Archive for November, 2010

Halloween 2010

My vision for an electronically-enhanced Halloween was something like this:

It is a dark and stormy night. The air is filled with the sounds of children…screaming in terror at the evils of the night, and oohing in delight at the candies received.Little Timmy approaches the next house…it looks plain enough, with just a couple of styrofoam tombstones in a corner. Unfazed, he continues approaching the door. As soon as he steps up on the front porch, lightning flashes almost blind him, and thunder claps almost deafen him. Startled, Timmy makes another step. A soon as the thunder dies down, a ghost rises from behind some gravestones, and a horrible screaming begins. Timmy, with tears streaming down his face, can’t tell if the screaming is coming from the ghost or from himself…

So yeah, that’s what I was going for. Instead, I only had time to get the ghost to rise up from the grave, and I had to trigger that manually. I did manage to make a couple kids cry, though. It was a qualified success.

The Trigger


Modifying a camera flash – part 1


A camera flash uses high voltage, and a large capacitor. Even if the camera has been sitting around for a “long time”, the capacitor can still hold enough juice to give a good shock. In order to avoid this, be sure to fully discharge the capacitor by shorting the 2 terminals with a screwdriver or something. (But not your nice screwdriver…the arc can actually pit the metal)


I was about to buy a disposable camera just to dismantle it, but it seemed a waste of $10 (or whatever it cost). I remembered a couple people mentioning that sometimes you could get them for free from photo labs, so I figured I’d give that a shot first.

So I went to my local grocery store’s photo department, and asking if they had any “spent” disposable cameras. I felt a little foolish asking for what was essentially trash, so I described how I wanted the circuitry from them. I don’t really think it mattered, though. The guy behind the counter just brought the bin out for me and told me to have at it. Awesome!

I ended up with 2 Kodaks and 1 Fuji.


Motor Controller

Design Goals

  • Variable speed using PWM
  • “Safe” circuit with no chance of a short circuit
  • minimal pin count


I don’t think I mention it below, but I chose the particular model of NFET because it was the same one that was used in the original drill that I pulled the motor from.  The NFET there also had a decent sized heatsink attached to it, so that could have been another source of problems.

I also considered using a TVS diode to suppress the motor surge.  Again, I have not tried this to see if it would have made any difference.

I found out much later that most of my problems with the solid-state solution were probably because of the way that I was driving the NFET gates.  A proper gate driver will be able to provide the necessary voltage to the high-side gates, will provide the necessary current to ensure fast switching, and will add a delay to eliminate any shoot-through current (where both NFETs on one side are active at the same time, creating a short circuit).

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