Voltage sensitive Mirror switch

[Whisper]

I need to turn on power to a 200 ma DC auto mirror/compass when the line voltage reaches 14V DC and turn it off completely when the line drops below 13V DC. I am looking for the simplest circuit to do this that wont drain the auto battery much.
I'm adding a 12v mirror with compass to my Prius. There is a constant 12V (Nominal) source in the dome light that I can easily access but I want to turn off the mirror when the car is not running.
I have measured the "constant On" line to the dome light at 14-14.3Vdc when car is running and 12.1-12.9Vdc with the car off- and this difference is what I'd like to use to energize the mirror/compass.
I know I can snake a new "Ignition on" lead up to the mirror but this is a nightmare to do in the Prius because of the way the trim is designed.
Any ideas will be appreciated.

 

[picbits]

Have a look for something called a window comparator.

Relatively easy to do with a couple of $ (£) worth of components

 

[Whisper]

The window comparator is a good idea- I found the LTC 1042 that seems to
fit the bill. Thanks for the suggestion.
I'm going to try to make something work with a zener and an op amp
that I have on hand first.

 

[Roff]

You don't need a window comparator. You're on the right track with the zener and op amp (a real comparator like an LM393 is generally easier to use).

 

[Whisper]

I found a LM 393 (quad Comparator) in my parts bin and also an 8.4 V Zener diode and a 100K variable pot. Hooking the ends of the pot from +V to ground and the center tap to the neg input (IC pin 4), and a 10K resistor from +V to IC pin 5 (Pos Input) and the zener from there to ground gives me the switching I need on IC pin 2 (Output) with enough power to drive a reed relay.
The pot allows fine tuning the turn on voltage with plenty of leeway on each side.
Now I have to make sure the Prius +V doesn't sink below 12.5V when the engine shuts down while driving. If it does, I'll have to add a time delay to the turn off.
Thanks to Picbits and Roff for the inputs- it was what I needed to start my somewhat rusty thinking engine.

 

[Roff]

Why are you doing this, and how are you going to control the mirror? Are you going to use a relay?

 

[Diver300]

Where does the 12V come from on the Prius?

The main battery on a hybrid vehicle is at a much higher voltage than 12 V. The 12 V systems are fitted to allow standard 12 V accessories without the manufacturer having to design the whole lot from scratch.

There does not need to be a 12 V battery at all. There could be a regulator from the main battery to run the 12 V systems even when the ignition is off. If so, the battery voltage may not change when the engine starts. There is also the added fun that the engine stops and starts on a Prius, so if the battery voltage does go up and down as the engine starts and stops, your circuit will switch on and off while you are driving.

 

[Whisper]

To Roff- Yes-I will use a reed relay driven by the comparator circuit to power the 200MA compass and mirror.
To Diver 300- This is all from the regular 12v battery circuit. The dome light doesn't connect to the 277V battery. I have been driving around with a DVM connected to the dome light +V. The lowest voltage I've seen is 12.91 while driving- with engine running or not*. The highest I've seen with the car totally shut off is 12.52V. My circuit will turn the mirror on at 12.8V and off at 12.6V

*you're right- the engine shuts off frequently as I'm driving around but the voltage doesn't seem to go below 12.9. But even if it did do so briefly it would only cut the compass power briefly and it would come back again at 12.8V