Taping a cardboard support under the soldering fixture helped hold all the parts in place:

Astable – 2N7000 soldering

The struts fit neatly into an NP-BX1 battery holder and the circuit began blinking merrily:

Astable – 2N7000 assembled

My photography hand is weak …

The circuit schematic / layout resembles this:

LED Schematic – MOSFET transistors

The missing 1 MΩ resistor at the LED would serve as a physical support to tether the loose end of the 100 (-ish) Ω resistor, which desperately needed some stabilization under the LED spider.

The simulation says it should blink about every 4s:

Astable – 2N7000 buffered – true model

The 2N7000 MOSFETs use a SPICE model from the Motorola ON Semi downloads, although they behaved about the same way using the LTSpice 2N7002 model.

It really does blink every 4s:

Astable – 2N7000 overview

The LED pulse width should be about 50 ms:

Astable – 2N7000 buffered – LED current – true model

The voltage at the bottom of the ballast resistor is directly proportional to the LED current:

Astable – 2N7000 pulse detail

So the pulse is actually 80-ish ms, which is Close Enough™ for my purposes.

The key advantage here is making both the astable’s period and the blink’s duration (roughly) proportional to the component values, so I can tweak them with some confidence the results will come out more-or-less right.

I love it when a plan comes together!

Some poking around revealed an astable multivibrator using now-obsolescent ZVNL110A MOSFET transistors. The key idea seems to be large gate resistors putting the DC operating point exactly at the voltage required to hold each transistor in the linear region, pretty much guaranteeing the astable will eventually start up.

A bit of simulation suggests this variation ought to work:

Astable – 2N7000 buffered

Well, after the kickstarter in the lower left shorts the transistor for millisecond to enforce some asymmetry, whereoupon the simulation ticks along just fine.

The yellow trace shows the voltage across C2 ramping back and forth between ±1.3 V, with a period just over 4 s and almost exactly a 50% duty cycle: much better than the bipolar version, with sensible component values. As before, the cap sees both polarities, so an electrolytic cap isn’t appropriate.

The red trace is the drain voltage at M2 (presumably, “M for MOSFET”, rather than a plebeian “Q” or “T”), which is firmly at 0 V when it’s ON and ramps upward as R4 pulls C1 higher to turn it even more firmly OFF.

The green trace shows the LED current pulse when M2 turns ON at the end of each cycle. Rather than contort the astable into a very low duty cycle, I generate the pulse by dumping current through a smallish cap into the gate of M4. A few tens of milliseconds makes a perfectly serviceable blink and keeps the average current drain down around a milliamp or so.

In between, M3 buffers the astable’s output to deliver enough current to C4. Without the buffer, the cap draws enough current to mess with the oscillations; that’s how I got backed into this corner.

Figuring the LED at 20 mA for 50 ms, the astable at 10 µA, and the buffer at half of 40 µA, the average current of 1 mA comes entirely from the LED, so even a weak lithium camera battery should last a good long while.

If the low average drain ekes 1000 mA·h from the battery, the LED should blink for a month or two before the battery shuts down.