General Questions

[DeltaHat]

You can complete any of the 10 tasks in as few or as many steps as you need. Some of the tasks are VERY specific about how they are to be completed. Between steps, you can do whatever you want.

I can't speak for next year, but this year, the bonus applies to all levels of competition.

[tmanneo]

Oh, ok. Thanks I've already come up with some ideas. So excited!

[Paradox21]

For usable electronics for this event, the last item is mechanical relays. Can some one explain what exactly one would use this for in MP and what kinds of relays are and aren't allowed (I wikipediaed mechanical relay and it took me to the plain old relay page, so I don't really know how to differentiate between kinds of relays).

[andrewwski]

Most relays are mechanical - they have an electronic coil that when a current is passed through them draws a switch to a different position. You'll be able to hear it click when this happens.

Relays are used for a lot of things - opening or closing a circuit depending on whether or not voltage is present, etc.

[Dark Sabre]

Well, the distinction they are drawing when they say "mechanical" is between ones that are really just switches driven by an electromagnet like andrewwski was talking about and solid state relays, which are kinda just big MOSFETs.

A solid state relay (fairly representative, at least for larger ones...smaller ones can look different):

^^ you can't use these and you don't want to...they are more expensive than the mechanical ones ^^

Some examples of "mechanical" relays:
A 12V Automotive relay...like you would find in your car for some things:

A big relay, probably for an appliance or something?

A couple relays that are meant to be soldered onto a PCB board, hence the pins:


Anything you would use would probably be like one of the automotive relays or one of the PCB relays, depending on what voltage you need it to trip at. While automotive relays are rated to trip at 12V, their actual trip point is lower. For example, this RadioShack "12V" relay actually trips at 7.8V, according to the tech specs. You could therefore safely use it in a 10V circuit.
If you are doing a 5 or 6V circuit, you'll probably have better luck with the PCB/PC relays, like this one from Radioshack

I never used any relays, but there are reasons you might need one...if for example you had a 10V circuit that was completed in one action, but the motor that drove the next transfer was a 3V motor, you could use a relay to make the 10V circuit complete the 3V circuit without ever exposing the 3V motor to a higher voltage. People also used to use relays for the "chemical battery" transfer...since the chemical battery itself couldn't power a motor to do anything, you could try to use a relay whose coil used little enough amps that the chemical battery could operate it. Then the relay could complete the circuit that could source current from a real battery for the next transfer.

If you ever do go relay shopping check:
Coil voltage (the voltage that is needed to make the electromagnet inside pull the contacts for the other circuit together)
Amp rating (how much current the relay can pass through for the "load" circuit...a .1A relay will burn up if you try to run nichrome wire off of it)
Contact arrangement (how many circuits are present and how they are triggered...just read http://en.wikipedia.org/wiki/Switch#Con ... rangements)
Normally Open or Normally Closed (normally open is a pretty safe assumption)

If you are not sure what kind a relay is based on looks:

You'll be able to hear it click when this happens.

100% true. always.

[fleet130]

Electromechanical Relays

Relays have many uses. An electromechanical relay uses the magnetic field produced by an electrical current flowing through a coil to close one or more mechanical switches.


A few uses for relays are:

1. Control a larger/smaller voltage/current with a smaller/larger voltage/current.
2. Holding or Latch circuit to “capture” a momentary input signal
3. As building blocks in basic logic circuits
4. Isolate the input circuit from the output circuit

The Holding Circuit (or Latch) has commonly been used in MP to turn on a device with a momentary signal. A brief electrical pulse actuates a holding circuit which keeps power applied after the pulse has ended. For discussion, we’ll use a ball rolling into an elevator to be raised to the top of the device. As the ball enters the elevator, it depresses a lever closing normally open contacts of a switch mounted on the floor under the elevator. As the elevator rises, it lifts the ball off the switch, releasing the lever and allowing the contacts to open. The drawing below shows the basic holding/latch circuit. To simplify the diagram, both the relay coil and Device Under Control use the same battery.


First Diagram: Electricity cannot flow to the Elevator (Device Under Control) because of the normally open contacts of switch (S1) and the relay.
Second Diagram: The ball rolls into the elevator, closing S1, energizing the relay coil and starting the elevator.
Third Diagram: The elevator lifts the ball off S1, allowing its contacts to open. Normally this would break the circuit and the elevator would stop; however, the closed relay contacts continue supplying current to both the relay coil and the elevator.

This works great, but sooner or later the elevator reaches the top. If it continues to run, it could cause physical damage or waste energy from the batteries. Our next circuit addresses the problem.


In this circuit we’ve added a normally closed switch (S2). Its purpose is to stop the elevator when it reaches the upper limit of its travel.

First Diagram: The ball has just rolled into the elevator closing S1 and starting the elevator.
Second Diagram: As the elevator rises, S1 opens, but the latch circuit keeps the elevator on.
Third diagram: When the elevator reaches the top, it opens the normally closed switch S2, de-energizing the relay and stopping the elevator. Since the relay contacts are now open, closing S2 will not start the elevator again.

A switch that prevents a device from exceeding specified parameters is called a “limit switch”, regardless of what type it is. Lever switches and magnetic reed switches are particularly well suited in these applications.

The Loop-Back Circuit
And finally, the last circuit! Sometimes it’s desirable to have several devices in series. The circuit below shows how each device can turn off the preceding device. This is sometimes called a “loop-back” circuit.


Switches S1, S2 and S3 and the normally open relay contacts prevent any of the devices from running.


A mechanical action closes switch S1, energizing the 1st latch, starting the 1st device.


1st device continues operation, even though S1 has opened.


The 1st device completes, closing S2, energizing the latch, turning off the 1st device and starting the 2nd device.


2nd device continues to operate, even though S2 has opened.


2nd device completes its action closing S3, energizing the 3rd latch, turning off the 2nd device and starting the 3rd device.


3rd device continues to operate, even though S3 has opened.


3rd device completes its action, openings S4, releasing the latch and stopping the 3rd device. Since none of the relays are latched, none of the devices will operate, even if S4 closes again.

Next: Add a circuit to return the elevator to the ground floor after the ball leaves the elevator at the top.

[gh]

One more thing you can do with a relay is to make a buzzer: wire it up such that the coil is powered through Normally Closed contacts, so that energizing the coil breaks the circuit. The coil loses energy, closing the circuit again and energizing the coil. The springed contacts flipping back and forth like that makes a buzzing noise.

Oh yeah, anyone got questions on a new topic (ie, relays) should just start a new topic, or this will become a 56-page topic, and it'll be the only thread in a forum just for MP.

[AlchemyRocks]

Just a quick rule clarification.

For mission, it is my understanding that as long as all our materials arrive within the box (50*50*80cm), that once building starts, we are able build outside those size limits. (Not just the flag, everything)

Is this correct?

[Dark Sabre]

Mission is not a build-on-site like junkyard challenge. You bring your completed device and it will be measured the the judges right before it runs.

[Sir_L_Jenkins]

Just a quick rule clarification.

For mission, it is my understanding that as long as all our materials arrive within the box (50*50*80cm), that once building starts, we are able build outside those size limits. (Not just the flag, everything)

Is this correct?

I think from the rules you have a 30 minute prep period to get all your materials ready, like you wouldn't want to transport your device with a live mousetrap or having all of your circuits already connected to your battery with hair triggers to start them off. I don't think that time period is for building necessarily, just time to set up your device.