Is this really off topic? It's really interesting anyway. Hmm. The capacitor charges through the resistor until the neon strikes. Since the neon now draws more current than comes through the resistor, the capacitor discharges through the neon until the neon extinguishes. Process repeats. Neat. I suppose that if the resistor is too small, the neon doesn't extinguish, but the capacitor merely discharges until it reaches the steady state voltage of the neon at the current through the resistor. I like it! Flip flop should be easy too. This one looks rather like an ECL logic gate: +V DC | < < | +--+--+ | | (:) (:) | | A--+ +--B | | < < < < | | GND GND Pull A to ground. Lefthand neon lights. Voltage on right hand neon too low to maintain discharge and it goes out. Same works for B - RH neon lights and voltage at the common anode too low to maintain LH neon, which goes out. Don't know the component values though. Outputs are also A and B. You may need pull-up resistors on them, depending on what you're driving. Fun. Keep neons in light-tight containers, though! Are there circuits in which optically coupled neons are useful? Philip.