Houston, we have ignition. Mazda Miata

The NA and NB Mazda Miata are fine examples of KISS (Keep It Simple Stupid) engineering. One corner I feel was a cut too far is the omission of an ignition key light. You just can't see where the key goes on a dark night or in an underground car park. Given the simplicity of this creature comfort, it doesn't seem like it should be too much to ask. A quick hunt on the excellent www.miata.net forum turned up a Brian Weltman's circuit and instructions.

Circuit

I happened to have equivalents to everything in Brian's Radio Shack bill of materials on hand. I ran into a little trouble however, with his design. The feed to the relay coil (the red/black) comes from a 12v source in the ignition circuit. For whatever reason, the reading in my car from that source to ground was closer to 14.5v. Given the heavy gauge of the red/blk wire (approximately 12AWG) I suspect that it is carrying a fair amount of current. The over voltage, coupled, I presume, with the high current, burnt out the relay coil on the first try. No relay coil means the LED wont turn off immediately when you start ignition. 

Replacing the relay with one that has a higher coil voltage would be one way of resolving this. However, I didn't have one on hand. Also, while I am not an electrical engineer, I am not convinced that it is a good idea to leave what amounts to a direct short via the coil to ground open while you have the ignition on. I therefore added an LM317 voltage regulator to the coil side of the circuit. This serves the dual purpose of dropping the voltage the coil sees down to 11.5v and limiting the current that passes through the circuit to ground.

The red/blk wire from the ignition feeds the V_in side of the circuit. V_out is sent to one side of the coil. Values used for components were:

C_in = 10uF (rated to 25v)

R1 = 270 ohm

R2 = 2.2K ohm

C_out = 1uF (16v)

Given the very small current requirement of the coil and the simplicity of the circuit, I suspect that the input and output filter capacitors are probably not necessary, but we are going with belt and suspenders today.

  Finished circuit.

As I had to cut the wires to fix the relay problem I added molex connectors so that the whole shebang can be removed or replaced.

EDIT - 2017 2 3

Complete parts list:

2 x 2.2k ohm resistor

1 x 275 ohm resistor

1 x 120 ohm resistor

1 x white led

1 x 1N4004 diode

1 x 1uf capacitor

1 x 0.1uf capacitor

1 x 4700uf capacitor

1 x LM317 regulator

1 x PRMA1A12 relay

1 x proto board

6 x 18" wire

shrink wrap

tie wraps

wire taps optional

Relay pinout

LM317 circuit and pinout



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Wire connection points

I can confirm that the wires are indeed exceeding awkward to get to without removing the driver's seat, but, by lying on your back on the seat with your head in the pedals it can be done. The light gauge (approximately 22AWG) blue/red & white/red wires that feed the dome light in my 2000 NB were aft, above and to the left of the dead pedal. They were held by a clip in the door frame above the hood release catch. [From what I understand, there is no dome light on earlier models so you will have to find another source of 12v that is activated by the door switch.]

The arrow pointing up indicates the hood release cable.

The arrow pointing left is where I attached my splice connectors.

Note - my NB has an (annoying) alarm/immobilizer that was previously spliced into the same place (the brown wire).

The heavier gauge (~12AWG) red with black stripe wire follows the steering column. It is immediately visible on the left side of the column close to the steering wheel once the plastic clam shell cover and the rectangular piece below it are removed as per Brian's instructions. 

Installation 

I ran into a further minor complication here as the finished circuit board is slightly too large to fit against the other components near the ignition in the position I had intended. I just flipped it around and tie-wrapped it to the wiring harness a little below the clam shell.