Electronic – Lighting metal halide lamp

An LED solution is almost certainly going to suit you better than using filament lamps. LEDs take somewhat more effort to use than lamps in this application - but you will always be happy with the result once you forget what it took to implement.

BUT - lamps first

• Digikey sell 28V and other lamps

• Including lamps from Chicago miniature lighting.
  This spec sheet from CML shows 7 x 28 V lamps in T-3 3/4 wedge base design with currents of 70 MA to 200 mA. That's only one Digikey inspired example. Many more.

• You could use 24 V automotive bulbs with a small series resistor (R~= 4/I ohms)

• You could use 2 x 12V automotive bulbs in series plus a small R (perhaps more available).

• You could use 1 x 12V bulb with larger R. (R~= 16/I. Power rating or resistor = 2 x P bulb.)

BUT

LEDS:

As you have MCU controlled relays, operating the LEDs from the MCU supply or similar low voltage supply and driving them from the MCU drive to the relays makes most sense.

Power at 28V and 0.2A = V x I = 28 * .2 = 5.6 Watts.
A 5 Watt lamp is about car tail light bright.
If you used 5 Watts for a LED you could utterly dazzle your viewers.
Far less power will be adequate.

I'd suggest that even 1 Watt would be very adequate for an LED (lights up a small room) and even much much less would probably suffice. If you wished you could use several LEDs in series, which has some advantages.

If you did need to operate the LEDs from 28 VAC, starting with a resistively driven LED, 50 mA would give you "lots of light". Something like a 270 ohm resistor and series diode and probably a capacitor at the LED would drive this. R value needs to be checked due to half wave AC effects on average current, For an LED an NSPWR70CSS-K1 from Nichia will produce approaching 20 lumen at this drive level.

Ideally you'd use a buck converter to reduce the voltage from 28 VAC. LED driver boards that will drive up to eg 350 mA LEDs can be had on ebay and elsewhere for under (or well under) $5. Deal Extreme sells a number of potentially suitable parts.

Example only - ebay 3 Watt LED driver, up to 24V in - use a series R and diode and cap.

$1.94 each.

Operation from eg 5 Volts requires an eg ULN2803 (costs a few dollars US) to drive up to 8 LED drivers plus a resistor per LED. ULN2803 input can be driven by microcontroller directly OR from 28 VAC using 1 diode, 1 resistor and 1 small capacitor.

Indeed, you need to at the very least replace the circuit breaker to some sort of signal instead, as otherwise you wont be able to power the lamp. If you were to send the signal as a brief lack of power, you need some to make some bi-stable circuit in the lamp to do the switching.

I would recommend using a micro-controller and bi-stable relay. I would power the µC from a diode bridge and a capacitor, and use the EEPROM to remember the state of the device, and use the brownout/power reset to trigger a switch, i.e. to actuate the relay.

Now any simple push-button in a series chain (i.e. both your wall button and cable button) will trigger a toggle.

I hope you have space for a relay in your lamp-side circuit, as switching mains AC in any other way will be much more complicated...