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# Help needed with circuit design

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#### djcodesy

##### New Member
Hi all,

Hopefully someone would be able to help me out with a quick circuit diagram as sadly it seems I’ve reached a point where I’m completely stumped as how to do something that seems like it should be a fairly an easy thing to do... I’ve spent countless hours searching, reading and learning yet still unable to get exactly what I’m after. I have managed to put together something that does what I’m after by using what I’d learned and found online but let’s just call that attempt ‘optimised for improvement’ at best!

So here’s a basic description of what I’m essentially trying to achieve:
A switch that when turned on will turn a light on as well as turn a dc motor one way until it reaches a limit switch, at which point the light remains on but the motor stops.
Then when the switch is turned off the light goes off and the dc motor goes in reverse until it hits a second limit switch.

I guess the simplest thing I can think of that is somewhat similar would be pop up head lights on a car.

My attempt used a couple of dpdt relays and micro switches but the relays became incredibly hot when the motor was at rest, leading me to believe I’m doing something wrong..

Any help would be greatly appreciated

Thanks

Big hint: Search for "chicken coop door"

In reality, you want a "stopped" indicator too?

it is loosely related to "power automotive door locks" too.

The "motor interface" without limits is relatively easy. Use of DPDT relays might help with the indicators.

Take two SPDT contact set from two different relays. Put the common contacts to the motor. One on each side.

Now take the NC (Normally Closed) contact and put them to ground.

So, with the relays de-energized, the motor is shorted. It's important later, because a moving DC motor is generator and shorting a generator makes the motor stop real fast.

Now, connect the NO terminals to the DC supply. Nothing will happen yet.

Now what happens:
1) No relays energized or both relays energized = brake
2). One relay energized = motor moves in one direction
3) The other energized = motor moves in the opposite direction.

Knowing a little bit about the motor and voltages will help here., but that's the starting point.

The limit switches typically can't handle the motor current and sometimes they have a hard time handing the relay current.
Quiesent current issues are another problem. i.e. Little or no current consumption except when moving.

Diode protection around the relay coils might be needed.

You have to decide if the system requires a closure to ground or a closure to positive to move.

Thank you so much for the reply.. I never would have thought of looking up chicken coop doors, but i just had a quick look now and I think you may have just solved my problem!

I’ve found this one diagram that I think may be right?? (hopefully the attach works off my phone) I assume that I just run the light straight to the switch?

Everything is 12v including the led light, and there’s no need for an indicator..

I really do appreciate the help and pointing me in the right direction..

Thanks again

You can do it far simpler than that, just one, two-pole relay
(Or two single pole with the coils linked).

https://electronics.stackexchange.com/questions/212522/back-emf-of-dc-motor-protection

Just use a relay in place of the switch. The light can be connected between one of the power inputs and one of the right hand contacts on the relay, depending which is most convenient (eg. if the lamp is grounded).

well i guess its about time to warm the old soldering iron up and have a play around and see which one works best for my application. thanks again for the help.

If i could though just ask one more question quickly in relation to the diagram i posted earlier. On the relay located at the bottom left of the picture(the one where the two limits both connect to) there's a little bit drawn in a thinner line essentially joining the three poles together.. I'm assuming that that's basically the on/off switch yes? if not then what is it? and if so then is the switch at the very bottom then necessary or just a safety precaution?

sorry if I'm being a pest, I'm still relatively new to this particular aspect of electronics.. I'm learning it as I go so i really do appreciate the help =)

In the diagram with the three separate relays, the bit in grey is just to identify the internal contact arrangement of the particular relays the author used.

Some types have the middle pin of the group of three as the common, that one has the pin nearest the coil as common (and next one up normally closed, then last is normally open).

The switch at the very bottom of the diagram is the operating switch, it controls the coil of the lower relay and sets which way the motor will run.

In the simpler one I posted, the contacts on just one relay would replace the "reversing switch" part of the diagram. then operating the relay runs the motor one way or the other.

I didn;t check the design, but the arrangement is probably.

Coil NC NO C for the rows of contacts.

The way some relays are designed, you have an arm and then two contacts, so the contacts are either the two left ot two rightmost ones. You know which one is the coil.

If the light and motor can run from the same DC power source, then:

1 - DPDT switch
2 - limit switches
3 - rectifier diodes rated for at least twice the voltage and current of the motor or light (whichever's greater) startup requirments
0 - relays

ak

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