I'm trying to tap into my car's brake circuit to switch between receiving a wireless camera signal and powering my device. For this purpose, I connected a relay to the circuit via a fuse tap. It should connect the phone to the receiver (displaying the camera feed) when the brakes are applied and charge the phone otherwise. Unfortunately, I'm having some difficulty figuring out how to get the relay to switch properly.
I wired the relay to the circuit in the fuse box that powers the car's instrument lights and the brake lights. The current draw for the instrument lights (which are always on) is 0.7 amps. The brake lights draw 1.15 amps. Without adding additional resistance between the car battery and relay the constant 0.7 amp draw will always keep the relay in the ON position (connecting phone and receiver). Hence, the need for additional resistor between battery and relay. The relay's coil draw is .5 amps.
Here is my most likely flawed resistor calculation. Please point out any errors.
First I calculate the internal circuit resistance
V=IxR 12 = .7xR R = 17.143 ohms // with the brake lights off 12 = 1.15xR R = 10.169 // with brake lights on
To keep relay in OFF position, I need to supply a current less than the relay's coil draw (.5 amps). Adding an 8 ohm resistor should do the trick:
R = 17.143 + 8 = 25.143 //brake lights off I = V/R I = 12/25.143 I = .477 < 0.5 // relay switch in off position R = 10.169 + 8 = 18.169 // brake lights on I = 12/18.169 I = .66 > 0.5 // relay switch in on position
It seems like 8 ohms is enough to keep switch OFF with 0.7 amp draw, but my relay always stays in the ON position, even after I increase resistance to 16 ohms. So what gives?