General electronics question

[tl45]

Not sure if this would be the best section to post this query as it's not PC related but can't find anywhere else obvious for it?

Am trying to jog my very faded electronics memory on best type of circuit to use for taking 2 voltage inputs then making logical decision on which is the output.

One input is from car instrument lighting dimmer switch, which is a varying form of pwm - the other would be a standard 12V feed. Output will be the 12V DC unless the PWM input is present, then this would over-ride the 12V feed.

It's for cluster lighting in my car where there is 1 set of gauges on the dimmer circuit but not back-lit like the main gauges when headlamps are off.

First thoughts were to use a relay with n/o and n/c contacts triggered off the PWM supply but got a feeling this would pulse the relay rather than switching it over. Maybe converting the pwm to steady state first would cater for this?

Any guidance to a simple solution would be appreciated, once I have some guidance on where to start from I should be ok

 

[Tealc]

Not sure how I'd do it.

Maybe a peak detector circuit or something. Will the pwm always be running and you want to switch over on low duty cycle?

Maybe a smoothing circuit and a comparator to switch on and off the steady 12v.

You could use a microcontroller as well to dial in a trigger level if you didn't want to go analogue.

 

[tl45]

The pwm wouldn't always be present - only when you turn the headlights on.

Some ideas for me to research a bit more there, not sure if a filter could be used either. Shocking my memory here is so bad, considering my qualifications in electronics

Briefly thought could just use the relay as even if it were to pulse the coil one of the outputs would then match the pwm waveform anyhow (more or less) but having it rattling away at high frequency would be far from ideal.