Warning signal from towed vehicle

Hi
I am towing a Ford Expedition behind a Motorhome. I have installed a braking device in the Expedition that is supposed to depresses the brake pedal when the brakes are applied in the motorhome. There is also a break away switch that will cause the device to apply the brakes in the Expedition if the switch is closed. I am running a wire to the cab of the motorhome that will have 12 volts only when the brake of the Expedition are applied. I will also have another wire from the brake switch in the motorhome that will have 12 volts only when the motorhome brakes are applied. If I have a malfunction, where either the brakes of the motorhome are on without brakes in the Expedition or vice-versa, I will have corresponding power in one wire and not the other. If there is no malfunction, either both wires will have power or neither wire will have power.

So here is the question: If I have a malfunction (i.e., power in only one of two wires), I would like a warning buzzer to sound. How do I do that?

I have very little experience in electronics, so I apologize if this is a trivial problem.

Thanks in advance for any help you can offer.

Best regards,
Charlie

Put a normal automotive relay between the two wires, and use the contacts to operate a buzzer.

Here is an explanation of relays:- https://www.americanautowire.com/relay-basics/

You should connect pins 85 of the relay to one of the wires you want to monitor, and pin 86 to the other one.

Connect an ignition supply (via a fuse) to pin 30 of the relay. Connect pin 87 of the relay to the +ve of a buzzer, and the -ve of the buzzer to ground.

You might get a short beep from the buzzer when you apply or release the brakes.

You might also get warnings a lot of the time. A year or so ago I was following a motorhome which was towing a car, and the brake lights on the two did not come on at the same time, although they were both on during moderate or harder braking, and both off when there was clearly no braking. I assume that the car's brakes were on an overrun (surge) brakes, and the car's brake lights were still operated by it's brake pedal, with no connection to the brake lights of the motorhome. If your rig behaves like the one I saw, you're going to hear the buzzer a lot.

If you use a low current buzzer, you may be able to connect it directly between the two bulb wires.

If the buzzer has plus and minus polarity, then you would need to connect a small bridge rectifier between the wires (AC input), with the buzzer connected to the bridge output terminals.

Diver300 and crutschow, thank you very much for your help. Diver300, I am hoping you can explain to me how your single relay scenario works. In particular, with one trigger connected to the ground pin (85) and the other connected to the trigger pin (86), there doesn't appear to me to be a ground to complete the circuits for either trigger.

Also, once you put me in the direction of relay's, I did some more digging and came across the concept of XOR circuits. Besides the single relay set up you described, there is also the idea of using two relays with crossed connections. Is there a benefit of one design over the other aside from cost and space to mount two vs one relay?

Again, thank you very much for your help.

CTaylor said:

Diver300, I am hoping you can explain to me how your single relay scenario works. In particular, with one trigger connected to the ground pin (85) and the other connected to the trigger pin (86), there doesn't appear to me to be a ground to complete the circuits for either trigger.

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I had assumed that the brake lights on the vehicle and trailer would provide the ground path for the relay. That would probably only work with incandescent bulbs.

Using two relays would make a system like that work with no need for the vehicle or trailer lights to be there at all, but if they were, it wouldn't make any difference.

You would need change-over relays, so they are 5 terminal relays with connections 30, 85, 86, 87 and 87a.

Wire connection 85 to ground on both relays. Wire on trigger input to connection 86 on one relay, and the other trigger input to connection 86 on the other relay. Wire connection 87 on the first relay to connection 87a on the second relay, and 87a on the first relay to 87 on the second relay. Wire power to connection 30 on one relay. Wire connection 30 on the other relay to +ve on the buzzer. Wire -ve on the buzzer to ground.

A sonalert and a full-wave bridge would be equivalent to the systems proposed so far.

Seems like a useful alarm system should also detect that the towed vehicle has broken away from the towing vehicle. If that happens, all of the systems proposed so far fail to detect that and generate an alarm.

Two relays would detect a break-away, as one relay would operate but not the other.

One relay, or a sounder with a full bridge, would rely on power or ground from the trailer, so would not work if there is a break-away.