Electric start gate release
The existing mechanical release requires the starter to press on the release lever in a direction that's sideways to the track. Sometimes this results in the track being pushed sideways slightly, especially given that the gate and release are about four feet off the floor, and the track legs are only 14 inches wide.
I took my cue from *5-J*'s project. Specifically, I used the same solenoid, and use an "elbow catch" to hold the gate closed. The solenoid and catch are connected by music wire.
The assembly body is made from 1" x 1/16 aluminum angle, held together by pop rivets. (I was pleased to find that aluminum angle in this size is quite inexpensive and available at the local Home Depot and Lowe's.) I originally planned on having two mounting "arms" to attach it to the track, but I found having just the one arm was fully adequate. The arm extends across the first two lanes only (note the two extra bolts in the left two lanes), but of course could be longer.
The solenoid actually comes with threaded mounting holes that apparently fit M3 (metric) machine screws. That's a size that seems to be difficult to find, but happened to be included in a collection of bolts I bought from Harbor Freight a long time ago, so by sheer luck I had a few matching bolts. I mounted the solenoid on homemade standoffs, as otherwise the piston travel doesn't align well with the motion of the elbow catch.
On the underside of the track, the leg hinge crowds the slots that the starting pins pass through, leaving barely enough room for the 1-inch mounting arm. I mounted the arm to the track first, and then used the mounted arm to mark out the placement of the release "body".
The elbow catch is engaged by another small "tongue," a piece of aluminum angle attached on top of the gate mechanism. One goal I had was that the manual gate start still remain functional. First, I wanted a "plan B" in case the electric release failed during an actual race; secondly, I wanted to be able to set up the track without electrics during our derby workshop. I drilled counter-sunk holes and used #6-size flat-head screws to mount the plate while leaving a flush underside that wouldn't interfere with the manual release arm. (Some of the pictures here show pan head screws attaching the tongue; the switch to flat-head screws was made relatively late in the project.)
During initial testing, I found that the electric release worked about 95% of the time, but occasionally was unable to provide enough pull to release the gate. The solution I hit on was to add an offset extension to the piece that engages the elbow catch. This serves two purposes: (1) it keeps the "tongue" engaging only at the "nose" of elbow catch, where less force is required to release, rather than all the way in the "mouth," and (2) it prevents the elbow catch from extending the last 1/8" or so forward, which in turn pushes the solenoid piston another 1/8" back, allowing the solenoid to produce more force. The extension is just two small pieces of scrap, attached with epoxy. Since making this modification, the mechanism hasn't failed once.
I added switches and connectors, as shown here. One concern I had is that an over-eager cub scout not be able to release the gate prematurely. Accordingly, I included provision for a second switch (controlled by an adult), with the switches wired in series. The two RCA jacks connect to the two switches. The small toggle switch shorts together the contacts for one of the jacks, to allow for single-button (i.e., adult-only) operation.
Here's the actual hand-held button: a pushbutton from Radio Shack and a piece of 1/2" PVC.
Once the solenoid and latch were working, I added a third piece of angle on the top, and used it to mount the microswitch which starts the race timer. This had previously been attached directly to the start gate, but not particularly well. This installation is a great improvement.
Finally, I attached the little circuit that allows closing the gate without reseting the timer (described here).