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Help needed please

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welshvicar

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Hi all,
I am new to electronics and need some help please
I have a project that requires a magnet to turn on a motor and stay on until the magnet is used to turn the motor of again
it is a small motor running of a 9 volt battery.
the plan was to use a read switch and pass the magnet over the reed to turn the motor on and off ( but I have been told I would be better off using a hall sensor ) the problem I have is how to keep the motor on. space is also an issue so would need to be as small as possible
 
welshvicar said:
but I have been told I would be better off using a hall sensor

Firstly..... Do we have real clergy here???

Secondly.... Who told you this.... If you can do this with a reed switch... Then do so... A hall sensor is basically the same thing anyway....Also you can get more current through a reed switch...
 
I would use a D Flip Flop like a CD4013 configured as a divide by two and let a reed switch pulse the flip flop clock input. Pass a magnet over the reed switch and the motor turns on, pass the magnet over the reed switch again and the motor turns off. The flip flop output would drive a transistor which would drive the small DC motor. I would use a reed switch and you include a a debounce circuit (only a few components) to clean up the reed switch. There are any number of ways to do what you seem to be looking to do.

Ron
 
Welcome to the forum!
Can the magnet be left in place on the reed switch while the motor is on, so that removing the magnet subsequently switches the motor off?
 
Thank you all for your replies
no the magnet cannot be left in place
could someone kindly post a drawing of a circuit as Reloadron has suggested as I have no experience of flipflop
many thanks
 
Thank you all for your replies
no the magnet cannot be left in place
could someone kindly post a drawing of a circuit as Reloadron has suggested as I have no experience of flipflop
many thanks

Give me a few hours. Maybe less if I can find one I have. :)

Ron
 
There are also latching, or bistable reed switches. To use those, you need to reverse the polarity of the triggering magnet to set or reset the switch.
 
Here is something that will work:
4013 Motor Start Stop.png


4013 Reed Switch.png


The circuit looks more complicated than it really is. As drawn V2 is the 9 volt power. V1 was only used to simulate the reed switch, the actual normally open magnetic reed switch is shown in the lower image. Where you see Vout in the lower image that would be connected to R1, R2 nd D1 junction. R1, R2, D1 and C1 make up the switch debounce circuit and drives the 4013 D flip flop. The Q output of the 4013 drives the Q1 transistor which drives the motor. The 2N2907 is a very common off the shelf transistor capable of driving a 800 mA load and your motor likely draws less than that. Passing a magnet over the reed switch will start the motor and passing a magnet over the switch again will stop the motor. C2 is there to clean up any brush noise from the motor and D2 acts as a flyback diode across the motor. The CD4013 is a dual flip flop chip. Here is the data sheet. A Google of flip flop 4013 circuits will bring up dozens of similar circuits.

This is how I would likely go about what you want to do. There are other ways and methods.

Hope this helps....
Ron
 
Shouldn't the transistor be a NPN type?
 
Hi,

I wondered too just as Ian did, what your church function might be.

As to the problem at hand, there are a number of interesting approaches depending on how you want to do it.

First, if you bias a reed switch magnetically you can get it to stay off until another magnet with the same polarity comes close and then it turns on and stays on. When you reverse the polarity of the magnet and bring it close it will turn the switch off. The catch is that to turn it on one way the magnet must be oriented one way, and to turn it off it must be rotated.

Second, if you use a normally open reed switch to turn a relay on you can wire the contacts to keep it on. A second normally closed reed switch would turn the relay off.

A magnet/pawl combination would work too but i dont know if they sell anything like that.

I was trying to come up with some very simple solutions based on your description of your experience so far in electronics. You probably dont want to tackle anything too complicated at first.
 
Shouldn't the transistor be a NPN type?

I guess it could be a 2N2222 and move the load up to the collector side. I figured it would work either way. Actually Alec, never gave it much thought but a better design likely would be a 2N2222 with the motor, flyback diode and the filter cap on the collector side. Still not sure what the OP wants but figured I would toss something out I figured would work. Don't even really know the motor current and I guessed just a little 9 volt hobby motor.

I also like MrAl's suggestion if two reed switches (N/O and N/C) can be used. Pass over the first switch N/O to turn it on (Latch) and the second switch N/C to turn it off (UnLatch).

Ron
 
I also like MrAl's suggestion if two reed switches (N/O and N/C) can be used. Pass over the first switch N/O to turn it on (Latch) and the second switch N/C to turn it off (UnLatch).
But if the first switch is magnetically biased to latch it on, won't it stay latched? It's a permanent magnetic force which makes it latch, not a current-controlled magnetic force; so you'd have to pass a reverse-polarised magnet over it to unlatch it. Unless, of course, you used the second switch to turn on a de-magnetising coil around the first switch.
 
But if the first switch is magnetically biased to latch it on, won't it stay latched? It's a permanent magnetic force which makes it latch, not a current-controlled magnetic force; so you'd have to pass a reverse-polarised magnet over it to unlatch it. Unless, of course, you used the second switch to turn on a de-magnetising coil around the first switch.

I see what you are saying Alec but the idea is we have a normally open reed switch and a normally closed reed switch. The reed switches don't latch but they do latch a relay. The relay would have two sets of contacts as in a DPDT relay. One set of relay contacts would be used to latch the relay and the other set used to power the motor. This is the portion of MrAl's post I was focused on:

Second, if you use a normally open reed switch to turn a relay on you can wire the contacts to keep it on. A second normally closed reed switch would turn the relay off.

Ron
 
You're right, Ron. My bad. I was focussed on this bit :
"if you bias a reed switch magnetically you can get it to stay off until another magnet with the same polarity comes close and then it turns on and stays on."
 
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