Neon lamps aren't just for lighting. They can actually be made to perform a variety of circuit functions. For example, a collection of interesting oscillators can be built using only neon lamps, resistors, and capacitors. No transistors, vacuum tubes, or such devices are required. The blinking neon bulbs in the goggles mentioned in the previous post are one such oscillator.
The GE manual on neon lamps contains a huge collection of neon lamp circuits and the detailed theory behind them. I'll discuss the relevant theory only briefly; for details, see the theory sections of that manual.
Here's a nice simple multivibrator built from a pair of neon lamps. Operation is explained with a simple model for lamp behavior: the lamp, in the off state, requires about 70V to turn on (for the NE2 types I'm using; others are different, and individual lamps vary noticeably). However, once on, the ionized gases improve conduction and the voltage required to maintain the operating current (of about 300uA) is reduced to approximately 55V.
Consider the case where N1 is on and N2 is off. Current flows through R1 and N1, and the junction between R1, N1, and C1 is maintained at 55V (N1's maintaining voltage). Since N2 is off, we know that the junction of C1, R2, and N2 is below 70V (N2's firing voltage), and no current is flowing through N2. Current flows through R2, into C1 (charging it), and through N1 to ground. The voltage on C1 rises toward the supply voltage as C1 charges. Eventually, C1 is sufficiently charged that the voltage on N2 passes its firing voltage and N2 turns on. As it does so, the voltage on its positive electrode rapidly falls to its maintaining voltage of approximately 55V. Since the process is rapid, the voltage across C1 is unchanged, dropping the voltage on N1 from its 55V maintaining voltage down to about 40V, which quenches N1. C1 begins to charge through R1 in the opposite direction, and the cycle repeats. Frequency is controlled by the supply voltage, the resistor and capacitor values, and the firing and maintaining voltages of the two bulbs.
Startup of the oscillator is straightforward: as the supply voltage rises, small component asymmetries cause one bulb to fire first. The first cycle begins with C1 at approximately 0V, and the oscillation described above begins at the second half cycle.
The ring oscillator shown here is only slightly more complicated. As N1 fires, the voltage on both N2 and N3 are reduced, inhibiting firing. C2 and C3 both charge until one of N2 or N3 fires. Assuming N2 is at a higher voltage than N3, it will be just below 70V before it fires, with N1 at 55V and N3 somewhere between the two (say 65V). As N2 fires, the voltage on it falls, and so do the voltages on the other two bulbs, thanks to the capacitors. The relative voltages, however, remain the same. N3 will still be at a higher voltage than N1. While N2 is on, the voltages on N1 and N3 are rising; N3, starting at a higher voltage, will fire before N1. The process will continue in a ring, with each bulb firing in sequence.
Obviously, the circuit can be extended. With five bulbs, operation becomes slightly more complex. Again assume that N1 is on, and the other bulbs off. Currents flow through R2-R5, charging the capacitors. R2's current flows through C1, and R5's through C5, and both flow to ground through N1. The current flowing through R3 and R4 head through C2 and C4, which also charge. Thus, C1 charges faster than C2; the voltage on N3, however, rises faster than voltage on N2, since the voltage on N2 is VM1 (N1's maintaining voltage) plus VC1, while the voltage on N3 is that plus VC2. N4 and N5 have the same relationship. As a result, one of N3 and N4 will fire first. Assuming it is N3, the same conditions apply as before, and the next bulb to fire will be either N1 or N5. However, since N1 was on most recently, the voltage on N5 was higher; it will fire after N3. Next will be N2, then N4, and then N1 again. This star patterned firing order will continue.
Both of the above oscillators are symmetric. They can oscillate in either direction. The determining factor will be component mismatches and startup conditions. In general, small ring oscillators will have a preferred direction, thanks to variations between the resistors and bulbs. Starting them with the capacitors partly charged, though (by briefly removing power to the circuit and then reapplying, for example) can sometimes cause oscillation to reverse.
Extending the circuit to greater numbers of stages will, with precisely matched components, keep the same star shaped pattern as the five-stage ring. However, unmatched components will break this pattern, and as the size of the ring increases the matching required becomes tighter. With slightly mismatched components, the pattern becomes chaotic — it is neither fully random nor fully ordered. With 9 bulbs and 5% tolerance resistors, the order was no longer rigidly preserved in my testing. For the goggles, I decided I liked the chaotic effect. In order to emphasize it, I intentionally varied the component values. I basically just pulled resistors at random out of the 1M-10M drawer. Some of the capacitors are 1uF, and some are 2uF — with all of them at 1uF, the oscillation was faster than I wanted. The resulting effect, where the bulbs fire without discernible pattern or rhythm, is one I like very much.
This is the circuit as constructed in the goggles. The central wiring is one supply rail, and the other supply rail is the loop of wire visible in front of the goggles in the video. In use, the power supply is placed in the left eye such that the neon lamp is in the right spot, the wires are folded up to fit, and it is taped in place with electrical tape. The exposed wiring on the right is also covered in electrical tape. Note that there is no connection between the capacitor lead and the common wires at the bridge of the goggles; that's covered by electrical normally as well.
Construction tips and modifications: Nothing in particular. The circuit works at a range of supply voltages (greater than the highest bulb firing voltage). Supply polarity can be reversed as well. The resistor values must be chosen to keep the bulbs within their rated current limit. If you make the resistors too large, with too little current flowing in the on bulb, it will cease to operate (details can be found in the GE manual). In general, values of a few hundred kOhms to a few MOhms are appropriate. With more stages, you'll need higher valued resistors, since current for the on bulb is flowing through all of them in parallel. Oscillation frequency can be tuned from a few kilohertz all the way down to below 1Hz with appropriate capacitors. The capacitors need to be ceramic or film types, not electrolytics, as they will be charged alternately in either direction. For the goggles, I used BC1162CT-ND 1uF ceramics.
