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Need help with relay circuit

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I have built an electric powered scale model torpedo for my 60" British PT boat. I need to figure out how to use a relay to turn on the torpedo motor as it leaves the launch tube. The perm. magnet is glued to bottom of launch tube. Magnetic reed switch is inside torpedo. I have a magnetic reed switch which can handle 1 amp and want to use it to energize the relay coil to switch on the 6VDC 11 amp draw motor. I have a SPDT PC relay to do the switching. My question is how do I power the coil circuit which uses the magnetic reed switch to turn on the torpedo motor. I fried the first reed switch by wiring it directly in the motor circuit. The motor is a Graupner Speed 300 6VDC little motor (1/2" dia 1" long) and it draws a max of 11 amps. Power comes from 6 AAA alkaline Duracell batteries wired in series. Am I missing something on how the coil is powered. The coil specs are: Nominal 12VDC, Nominal coil current 30mA, Pickup/dropout voltage 9.0/0.6VDC. The torpedo is 1.25" ID so things must be very small. See the boat and a video at: web.me.com/hookpilot. Current pictures of torpedo on my FaceBook page for Michael Jones.
 

BrownOut

Banned
I'm holding my jaw in place as I write this. How on Earth do you get a 6VCD, 11 amp battery in such a small place? Not to mention the realy and switch??? Then you also need 12 VDC for the realy. How do you make those things so small?

If you were to use a reed switch to energize a relay, you would have to make sure the relay coil is connected to the "open" terminal while the switch is near the magnet. I'm not sure if read switches have a normally open terminal. Maybe you can post a pinout of the switch.
 
I'm holding my jaw in place as I write this. How on Earth do you get a 6VCD, 11 amp battery in such a small place? Not to mention the realy and switch??? Then you also need 12 VDC for the realy. How do you make those things so small?

If you were to use a reed switch to energize a relay, you would have to make sure the relay coil is connected to the "open" terminal while the switch is near the magnet. I'm not sure if read switches have a normally open terminal. Maybe you can post a pinout of the switch.

The battery pack is 4 in-series AAA alkaline batteries. the reed switch is very small, the kind used in a house alarm on a door. The reed switch is normally open. The motor specs are maximum numbers. The little relay is 7/8" x 3/4" x 5/8". My question is how do I power the coil part of this? I'm not an electronics expert. I can't use the existing battery pack directly. I tried that and destroyed the reed switch.
 
I'm holding my jaw in place as I write this. How on Earth do you get a 6VCD, 11 amp battery in such a small place? Not to mention the realy and switch??? Then you also need 12 VDC for the realy. How do you make those things so small?

If you were to use a reed switch to energize a relay, you would have to make sure the relay coil is connected to the "open" terminal while the switch is near the magnet. I'm not sure if read switches have a normally open terminal. Maybe you can post a pinout of the switch.

Here's a link to photos of the torpedo build:Login | Facebook
 

marcbarker

New Member
does the reed switch activate breifly as it goes past the PM?
 
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k7elp60

Active Member
Mike I assume the torpedo runs until the battery runs down.
Here is a circuit that will do what you want. The reed switch energizes the relay turns on the motor and keeps the relay energized. When the battery gets discharged to the relay cutout point the the relay will deenergize.
 

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Torpedo relay info

The torpedo will run until I pick it up off the beach and place a magnet near the reed switch to shut it off, probably no more than a minute run time. When not in use, the torpedo is pulled apart and batteries removed. O-rings seal the tail cone to the main body of the torpedo.

I still don't see how to keep the reed switch from 'seeing' the load on the circuit. As I said before, I wired it up without a relay using 4 AAA alkaline batteries in series to the motor and the reed switch failed after a few minutes,meaning I couldn't shut it off using a magnet.
Mike in Honolulu
 

marcbarker

New Member
The torpedo will run until I pick it up off the beach and place a magnet near the reed switch to shut it off,

I still don't see how to keep the reed switch from 'seeing' the load on the circuit.

The reed switch can only initiate motor run. To stop, the power must be interrupted.


The Diode and Relay divert power away from the reed switch.
 

k7elp60

Active Member
Thanks guys for help with the munitions

It makes sense now. I guess the diode prevents the current from going back through the reed switch. What diode do I ask for at the Radio Shack store?

I'll post a few pictures and video when this thing is finished. Next project is operational depth charges, but very little electronics needed here, just paper tube, balsa end caps, black powder, waterproof cannon fuse and a model airplane engine glow plug. A servo will energize the glow plug, set off a few grains of powder to ignite the fuse and blow the depth charge clear of the rack on the PT boat. Several people have done this already. Do a search on YouTube for model ship depth charges to see short video.

Regards,
Mike in Honolulu
 

marcbarker

New Member
1N4001 or better. If not, the shop staff should be pleased to suggest a suitable alternative if you show them the circuit.
 
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