Voltage Monitor

Hello, I may not be posting in the right forum but I was wondering if there is a way I can deactivate a relay if the voltage goes down too low in my electrical system... I am installing the relays that leave the radio on until the door is opened and I was just wanting a way to kill the radio if the battery starts to get too low... Thank you! -Jason

Why don't you consider using a 15 minute timer relay?  Basically if the door is not opened within 15 mintues then the relay will automatically shut off.  Thats how most OEM RAP systems work.  Seems like it would be pretty easy to do it that way.Kevin Pierson

DIdnt really think about that..its been a long week...Thanks

By the way, which relay would you recommend and where can I get one? Thanks! -Jason

Just curious, why do you want to do this? Is someone going to be sitting inside listening to the radio during this time? Maybe put a meter on it and just keep an eye out manually, if you're worried about drawing the battery down too much. Or, you could use a voltage comparator to switch a relay off when the voltage drops below whatever threshold you select. Something like this comparator for $1.04 at mouser.com, TLC3702IP You would just set up one of the inputs with a reference voltage, and the other input with your battery voltage, and when it drops under the reference point, the comparator output will trip, and you could use it to toggle your relay off.

Using a voltage comparator wouldn't be very simple in the scenario, because of the lack of a regulated power supply. You would have to incorporate a 5vdc power supply to be your reference voltage, then you would have to scale your battery voltage through a voltage divider or something along those lines. You will have a VERY hard time using your battery voltage to create a reference voltage and a setpoint voltage. Another option would be an analog to digital converter. It will convert the battery voltage in to series of bits, and if you play your cards right you can get away with only monitoring one bit. The downside to that is you will still need quite a bit of circuitry to make it work correctly. There are a wide range of companies that make timer relays. You'll just have to look around and see if you can find one that will adjust all the way to 15 minutes. Keep in mind that RAP systems were not designed to allow the occupants of the car to listen to the radio for extend periods of time, but only to allow you to roll your windows up, shut the sunroof, and have some tunes while doing it!Kevin Pierson

You could use a zener diode to generate a reference voltage; they're an inexpensive way to get a stable reference. Just add a few resistors and a trimpot to set the threshold wherever you want.