I just finished building this tiny rocket altimeter.  It fits inside a standard 18mm (diameter) rocket motor case – in this case, a used Estes C6-7 motor.

It’s based on Robert DeHate’s PICO-AD4 altimeter.  It measures peak altitude and will fire the ejection system at either apogee or a main deployment point (a few hundred feet AGL).

The most difficult thing was finding an adequate battery that would fit – I ended up with a L1016 6v alkaline; they are widely available (online) and inexpensive.   It uses a 220 uF capacitor to fire a AG1 flashbulb for deployment (220 uF is enough to do this reliably).

Here’s the schematic – not much to it:

I drew a power switch, but I couldn’t get it to fit.  So I just take out the battery to turn it off.

I made the hole for the battery with a Dremel tool.

Here’s the back side:

The jumper on the back (J1) selects whether the ejection will happen at apogee or using the main deployment point.

The two bolts/nuts on the end are the attachment for the AG1 bulb (2-56 hardware), the battery clips were made from parts in my junk box.  The negative terminal is soldered onto two cut-off pieces of paperclip (paperclips are made out of good-quality steel), which are in turn soldered onto the perfboard.

(By the way, I’ve discovered the perfect tool for cutting PCB traces – a little battery-powered engraver. Harbor Freight sells the one I use for less than $10.  Just make sure you blow away all the tiny copper pieces afterward with canned air.)

The resistor is just to limit the charging current on the capacitor – that little battery really doesn’t like high currents.  The resistor value is not critical – anything from 500 to 3000 ohms or so should be fine.

Here’s a top view (with it running and the LED blinking):

The 220 uF cap is held in place with some hot glue (as are some of the wire-wrap wires on the back).  I used epoxy to hold down the jumper, screws and base nuts and to reinforce the thin bits of perfboard next to the battery – otherwise the whole thing bends too much under the off-axis tension from the battery springs (I broke an earlier version that way).

Here it is snug in it’s 18mm motor case:

The flashbulb wires are meant to go out the other end, thru the rocket nozzle hole.

As you can see, I had to ream out the inside of the motor case a bit to get things to fit.  (This was desirable anyway, to get out all the black soot left after firing – do NOT try this with a unused motor!!)

The reaming was done starting with a 1/2″ drill bit, then a 9/16″, finishing with sandpaper wrapped around the 9/16″ bit (use some double-stick tape to keep it on there).  It helps a lot to have a drill press.

I haven’t flown it yet, but it works well on the workbench.