How to wire a reversable motor with NO limit switches

[678hug]

Hi, I am trying to fit a motorised satnav housing into the top of my Mazda dash and really would appreciate a little help. The circuitry that drives the motor was all incorporated into the factory screen circuitry which has now been removed and replaced with a tomtom.

What i've got.... 2 switches, 1 up and 1 down. Each switch is the same with pins 1 and 3 NO until the switch is pressed when they then become closed. Pin 2 connects to pin 3 at 20 ohms and does not connect to pin 1. There is no difference on pin 2 when the switch is pressed.

I've also got the motor which drives the housing up and down via a small gearbox. It's 12 volt and the gearbox has 2 limit switches in the 1 housing triggered by a gear wheel. The 2 limits switches are at 11:55 and 12:05 clock positions so i guess one is down and the other is up. The limit switches are NO until contact is made where they then become closed with a 60ohm resistance.

What i would like help doing ........
Probably obvious, but i need to drive the motor in one direction by holding (or preferably single push) of the button and auto stop at the up limit. Reverse the motor with the other button but only with button held as it needs to be able to be nudged to avoid sunlight glare, stopping at the down limit switch. Please note these limit switches are NO.

I would prefer to do it with relays as i think i would find it simpler, but my head is spinning trying to work it out.

Any help would be greatly appreciated.

(PS i am looking at H bridges but can't get my head round the NO limit switch thing)

 

[KeepItSimpleStupid]

Here is a hint:

If you use the limit switch as a "If limit A, then disallow direction A" and "If limit B then disallow direction B"

So, without drawing the schematic, you basically insert a diode in the power line for each limit switch.

The two relays that make up this decision, will either be a wire or a diode for each case. Like C would be in series with the power, NC and C would have a diode between them. When the relay closes (hit a limit) then the diode is inserted. When at a no limit, the diode is shorted out.

The limit relays are effectively activated by the NO contacts of your limit switches.

 

[cr0sh]

As an addendum to KISS's description - see this link: **broken link removed** (particularly the schematics at the bottom).

 

[678hug]

Hi thanks for the replies.
I'm getting closer but i'm not sure that the circuits shown will work as both limit switches are open at all times except at the up and down limits.

Therefore for positions between full up and full down both limit switches are open circuit preventing power reaching the motor.

 

[KeepItSimpleStupid]

Each limit switch "drives" a relay, which will give you an SPDT contact to work with.

Let's temporarily make it a DPDT relay (2 SPDT contacts labeled a and b) and make Ca and Cb the terminals that would be in series with the motor. Take NC(a) and NC(b) and short them together. Take NO(a) and NO(b) and put a diode between them.

For the second limit, reverse the order of insertion into the power line. e.g. Power...C1(a)...C1(b).......C2(b)..C2(a)...motor....GND

So, between C1(a) and C1(b) there will be a short or a diode and similarly for C2(a) and C2(b). The diodes in the mini circuit above, would face opposite directions.

You can realize the same behavior with an SPDT relay for each limit.

If you still can't understand it, I can try to draw a schematic.

 

[678hug]

Hi, thanks again for the reply. If you could do a small drawing, i think i'd be able to get my head round it.

 

[KeepItSimpleStupid]

OK, this is more complicated than it needs to be, but it should be simpler to understand.

Note, I used a logic supply (+5) for the limit switches. That's so they stand out a bit.
V is the motor supply.

The motor circuit is a series circuit, so it doesn't matter what order the parts are drawn.

Now, you should easily be able to reduce the DPDT relays to SPDT ones.

Note: No means of reversing the motor power is provided.

 

[678hug]

I was just in the verge of replying and asking a few questions and............... the penny dropped. I've finally understood thanks to the schematic and your previous descriptions.

I might be back with a question or two tomorrow, but for the moment thank you very much.

There are some really clever people out there!!!!

 

[KeepItSimpleStupid]

Briefly, let's look at the "H-bridge".

You typically have a DIRECTION signal and a NOT ENABLE signal, ignoring (fwd, rev, brake and coast logic for now).

You can look at it as if limit A, force direction B and if limit B force direction A. There might be an issue if both limits are on at the same time due to a fault.

But, the logic; invert the direction if limit A or limit B is set may also work. If limit A and Limit B, then force enable off would be a safety.

If you are interested in doing a processor based version, check out www.comfiletech.com, **broken link removed** and www.cpustick.com I haven't used either, but I did try the cpustick simulator