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On my boat I would like a warning buzzer to sound if the battery charger is on and the engine batteries are not isolated. Because if you start the engine with the battery charger on, the diodes in the engine alternator blow (been there, done that..!)

The unit could be 240 V or 12 V powered.

So on an abstract level the logic would be: IF 240 V to charger is live AND 12 V from engine batteries is live THEN SoundBuzzer.

I can imagine how to do this with two relays, one 240 V, the other 12 V. When both 240 V and 12 V are live the circuit is complete and the buzzer sounds. But I don't like the idea of a 240 V relay being energised for weeks on end while the battery charger is on and the boat is unattended.

I would prefer a neater solid-state solution, Is there one?

ocrdu
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Lance Kneeshaw
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    Put a relay in the wire to the starter solenoid such that if that charger is on then the solenoid cannot be activated. – Solar Mike Jan 22 '22 at 10:33
  • I don't understand your concern about the 240V relay - relays are pretty reliable. If you're concerned about thee relay, why are you not concerned about the charger? It is more likely to fail. I dare say your problem could be solved with relay logic. I would design the system to mititgate the problem AND sound an alarm or give some sort of indication. – Kartman Jan 22 '22 at 10:39
  • @Kartman, the PSU will have a larger MTBF than the mechanical relay but contact burn-out is even more aggro' with mech relays. But OP can avoid all that malarkey by using an SSR anyway, which needs no snubbers and needs lower drive current. Costs more but it's a one-off project and no mech relay is their preference. – TonyM Jan 22 '22 at 10:45
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    You could also go for a simple, failsafe, KISS solution (I own a boat myself and that would be my preference). You could use a SPDT On-Off-On switch that either connects the batteries to the engine or to the charger, but never both at the same time. It would have to be a robust switch that can handle >300A (or whatever is needed to start the engine). – StarCat Jan 22 '22 at 12:31
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    You don't need 2 relays - one will suffice. Used a 240V buzzer, and switch it using a 12V relay. – brhans Jan 22 '22 at 13:52
  • Looks like something in your boat is profoundly miswired or badly engineered. – fraxinus Jan 23 '22 at 09:43
  • See answer update with schematic. – AnalogKid Jan 23 '22 at 20:09
  • Thanks all for your help. The DPDT switch idea is elegant, but I don’t want to mess with the aesthetics of the fascia if possible (and would never find a matching switch). It’s possible I was being unnecessarily wary of normal relays! Especially if used so they are normally de-energised. The suggestion of a 240v buzzer is beautifully simple. 240v -> 12v DC relay, NO -> 240v buzzer. When batts on, relay closes, buzzer sounds. On the other hand automatic isolation has the merit of taking the user out of the equation. A 12V DC Coil Non-Latching Relay SPDT, 240v AC, 16A Switching Current. Configur – Lance Kneeshaw Jan 23 '22 at 06:28

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Without knowing more about the various grounds running around, this sounds like a three circuit problem: The AC being sensed, the DC being sensed, and the power source for the beeper.

I would use two opto-isolators, one for the AC input and one for the DC input, with the two output transistors in series with a loud piezo beeper and its supply. No logic chips, no opamps or comparators, no Arduino, and no 555's. <gasp!>

The DC input needs a current limiting resistor, and I would add a small bridge rectifier so the input polarity doesn't matter. The AC input needs a current-limiting capacitor, a rectifier, and a small filter capacitor so there is a constant DC through the opto's input LED.

Power quality on a boat is very poor. A Zener diode in series with each opto input will increase noise immunity.

If the 12 Vdc power source for the beeper is the same as the one being sensed, I'd stay with the optoisolator. It's actually less messy than sensing the 12 V with a transistor or comparator.

I can whip up a simple schematic if you need one, and add it to this response. Or you can take a swing at one and we will advise.

UPDATE - Schematic

Here is a first-pass at a schematic. I came to my senses, and replaced a bunch of semi-dangerous stuff with an off-the-shelf 12 V wall wart. It is a cheap way to get certified isolation, noise filtering, and safety.

A range of component values will work for the resistors and Zener diode. The only real requirement is that the piezo beeper current be less than 20 mA to limit the current through the LED. Without the LED, the piezo current can be higher. The opto-isolator output transistor is rated for 50 mA, but I wouldn't run it at that level continuously.

Note: Both 12 V inputs are polarity-sensitive.

Note: There is no GND symbol in the schematic. I did this on purpose. All three circuits are completely floating with respect to each other.

enter image description here

AnalogKid
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