no problem at all.
The first comparator flips state to HI when the input rises above the setpoint and turns the coil on, and goes LO when the coil goes off. The second comparator behaves in reverse, it goes HI when the input falls below the setpoint and the coil goes off.

So all that's needed is three little wires to hook up the second comparator in the TL082, and another 4017/555 combo, which could use the second half of a 556 dual timer, probably. Or one can simply swap the input connection to the one 4017/555 over to the second comparator output, as I am doing at the moment.

[MileHigh]

Hey TK:

I am really tired today so all I can offer is a Google search that looks pretty good, "555 falling edge trigger."  You might find some good circuits that demand as input a brief negative going pulse.  That might no be too compatible with what the comparator output is doing in the sense that it stays low for a "long" time.  However, to get around that you just need to put a small cap between the comparator output and the 555 circuit that expects a brief negative going pulse to trigger the monostable.  So the cap will couple the falling edge to the 555 trigger input.

Also, you put a pull-up resistor on the 555 trigger input.  (You may also want to put a diode connected towards +12 volts to snub out any over-voltage on the 555 trigger input when the comparator output transitions from low to high.)

So when the comparator output goes from low to high, the capacitor has roughly +12 volts on both sides and it's not charged.

Then when the comparator output goes from high to low, the 555 input sees that high to low transition and that triggers the 555 monostable to flash the second LED.   Then while the comparator output stays low the pull-up resistor charges the cap and the 555 is "satisfied" because it sees a HIGH on the trigger input again - i.e.; the 555 trigger input (possibly) only wants to see a very brief negative pulse that is much shorter in duration than the monostable output that flashes the LED.

Typically it's a very tiny capacitor that will do the trick.  Perhaps a 0.1 uF cap.  Then you might want the falling edge trigger to be low only for a few hundreds of microseconds.  So you choose an appropriate pull-up resistor using the RC time constant.

It's a "quasi kluge" that should work just fine, and all you need to implement it is a negative edge triggered 555 monostable circuit.

MileHigh