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Variation on the Light Bulb Sequencer Project

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AZDean

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Hi everybody!

I'm helping my son do a variation of the 10 Channel Light Bulb Sequencer project for a science project. The variation is that instead of switching light bulbs on and off, we need to switch electro-magnets on and off.

Our electro-magnets are the simple kind where you just wrap wire (about 20 gauge) around a four inch long bolt and attach the wires to a nine volt battery. It seems I have to add two batteries in parallel to get enough magnetic pull, so first question is, what's a better way to make an electro-magnet?

But worse was trying to figure out how to switch the electro-magnet on and off. I bought a kit board that is very similar to the above mentioned project, except that it simply drives a series of eight LED's directly from a 4015 IC (through 3K resistors).

I tried adding an NPN transistor (like the above project shows), and simply wire my electro-magnet/battery thing across the collector and emitter of the transistor, while the base is connected to a pin on the 4015 IC (and the emitter also grounded to the kit board’s ground).

But I'm afraid I don't know much about transistors (or electronics) and I couldn't get it to work. The kit board works fine and drives the eight LED’s in a nice sequence. The electro-magnet works fine by itself, but I don't know how to get it connected to switch on and off with the LED's.

Oh, here's a link to the kit I bought (bought locally):
LED Runing Light Kit

Any help would be MUCH appreciated! Thanks!
 

Nigel Goodwin

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Most Helpful Member
The first thing you need to do is find out how much current the magnets take - depending on how you've made them it could be considerable.

Once you know that, you can find transistors which will handle the current, with a reasonable safety margin.

Also, and absolutely crucial, you must fit reverse biased rectifiers across the coils - when you turn a coil off, it produces a high back-EMF which will destroy the transistor.

But I think your initial work should to experiment with your electro-magnets, number fo turns, thickness of wire, and find out what gives the best results. A great deal depends (obviously) on how much power they need to have, and what they are being used for.
 

AZDean

New Member
Thanks Nigel for replying to my post!

I don't know much about "reverse biased rectifiers". Is a diode sufficient here to prevent the reverse EMF?

I also don't know how to calculate the current. We simply found that two nine volt batteries in parallel seemed to provide enough magnetic force. But how much current is created by two nine volt batteries in parallel?
 

AZDean

New Member
Okay... I've been looking this stuff up and I'm starting to get the idea...

A website here has a calculator that tells you how much resistance is in a wire of a given gauge and length. Then given the formula:

Current = Voltage / Resistance

I can see that my current is quite high! Much higher than I realized. So, I needed to get a transistor rated much higher, or try a smaller gauge wire and wrap a LOT longer length of it.

Hmm... I so new at this though, I'm afraid I'm missing something...
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've attached a circuit to show you what I mean. I see you've realised now about winding the coils! - it's not as trivial as it first seems :lol:

You don't calculate the current, you measure it using a multi-meter.
 

Attachments

Nigel Goodwin

Super Moderator
Most Helpful Member
grrr_arrghh said:
Nigel, can i ask what program you use for your circuit diagrams?
Abacom sPlan 5.0, you can download a demo version from their website at http://www.abacom-online.de/uk/. When you order from them they sent you a CDROM along with an invoice, and you have to send them the money back. The only problem is they don't take credit cards - you have to do a bank transfer or send a cheque in Euros. I found this very difficult, and was going to cost a fair amount to do - but luckily my boss has a Spanish bank account, with a Euro cheque book, so he was able to do me a cheque.
 

AZDean

New Member
Nigel Goodwin said:
You don't calculate the current, you measure it using a multi-meter.
Ah... I'm afraid I had a cheap multi-meter that couldn't measure that high of current... I now see I need a better one...

Thanks for your help with that diagram!
 
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