Need help with unique limit switch wiring (total noob) - Page 1
Forum section: electronics / beginners

author: blackcorv11
author_rank: Newbie
timestamp: February 10, 2025, 10:12:00 am
content: Hey guys! I'm hoping one of you can "fill in the blanks" on the attached layout consisting of two 3 prong / double throw limit switches, a DC motor, and a battery...I'd like to know how to wire things to achieve this functionality:1. Motor turns clockwise when s1 is open and s2 is closed.2. Motor turns counter-clockwise when s2 is open and s1 is closed.3. Motor turns counter-clockwise when both limit switches are open.4. Motor stops / no bat. juice is used when s1 and s2 are both closed.Thanks a ton for any and all help!

author: SteveThackery
author_rank: Super Contributor
timestamp: February 10, 2025, 11:24:32 am
content: Well now, tell us how far you have got with it.Just dumping a problem onto the laps of the forum members won't usually get the best response. People will be much more inclined to assist if you have already made an effort and then run into problems.How close have you got to meeting your design requirements?

author: Ian.M
author_rank: Super Contributor
timestamp: February 10, 2025, 11:43:16 am
content: Satisfying conditions 1, 2, and 4 is trivial (hint: wire the switches as an H-bridge), however, 3 cannot then be satisfied without extra parts.

author: blackcorv11
author_rank: Newbie
timestamp: February 10, 2025, 12:15:08 pm
content: I appreciate the reply! I'm not sure what an H bridge, unfortunately. I looked up diagrams but do better with basic drawings being that I'm not familiar with the symbols etc used in the diagrams. I've tried AI (Chat GPT) and it kept generating different wiring instructions. I've successfully wired limit switches to control a motor in forward and reverse when the switches are pushed in, but in this scenario I need the "opposite" set up where open limit switches power the motor on...Regarding #3, if it's more simple, I'd actually prefer the power to the motor to be off when *both* switches are open.Thanks again!

author: xvr
author_rank: Frequent Contributor
timestamp: February 10, 2025, 12:40:14 pm
content: Are your switches are latching types? Do they keep closed until motor bumps to opposite switch? If not than your logic will produce system that will bump motor by one of the switches forewer (or stack still if #3 will be changed to 'motor stop')As I can guess you use ordinary momentary switches, which close momentary when motor hit them and turn to open when motor release them. In this case your initial logic is incorrect - you have to introduce memory element (trigger) that will control motor movement. It will be switched by your limit switches.

author: ledtester
author_rank: Super Contributor
timestamp: February 10, 2025, 01:04:13 pm
content: Quote from: blackcorv11 on February 10, 2025, 10:12:00 amNote that 2 and 3 means that the motor runs CCW when s2 is open regardless of the setting of s1.If one of the switches was a DPDT (double-pole double-throw) switch you could do it like this (see attachment).Update: Also see these links:https://electronics.stackexchange.com/questions/667077/how-can-i-wire-a-motor-to-operate-in-forward-and-reverse-using-a-dpdt-switchhttps://www.robotroom.com/DPDT-Bidirectional-Motor-Switch.html

author: blackcorv11
author_rank: Newbie
timestamp: February 10, 2025, 01:13:51 pm
content: Thank you! It looks like this forum may be beyond my experience level (which is none), as I'm not able to decipher electrical diagrams. What I thinkI will do is hire an electrical engineer (via Fiverr or similar) to do a really basic "kid drawing" for me, lol. I appreciate the responses very much! I can pass some of this on to whoever I end up working with.

author: blackcorv11
author_rank: Newbie
timestamp: February 10, 2025, 01:15:51 pm
content: Oh and I will definitely check out the links as well - thanks again!

author: SteveThackery
author_rank: Super Contributor
timestamp: February 10, 2025, 07:12:29 pm
content: Quote from: blackcorv11 on February 10, 2025, 12:15:08 pmIf that is the case, then your requirements are trivial to meet. I notice that you seem to be using lever-operated microswitches, and the three terminals on the switch tells me they are changeover switches. That means you should not talk about the switches being "open" or "closed", because they both have an open contact and a closed contact inside. That sounds confusing, I know but the diagram should make it clear.Rather, it is better to use the term "operated" (the lever is pressed inwards) or "released" (the lever is not pressed, so springs outwards). Again, I'm hoping the diagram will make it clear.If wired as the diagram:S1 operated, S2 released: motor turns in one directionS1 released, S2 operated: motor turns the opposite directionS1 operated, S2 operated: motor doesn't runS1 released, S2 released: motor doesn't runI've tried to show how the physical terminals on the switch relate to the changeover mechanism inside.

author: blackcorv11
author_rank: Newbie
timestamp: February 11, 2025, 01:20:30 pm
content: Thank you very much, steve! The diagram makes sense (particularly the drawing), as does the terminology. This will give me something to go on as I try to figure out the full circuit. Appreciate it!

author: SteveThackery
author_rank: Super Contributor
timestamp: February 11, 2025, 02:07:09 pm
content: You are welcome. Remember, though, that this arrangement only works for your modified requirement #3 (modified to remove the need for the motor to run).By the way, the terminology and schematic symbols are those used in the telecommunications industry, because I think that terms like "make" and "break" are much more descriptive than "normally closed" (NC) and "normally open" (NO). Each switch (or relay, for that matter) can be considered to be "operated" or "released", and the contacts inside can be either closed or open, or a combination of both. That's why it's better to talk about the state of the switch/relay (operated or released) rather than the state of the contacts (open or closed) because you can have both open and closed contacts at the same time inside a switch or relay.In the next post I'll draw a diagram of the contact types.

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 02:38:40 pm
content: How big is the motor? What sort of load does it drive?Bear in mind that the switches need to handle the motor inrush current. It might be better for the switches to drive relays, rather than directly switch the motor directly.

author: SteveThackery
author_rank: Super Contributor
timestamp: February 11, 2025, 03:11:38 pm
content: Attached are pictures showing the four basic switch (or relay contact) types. I've used the drawing conventions of the UK telecomms industry, but there are other standards and it's the concepts that are important.

author: blackcorv11
author_rank: Newbie
timestamp: February 11, 2025, 03:26:25 pm
content: It's a 12V 10,000 RPM "pancake" motor (about 27mm wide and 10mm "long").... It drives a lead screw and about a 3-4 oz load.I'll work on a "scheme" based on two single pole double throw switches... Hopefully that'll do the trick because my mechanismdoesn't have much room for more electronics (eg relays)... Thanks again for the help!

author: blackcorv11
author_rank: Newbie
timestamp: February 12, 2025, 01:10:33 pm
content: I ended up hiring an electrical engineer consultant for $100 and from what he said, this set isn't possible to do without adding two relays... shucks! Thanks again to everybody for the help - still good to have learned a few things!

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 01:36:13 pm
content: Quote from: blackcorv11 on February 12, 2025, 01:10:33 pmI agree. That's what I was hinting at in my previous post. Limit switches typically aren't meant to switch loads. They are designed for connecting to a control circuit, which is typically relays for such a simple task such as this one.

author: benseno
author_rank: Regular Contributor
timestamp: February 12, 2025, 02:05:13 pm
content: i can be done if supply is bipolar and there will be double motor speed for one of the S2 closed cases.

author: calzap
author_rank: Frequent Contributor
timestamp: February 12, 2025, 07:34:35 pm
content: Moving a 3-4 oz load with a lead screw with a 10K rpm motor sounds iffy. Even if the lead screw has a very fine pitch, the load will have quite a bit of momentum very quickly if the lead screw rotates at 10K rpm. Maybe there’s a step-down transmission that hasn’t been mentioned or the motor goes slower under load.Mike