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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #11  
Old 04-14-2008
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Dunno, John. If it is just a "simple" circuit, there's an oil pressure switch. But if it is hooked up with a pressure gauge and a sender, that's a variable resistor. Given that and the two unknowns (the bulb and buzzer)...the relay allows it to be treated with blissful ignorance.

In either case--if the bulb has near-zero resistance, as most panel bulbs do, it will act exactly the same way as a plain wire, effectively shorting out the terminals of the buzzer that is in parallel with it. That dead short ensures that the buzzer will never work (until the bulb burns out) because there is near-zero current available to it, regardless of the voltage. No?
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Old 04-14-2008
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Quote:
Originally Posted by hellosailor View Post
Dunno, John. If it is just a "simple" circuit, there's an oil pressure switch. But if it is hooked up with a pressure gauge and a sender, that's a variable resistor. Given that and the two unknowns (the bulb and buzzer)...the relay allows it to be treated with blissful ignorance.

In either case--if the bulb has near-zero resistance, as most panel bulbs do, it will act exactly the same way as a plain wire, effectively shorting out the terminals of the buzzer that is in parallel with it. That dead short ensures that the buzzer will never work (until the bulb burns out) because there is near-zero current available to it, regardless of the voltage. No?
He has a light, which means he doesn't have (better not have) a variable resistor type of sender. With oil pressure it should be open (infinite reistance) and without pressure it should be a dead short to ground.

Like I said before, if the light bulb remains at or near zero resistance and acted like a plain wire, the fuse would blow. At .5 ohms and 12 volts you'd have a 24 amp 288 watt warning light.

The bulb only has near zero resistance when unlit. The whole inductive resistance thing is difficult to explain, and even harder to grasp. The concept is the same as with an electromagnet. If you take plain wire with a fuse in line, and touch one end to power and the other to ground the fuse will blow. Take the same wire and wind it tightly into a coil, and you get an electromagnet and the fuse doesn't blow. That's because in a wound wire you get a certain amount of inductive resistance when current flows. An ohm meter doesn't supply enough current to cause this, so you read whatever resistance the wire has if it were straight.

When dealing with inductors and light bulbs, you have to look at the power (watts) output. In this case we would have about 3 watts. If the light bulb is wired in series, the total amount of power available for the whole circuit is 3 watts. Enough for the bulb, but nothing else. A friend of mine showed me a great example of this many years ago. He was a part time TV repair man. When working on a TV that pops the fuse in the power supply, he would plug the TV into a special outlet that he made with a light bulb socket in series. He could limit the total power consumption of the TV with the wattage of bulb he put in the socket. A 100 watt bulb would power a 25 inch TV enough to get a picture about half the normal size right in the middle of the screen. It didn't have enough power to stretch the picture all the way to the edge. Same thing in here, except the wattage available is so low, the buzzer does nothing.
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At the risk of sounding like the Old Man of Loch Ness.... been there done that.
You need a relay there my friend.
The internal resistance of the switch and bulb is limiting the current of the buzzer so that it is choked off when the buzzer load is needed. You simply have too much in-line resistance and there is not enough current in series to sound the buzzer.

This circuit works for me...

Image of oil pressure switch alarm with relay - Photobucket - Video and Image Hosting

... and has done for 15 years.

The oil pressure switch is from an old Mazda, the relay is from an old BMW, and the horn from an old Austin.

When the oil pressure is low (or zero), the oil pressure switch completes the circuit to engine ground. There is an audible click from the relay (if you have yor ear to it), then you are deafened as the relay calls in the horn.

Crucially, the circuit from ignition switch to horn is designed to take the full load of the horn.... the other wee circuit cannot, so it is used as a trigger only.... it only has to carry a wee current to close the relay, and it is capable of that.

Total cost, about $5, in 1992.


Rockter.
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Oh, and I added a wee switch between oil pressure switch and relay to kill the alarm if I have my ears in the engine room and I have to shut off the motor at the motor kill lever.
If I did not, I would get deafened every time I shut down the motor at the engine, or I would have to run up on deck quickly to shut off the ignition.
It is not absolutely necessary, but it is desirable.
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Old 04-14-2008
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Quote:
Originally Posted by sapadeni View Post
I'd be happy to use a "squealing pig alarm" if I can get it to work. Do you know how they wired it?
I believe it was in line with the ignition key, so it would go "eeeeEEEKKK" at start for a second or two and then stop. If it went on again, you were below 10 PSI or something along those lines.

Check out entry #4 in this thread. The poster is a friend of mine and one clever fellow:

Checklist for a troublefree spring startup - Moyer Marine Atomic 4 Community

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Old 04-15-2008
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Wow. I would not have thought of using a car horn for a buzzer. In that case, by all means use a relay.
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Old 04-15-2008
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Why would you use a car horn, when you can get one of these instead? McMaster-Carr Put part no. 56965T13 in the search box on the left. I couldn't get the link to work.

At only $7.06 and it draws 0.015 amps, any oil switch in the world will operate it with no relay required. A 90db tone should be able to be heard easily even with the engine running. If that's not loud enough you can move up to a backup horn and get to 107db, but the price is high.

Unlike a car horn, you can easily mount one of these to the back of a panel, or could easily be hidden anywhere you like. At such low cost, low amperage and small size, you could mount a couple of them, one inside and one outside.
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Old 04-15-2008
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US..
Having suffered the chagrin of twice pumping the sump dry, and that wee light, unseen glowing in the daylight, I was trying to make sure that I would hear the next one.
It was raining, I was in a wee villiage called Ardrishaig in West Scotland, and there was a scrapyard there.
If you are content with the 0.015 amp buzzer, then go for it.
The good point about the relay is that it does not have a current limitation on it, this side of sanity, anyway..... you can call in horn and light if you wish. I added a pressure guage also, below decks.
There is no way that you cannot hear that horn.
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Wow, I'll work on digesting the information you guys provided. As I see it, I'm looking at several options:

1. Try reversing the polarity of the buzzer in parallel with the panel light. (I originally wired it with the buzzer positive to the low oil switch, neg to the black leading to the bulb).

2. Add a relay in place of the present light and tap the light and buzzer off the relay.

3. Run the pos of the buzzer to the ignition "on" wire, then the neg of the buzzer to the lead running from the switch.

Did I miss anything? I'll try these options out this weekend. Thanks a million for taking the time to help me out. It's definitely been informative. I'll let you know how I make out.

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Old 04-15-2008
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Quote:
Originally Posted by Rockter View Post
US..
Having suffered the chagrin of twice pumping the sump dry, and that wee light, unseen glowing in the daylight, I was trying to make sure that I would hear the next one.
It was raining, I was in a wee villiage called Ardrishaig in West Scotland, and there was a scrapyard there.
If you are content with the 0.015 amp buzzer, then go for it.
The good point about the relay is that it does not have a current limitation on it, this side of sanity, anyway..... you can call in horn and light if you wish. I added a pressure guage also, below decks.
There is no way that you cannot hear that horn.
I actually have one in my auto shop attached to my alarm system. It works as a door buzzer so that I know when someone comes in the front door. I can hear it 80 feet across my shop with the air compressor and air tools running. I would trust it.

BTW this buzzer puts out 90db, the same as a car horn.
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