Electrical Circuit for LC Project

congo_90

Hi, I'm in the middle of doing my L/C project. In my design i have 3 motors to run
also minimum 2 LED's limited to 9v power supply.
i've attached a rough idea of my circuit. Basically the problem i have is those motors must be able to all go in one direction then go the opposite. MY solution was to add a second ciruit (on the right) which is reverse polar. I have a switch just after both batteries there. When changing circuits one will be off and the other on. Thus completing the circuit but never allowing both on at the same time. Can somebody give me some tips/Advice on how to improve this design? Maybe something more practical? Also i think i can only use 1 (9v) batttery. I'm not sure so if somebody could check this? i have until monday to get this sorted so all ideas are welcome!
This seems a good idea because so many college level engineering students i'm sure together we could sort this out.
Thanks,
dan.

Taildragon

Hi congo_90

Just a very quick reply before I go out for the evening.

Some thoughts for you (I'm not going to do all of the work for you )

Consider using a double-pole double-throw switch instead of two single-pole single-throw switches.

Consider placing the LEDs across the motors, also consider using two different colour LEDs to indicate polarity, and thus direction of motor rotation.

Michael Collins

Hi. First of all, you have connected your LEDs in series with your motor...this isn't very likely to work. This is because the max current you'll want to put through your LEDs will be 20-30 mA, which would hardly be enough to run the motor...

There's one slightly simpler arrangement which still requires two batteries. You need to connect the two 9V batteries in series (connecting the + of one to the - of the other), then take the point where they join and connect it to one side of the motor. Then using a Single Pole Double Throw (SPDT) switch you can connect the motor to either a +9V or -9V supply. See the diagram attached.

If you want to do it without two batteries you need a Double Pole Double Throw Switch.

congo_90

Michael Collins wrote: »

Hi. First of all, you have connected your LEDs in series with your motor...this isn't very likely to work. This is because the max current you'll want to put through your LEDs will be 20-30 mA, which would hardly be enough to run the motor...

There's one slightly simpler arrangement which still requires two batteries. You need to connect the two 9V batteries in series (connecting the + of one to the - of the other), then take the point where they join and connect it to one side of the motor. Then using a Single Pole Double Throw (SPDT) switch you can connect the motor to either a +9V or -9V supply. See the diagram attached.

If you want to do it without two batteries you need a Double Pole Double Throw Switch.

Just noticed. I actually meant to put the LED's Where the main switches are and the Main switches for the batteries where the LED's are. the (SPDT) switches, are these the type where if the switch is in the middle its off. But if put either up or down is on? My original idea was have just simply the left circtuit with an extra switch which could simply reverse the polarity to the motors. i will attach a fixed copy of the circuit in a moment....

congo_90

**edit**

@michael collins,
Attached is a copy of the circuit revised with the SPDT. if it is one of those switches where it's simply on-off-on then i'm sorted because i have one here. if not then i'm hoping this switch could still work and the LED's would mean the circuit will simply follow one way only.

Taildragon

Dan

You're on the right track. The switch that you've added is a SPDT - imagine two of these in parallel, and you have a DPDT.

Think about this: What would happen if you connected your power source (9v battery) to the COMMON terminals of the switch, and your load (motor) to the switched terminals?

Sketch attached - I couldn't call it a drawing!

DublinDilbert

congo_90 wrote: »

Hi, I'm in the middle of doing my L/C project. In my design i have 3 motors to run
also minimum 2 LED's limited to 9v power supply.
i've attached a rough idea of my circuit. Basically the problem i have is those motors must be able to all go in one direction then go the opposite. MY solution was to add a second ciruit (on the right) which is reverse polar. I have a switch just after both batteries there. When changing circuits one will be off and the other on. Thus completing the circuit but never allowing both on at the same time. Can somebody give me some tips/Advice on how to improve this design? Maybe something more practical? Also i think i can only use 1 (9v) batttery. I'm not sure so if somebody could check this? i have until monday to get this sorted so all ideas are welcome!
This seems a good idea because so many college level engineering students i'm sure together we could sort this out.
Thanks,
dan.

As some of the others have said a DPDT switch for each motor would be good in this case.... if you require the motor to go in either direction and stop, you'll need a DPDT switch, with an off position in the center, shouldn't be too hard to track down...

For info on how to use a DPDT switch check out:-
http://www.eleinmec.com/article.asp?12

If i was doing this project and wanted it to look cool you could wire some Bi Colour LEDs ( with series resistors ) across each of the motors. You'll need the 2 legged bi colour leds where the diodes inside are connected in inverse parrallel, but depending on the direction of the motor the LED color will change. http://www.kpsec.freeuk.com/components/led.htm

Michael Collins

congo_90 wrote: »

**edit**

@michael collins,
Attached is a copy of the circuit revised with the SPDT. if it is one of those switches where it's simply on-off-on then i'm sorted because i have one here. if not then i'm hoping this switch could still work and the LED's would mean the circuit will simply follow one way only.

Yep, the on-off-on switch your talking about is a type of SPDT switch. However in your diagram you still have your LEDs in series with your motors. LEDs don't work quite the same as normal diodes, since the current that flows through them when they are forward biased is around 20 mA, depending on the kind. This won't be enough to run most motors...If you're just worried about a short circuit, notice that with the SPDT switch this can't happen...

DublinDilbert has given you a great link showing how DPDT switches work, using this you could get rid of the second battery. It's up to you. Also Taildragon's idea of using different colour LEDs for the direction of the motor is a nice idea...

congo_90

Taildragon wrote: »

Dan

You're on the right track. The switch that you've added is a SPDT - imagine two of these in parallel, and you have a DPDT.

Think about this: What would happen if you connected your power source (9v battery) to the COMMON terminals of the switch, and your load (motor) to the switched terminals?

Sketch attached - I couldn't call it a drawing!

DublinDilbert wrote: »

As some of the others have said a DPDT switch for each motor would be good in this case.... if you require the motor to go in either direction and stop, you'll need a DPDT switch, with an off position in the center, shouldn't be too hard to track down...

For info on how to use a DPDT switch check out:-
http://www.eleinmec.com/article.asp?12

If i was doing this project and wanted it to look cool you could wire some Bi Colour LEDs ( with series resistors ) across each of the motors. You'll need the 2 legged bi colour leds where the diodes inside are connected in inverse parrallel, but depending on the direction of the motor the LED color will change. http://www.kpsec.freeuk.com/components/led.htm

so i'm looking at 6 spdt switches which are the same as dpdt? also all these have to be invidually wired to a battery source in paralell? i will attach sketch in mo.... just wondering if this is the case. Would it not be easier to have one DPDT and then the 3 2-way switches for the motors? (they control i/o of motor) and the DPDT just reverses polarity like in taildragon's sketch?

I'm not looking for you's to do all the work. i'm just trying to get ideas suggestions and so far everyone who has posted has been a great help to me so far. I really appreciate this.

congo_90

**edit**
as promised a very quick sketch of just the 3 motors wired using the DPDT and 3 2way switches the (on-off) type. can this be done? they're used to simply allow/stop current flowing. thus making the motor move/stop. would save a hell load of wiring. if not i will use Taildragons idea. If this is ok then i will sketch up the final thing with LED's ran in paralell to the battery (with resistors) this shouldn't harm the current going to the motors? i plan to use only 1 motor at a time.

@DublinDilbert, I noticed flashing led's on that site which don't require any series resistors. And because i'm using 9v it should be ok. This could work to my advantage two ways: less wiring and the idea is its a construction vehicle so it can become a becon.

Taildragon

Hi Dan

You've cracked it!

Re the LEDs, is this circuit theoretical or practical? In other words, will you be expected to assemble this circuit and repeatedly demonstrate it?

Taildragon

Hi Dan

Sorry, but I have to go to work. I *think* you said that this assignment was due in today, so here's another tip for you.

Every diode (LEDs included) has a rating known as PIV (Peak Inverse Voltage). This is the limit of reverse bias that the PN junction can withstand before breakdown. Most common LEDs have a PIV of 5 volts or so.

In other words, if you reverse bias your LEDs with a 9v battery, you will probably zap them.

I've quickly knocked up the attached drawing to show you a possible solution - the diodes in black are nothing special, something like an 1N4148 would do, or any small switching diode.

This circuit should work fine with your flashing LEDs, just place it across the motor terminals.

Hope this makes sense to you!

congo_90

it did make sense thanks very much for your help! i needed to get a sketch in today but i managed to get an extension till tommorow. I'll do up my final sketch tonight and post up to show the end result. It must be assembled onto my work piece and will have to run at least twice (once to test then once by examiner). With a bit of luck i won't mess it up.
thanks again...
will post sketch later.

congo_90

anybody here do Engineering for LC? if so, do you know if it is ok to use a bread board as final wiring? it would save me a hell lot of soldering and also means i can organise the wires without having to go through a maze of wires.

Also the DPDT.... this is literally two SPDT switches yes?

10-10-20

I don't know about in the LC, but breadboard is only for test/development and you should consider using stripboard to show that you've mastered the use of a soldering iron.
There's more to soldering than making blobs...!

Another piece of guidance I would give you is that you appear to be designing around two 9v (PP3) batteries. These type are not rated to provide a lot of current, only a couple of hundred milliamps, while normal AA batteries can be from 1000mA (1A) to 2500mA or more, therefore allowing more power to your motors. But, maybe you have a reason to use 18v?

Anyhow, sorry about the confusion in my message earlier, I didn't realise that you had posted in here!

congo_90

**EDIT**
@10-10-20,

just checked out the rules there for the LC. Max voltage is 9V regardless of design. My original i figured i could get away with this because it's technically two separate circuits. I will double check with my teach whats the story there...

Anyway so 6AA batteries can provde more current?

I think i will still use the breadboard just to make sure everything will work ok on the circuit. I checked out electric motors in Maplins, Blanchardstown today. They're huge! i need to source some smaller motors but still have the ability to work. This is slightly off topic so no worries. I'll deal with this in time. Could the extra current do any harm to the motors? I wouldn't think so. Also would i need a bigger resistor for my LED's? can't remember off the top of my head but i think they're 220ohm's at the moment.

Final circuit is attached. Note, my sketch isn't the best. one LED should suffice. It's running from the battery terminals. Shouldn't rob any current from the motors no? I set it so regardless of DPDT switch position it will be on unless the ciruit is off.

Taildragon

@congo

Yes, a DPDT switch is basically 2 SPDT switches which are mechanically coupled, but you can buy them ready-made
Maplin do them, among others.

As for motors; well, your project is designed to prove a concept, and the motors won't have to do any actual work, right? In which case you might want to round up three scrap mobile phones. The "vibrate" function on these phones is provided by a tiny DC motor with a little weight fitted to the shaft. These would be ideal for your project, and work down to a couple of volts.

As for breadboard; I have absolutely no idea whether that is allowed under the LC rules, but I'll tell you this - I wouldn't use one. They're fine, and they have their place, but if I was sitting in front of an examiner with one shot at impressing him/her with my technical prowess I'd be very sure to have my model 100%. An intermittent bad junction could ruin your whole day!

10-10-20 makes a good point about stripboards. You might also want to Google "ugly construction" and "manhattan construction"

I have one last tip before tomorrow .......

If you are asked about your design, then you can also mention that the LED/diode combination which is fitted across the motor connections (assuming you go for that suggestion) has a practical benefit other than indicating motor activity.

Bear with me here for a second, OK?

Every time we de-energise (i.e. remove the power supply) an inductor such as the motors in your model we see a curious effect. As the magnetic field in the inductor collapses, there will be a voltage "spike" from the inductor, often referred to as "Back EMF". This spike can be very destructive to the circuit around it, so it is common practice to fit a (reverse biased) diode across the inductor - which effectively kills the spike.

If you use the LED arrangement as suggested, then you can legitimately claim that as a secondary design feature as they are protecting the switches from damage cause by arcing

Blind them with science! Good luck tomorrow, and be sure to let us know how you get on...

congo_90

tomorrow i begin my project. from here on in its stressful building and cutting metal pieces to work with it Aesthetics all the way. You suggested using mobile phone motors, i don't think these would have to power i need. The motors are driving the model. moving an alluminium arm up/down, extending/retracting part of the arm. So gearing is essential. I know that most of these motors have little to no torque. So i will be shopping around. Thanks for the suggestion. And i may take you up on that idea with the LED's. I will not be demonstrating infront of the examiner. They will come in on a given day. Mess around with everyones project. Mark it and go home.

DublinDilbert

Another approach for the motors would be to use Servo motors from model planes... as these have all the gearing + position control feed back built in... you could be able to get them for about €15 each...

you would have to build a circuit to generate a variable width pulse, but a 555 timer should do this nicely...

this approach is probably more complex than you need, but would also work...

congo_90

DublinDilbert wrote: »

Another approach for the motors would be to use Servo motors from model planes... as these have all the gearing + position control feed back built in... you could be able to get them for about €15 each...

Servo's have a limited amount of movement no? or can they do full revolutions?

DublinDilbert wrote: »

you would have to build a circuit to generate a variable width pulse, but a 555 timer should do this nicely...

this approach is probably more complex than you need, but would also work...

Seems more complicated. sorry, you completly lost me in that section there...
My knowledge of electrics is limited to a JC science and the odd domestic appliance in a house. Thanks for the suggestion. I'm going to try source smaller electric motors unless the servos can take the load and be attached to the parts of my project easily.

10-10-20

Servos can do 360, if you want them to do 360.
There is a controller available for them, SAA1027. I wouldn't bother to be honest, it's beyond the scope of what's being attempted here.

Just in relation to the 9v battery comment from earlier, the reason I made an assumption that you were using 18v was that the circuit diagram representing the 9v cell had the following: ---|i---|i---. While this is correct for a 9v cell due to the fact that 9v is made from 6 x1.5v cells stacked, it can be somewhat misleading - to me anyhow! I'd just go with a single cell representation... ---|i---

Anyhow, let us know how it goes.

congo_90

@10-10-20. sorry about that confusion. I thought you meant the second circuit i had from the original sketch.Just an update, the teach said it was a good idea especially the DPDT. I credited yourself and others for it. Now its just design and fit the electrics onto my piece (still on the drawing board with this lol).
Also now just a matter of getting the parts (my school won't provide anything more than the soldering iron).

Anybody know where i can get these parts cheap? I already have my LED's, Wires and some switches for experimental purposes. What i need to find now is the motors and a power pack. Strip board should be easily sourced.

The board itself doesnt have conductable (*spelling) strips like a PCB? its just a board with holes for devices to be fitted, Is this correct?

10-10-20

OK, two places are realistic.... Maplin (Blanchardstown and City C) or Peats of Parnel St.

Stripboard...
http://www.maplin.co.uk/Module.aspx?TabID=1&ModuleNo=1917&doy=27m11
Stripboard has rows of copper ono the rear to which you can solder the legs to. Components go in from the non-copper side. You can then cut the strips as required to make or break a connection. I'd suggest using a 5mm drill bit instead of a 'spot face cutter' as they are >€10.

Motors...
http://www.maplin.co.uk/search.aspx?MenuNo=12553&MenuName=Motors+%26amp%3b+Gearboxes&FromMenu=y&doy=27m11

It's simple to search, just jot down the 5 digit reference and away you go. Don't forget to match the voltage of the motor to the battery pack... For example, a 3v motor only needs two 1.5v cells... so get a suitable battery clip/holder.
http://www.maplin.co.uk/Module.aspx?ModuleNo=31427&C=Maplin&U=SearchTop&T=battery%20holder&doy=27m11

Be prepared to find that Maplin are out of stock on some components, so have a backup plan...

Taildragon

The board you need is often called Veroboard (although I'm not sure Vero actually make it anymore, but there are other brands on the market), and is available in plain or with copper strips on a .1" grid. Maplin sell this.

For cheap parts, I'd recommend http://www.bitsbox.co.uk/index.html who have very reasonable P&P charges to Ireland - I've had good service from them.

One of the oldest businesses in the game (since 1944 according to their website) is Bardwells http://www.bardwells.co.uk who often have very good deals in their "everything for £1" section. At the moment they are offering some motors/gearboxes that might interest you:

http://www.bardwells.co.uk/subcats.asp?catid=MOT02

I heard at the weekend that founder Norman Bardwell has recently passed away, so not sure if they are currently open for business - they are a small family concern.

10-10-20

Good links Taildragon, never heard of these people, but welcome the input!

congo_90

thanks!! both sets of links seem handy. I will start with Maplins. Failing that i will go on to the links. Picked up a DPDT switch today it's an off-on-off switch. But here's the thing: it has only one switch on top. It has the 6 terminals so is this the only switch i need? or do i need to get one more to make the DPDT?

It says DPDT on the label.. also checked the site out:
http://www.maplin.co.uk/module.aspx?ModuleNo=2341&doy=27m11#overview

Refernce: FH04E

just verifying so far that i haven't made mistakes

Taildragon

From what I can make out on the Maplin website, that switch is a DPDT with an ON-ON action (in other words there's no "centre off" position) which is fine for your circuit as you've drawn it, since you have individual SPST switches for each motor.

DublinDilbert gave you a good link http://www.eleinmec.com/article.asp?12 showing you the mechanical detail for wiring this.

congo_90

hey,
went to Maplin's tonight, Bought all my components including the strip board. It seems the copper runs all down in one direction the breaks (see attachment)
in this sketch the white lines is what appears to be the breaks. ie; if battery was connected to the top right and top left the whole top line will become charged but the others wouln't. Is this correct? have to go back to Maplins tomorrow. One of the switches terminals for the motor is damaged.