New Weapons Technology

A whole micro to make ONE PULSE? Why not just a swtich and "condenser"?

Have Fun! Rich

... not to mention the 220 Ohm resistor is going to run down that 9V battery pretty quick!

I had a similar thought, Rich, but I suspect he might make a bunch of pulses to ensure ignition... at which point I got to thinking, "well, still... how about a 555?" Realistically, if you look at Luhan's web page, you'll see that he surely has a lot more PICs lying around than 555's and can probably program them in his sleep by now so, hey, why not?

Here is why the PIC.

It generates a nice set of pulses, then it quits, even if you hold the button down. The 220 ohm resistor only conducts while the button is down. The current is a small fraction of the 600 ma the coil draws when operating.

I have not used a 555 in ages....

Luhan

I tried that, you still need a relay to time the pulse and it tended to burn out contacts.

Luhan

Well, once you turn on the main power switch, either the 220 ohm resistor is across the battery, or the FET is ON. So it sure looks to me the resistor is drawing current as long as the power switch is on.

BTW the old Model-T spark coils worked this way-- except instead of a PIC there was a little buzzer on the side to chop up the DC.

Check it out: thats not a 'power switch' - its the (momentary) firing button.

Luhan

Look again: the schematic shows a power SWITCH that turns on the main power. Once that switch is ON, you're drawing 9/220 amps, about 40 milliamps from the battery.

When you push the FIRE button, the 40mA draw goes away, to be replaced by the draw of the spark coil.

I think Rich is alluding to the standard old car spark coil circuit, which is a set of teensy contacts, with a small capacitor across them. These contacts typically last several hundred million ops (let's see: assume 3,000 RPM at 60MPH for 20,000 miles, that's 60 million revs, on a V-8 theres four bangs per rev, so 240 million sparks every 20,000 miles ) before needing replacement. Your finger will probably wear out first.

There is no seperate fire button. The circuit has a single push switch, which turns it on. As soon as it is on, it starts pulsing. It pulses for a short while, and then stops. It only carries on drawing power for as long as you push the button. Given that the pulse draws hundreds of times the quiescent through the 220R resistor, and that as soon as it goes 'bang', you presumably release the button, the resistor loss is minor.

Best Wishes

The relay is needed because the coil 'fires' on the release. This would not be good for a trigger response. The relays I have that fit the project had insufficient contact rating.

Luhan

Marshmallow Gun - Useing 1000V N-Ch FET driving a standard ignition coil.

(for anyone that don't wanna bother to open the page :)

Yep, there will be a big inductive kick. Try putting a capacitor across the contacts, just like in every car made between 1915 and 1970. About 0.1uF IIRC.

How about a combination of ingredients? A swtich that triggers a one-shot, which turns the MOSFET on for the dwell time, then off - that's what the PIC is doing now, right? And a capacitor might help by helping it ring.

Of course, if you've already got the PIC programmed, and it's working, then this is pretty much moot. :-)

Thanks, Rich