Latching relays circuit

[UKenGB]

It is indeed a clever use of electro-mechanical devices to implement state logic.

However, I'd be concerned on a few fronts:
- voltage division between the two coils, as already mentioned
- momentary impedance of inductor d2 causing corresponding voltage/current sag in d1; if d1 de-energizes (however momentarily), we're back to stage ZERO.
- I wouldn't feel quite right about a switch that requires real power to maintain state - controlling a safety-critical device.

Picture a spontaneous lite's-out as a result of a negative transient...
... induced by, let's say ... panic braking ...
Even with an electronic solution (ie. with improved transient immunity) such a default-off/auto-reset switch may raise safety concerns.

Voltage requirements of relay coil can be allowed for, as explained. I'm not concerned about safety implications. Most modern vehicles have headlights switched by relays that are held on to produce the light and any failure to hold on the relay will cause a 'lights out' problem. Anything can of course fail, but I don't see this as being any more prone to that than current vehicle design practices.

 

[Gaudeamos]

Most modern vehicles have headlights switched by relays that are held on to produce the light and any failure to hold on the relay will cause a 'lights out' problem.

Yes, modern vehicles' headlights are switched through relays, but the headlight on/off state is retained by a mechanical (usually rotary) switch and/or the vehicle's ECU.
Critical electronic control components (like ECU) are protected from failing and resetting due to brownout conditions (like cold cranking).
A momentary relay failure due to brownout may cause the headlights to turn off momentarily, but they'll recover to their 'on' state once the brownout has passed.
That's not the case with the all-relay circuit we're discussing.

In any event, I hope the flip flop module that you intend to use will fit your needs.
cheers