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switching a relay - but on second switch. ( if that makes sense? )

Discussion in 'General Electronics Discussion' started by Sion, Aug 4, 2016.

  1. Sion


    Jan 4, 2016
    Hi Guys, i wonder if anyone can help?
    I am working on a project, and part of it is essentially a rod moving up and down.
    I have tried my best to explain below.

    When the bottom of the rod reaches 15cm above the ground on its downward vertical travel, it passes a [ magnetic reed switch 1 ], which activates [relay 1].

    When the bottom of the rod reaches 5cm above the ground on its downward vertical travel, it passes [magnetic reed switch2], which activates [relay 2].

    When the rod reaches the ground, i then starts to move back up again.

    It first passes [magnetic reed switch2] , and it doesnt matter if it activated or not - so no problem here.

    However! when it passes [magnetic reed switch1] on its upward travel - i dont want the relay to activate.
    relay 1 shold only activate on the DOWNWARD travel.

    Has anyone got any ideas?

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    With relays ony this is going to be difficult.

    Using other electronic components this can be done. How - that depends on further circumstances. More information is required.
    For example:
    • How is the rod moved?
    • Do you have any electrical/electronic indication available in which direction the rod moves?
    • How is the direction of movement changed?

    If, for example, the rod is moved by a DC motor and the reversal of direction is introduced by changing the polarity of the motor's power supply, you could easily use this same supply, connect it to relay 1 via a diode such that current can flow only if the rod is moving downward. On activation the relay will then pass the voltage with correct polarity through diode and contact and the voltage can be used to control other relays (for example).
    When the rod moves upward and even though relay 1 will be activated by the rod passing it, current flow will be blocked by the reverse polarity of the power supply voltage which will be blocked by the diode. Thus the attached components do not se a voltage).

    'this just an idea.
    More can be done using some digital logic or even a small microcomputer (e.g. arduino).
    BLAUJUNK likes this.
  3. Sion


    Jan 4, 2016
    Hi, THanks for the reply.

    The rod is moved mechanicaly with a lever & cam setup - connected to a continuous belt driven motor - the motor never changes direction, but rocks the lever - which in turn, moved the rod up and down.
    - so the direction of movement is automatic.

    There is no electronic indication available as to which direction the rod is moving.

    Basicly, i think i am looking for relay1 to only work half the time.
    1st activation - switches.
    2nd activation - does nothing.
    3rd activation - switches.
    4th activation - does nothing.


    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    This purely curiosity... What is the purpose of this mechanism?

  5. Sunnysky


    Jul 15, 2016
    If only the tip of the rod activates the reeds momentarily, it MUST do so in both directions for this logic to work, and at no other time in between.

    If this can be done, then the logic needed might use 1 FF and 1 NAND or AND gate.

    SW1 Reset FF , Q=0
    SW2 leading edge clocks a flip flop , D=1 then Q=1
    SW2 AND Q=1 the. output activates transistor switch to sense tip going down only.

    If the reed switches are not reliable here, then optical interruptor or reflector transistor is needed.

    make sense?
  6. Minder


    Apr 24, 2015
    There are special one direction limit switches if you can use them, but they could be pricey unless sourced on ebay.
    Another alternative is a regular limit and make your own unidirectional striker.
    AnalogKid likes this.
  7. Minder


    Apr 24, 2015
    Also using the arrangement you mention, the relay will just pulse for the sw sense duration. if this is what is needed.
  8. Sion


    Jan 4, 2016
    Yes, only the tip of the rod activates the reed momentarily.
    They also seem reliable.

    Unfortunately its been a very long time since i have used logic gates, and dont really understand much of the rest ;)

    Yes, just pulsing the switch is fine for what i need.

    We are actually working on a model beam engine. ( steam engine ).
    The reed switches are on one of the linkages, so that we can activate a smoke machine to fire off at the right time in the cycle. I have though about putting the magnets / reeds on the wheel - but this does not stay 100% in time.
    I have attached an image of what we are working on :)

    As you can see, my main background these days is more model making. my electronics is very rusty - but i do love having a go!

  9. Sion


    Jan 4, 2016
    Hi. Been having a think on this one.
    (On iphone keypad now so bear with me )

    we can simplify this by just talking about the one reed switch on the rod. On the first pass- it needs to activate, and on the second pass it needs to do nothing.

    Do you think it would work if I add an additional reed above and below.
    When the magnet hits the top reed, it triggers a Relay which allows the middle reed to be used.
    The magnet passes the middle reed, and triggers it.

    The magnet then hits the bottom reed, which unmatched the relay, thus deavtivating the middle reed, until the magnet has safely passed over it, and all the way to the top, where it is re activated.

    I have attached a quick sketch.
    As I said, my electronics is rusty at best so not sure if this will work?


    Mar 28, 2011
    contact bouncing could also be a major problem, an all electronics device is necessary.
  11. AnalogKid


    Jun 10, 2015
    If you replace magnetic reed switch #1 with two reed switches very close together, the order in which they close will generate a pulse signal to the external circuit in one direction only without any logic circuits. Two switches, two resistors, and some DC power to create the output pulse.

  12. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    I am a little confused about how your model will work. Are you going to polish up that old warhorse shown in your picture, apply a fresh coat of green paint, and drive it with an electric motor? Yikes! What a sad ending for such a fine example of 19th Century engineering. Do you own this engine? Or perhaps are you preparing it for public display?

    Since it will be motor-driven, I assume it is not a functional model using live steam. Could you explain a little bit about the purpose of the reed switches? Are they to provide some sort of visual display for the model? Are they used to actuate your smoke generator somewhere in the up/down cycle of the beam? Where, exactly? Why two switching points? One to start the smoke, a second to turn the smoke off? Will you eventually re-build the engine shown in your fine picture and operate it with live steam? That would be truly awesome!

    The engine shown has six connecting rods, including the main rod that originally provided motive power to the flywheel. Can you tell us which of these rods has a permanent magnet attached that will be used to actuate the reed switches? I think I can identify the steam cylinder and the air pump cylinder that was used to re-circulate condensed steam back to the boiler, but what are the other three (smaller) rods near the pivot used for? Just curious. I'm sure you will sort out the reed switches and relays.

  13. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Assuming that the fog machine fills in for the ersatz "steam" aspect.
  14. Sunnysky


    Jul 15, 2016
    the relays you show will not work as they need to be latching with twin coils.
  15. Sunnysky


    Jul 15, 2016
    S1 Reset FF , Q=0
    S2 trailing edge clocks a flip flop , D=1 then Q=1
    when S2 closes AND Q=1 the. Q1-c output transistor turns off. THis can be inverted if desired or drive a relay

    Reed relays may need 100pF across inputs to suppress noise.
    5V needs decoupling cap as usual.
    Q1 any NPN darlington to suit output.
    D1,D2 any signal diodes.
    I just realized you want output on for the whole down stroke.
    or maybe not...
    how long a pulse ? "Yes, just pulsing the switch is fine for what i need"
    Last edited: Aug 5, 2016
  16. Sion


    Jan 4, 2016
    Hi guys.
    Thanks for all the replies.

    Sunnysky: to be honest, I have a bunch of those relays here some where ( just need to find them ;) ) so was hoping that i may just be able to get away with them - but will look at latching relays. Will also have a look over your other post - thank you for the input :)

    Heavens1944 : it's actually a model beam engine we made some years ago. It's mainly made from styrofoam. It has full movement just driven by a motor on the right hand linkage. it was used mainly for exhibitions, and theatrical use etc. We are now setting it up for display in our studio/ workshop to create some atmosphere :) . We actually made one nearly twice this size for the Messe in duseldorph (some 20 years ago. )
    I may have a video of that one of you are interested to see?

    The reeds can be fixed to any of the rods.
    The base part of the engine is hollow, so fixed in there out of site.

    we haven't fully setup the engine for some time, and sadly all the original electronics inside were removed for some reason- the the person who wired it up has since left, hence I am trying to figure it out.

    I have 2 "switch connections" on the smoke machine remote. If you make a brief connection on the "on" its stays on. And then a brief connection on the "off" and it stays off, until the on is triggered again.

    At the moment I have it working with just 2 reeds on one of the left hand rods.
    On its decent,
    It triggers the first one " steam on" soon after it starts its decent ( but not at its heighest point).
    It then reaches the bottom, and hits the steam off reed. -- so far so good.
    Problem is, it re triggers the "steam on" reed too early, as it passes it on the way back up - and we only want it to trigger on the way down.
  17. Sunnysky


    Jul 15, 2016
    so fix my timing diagram if you have 2 outputs and 2 inputs.
  18. Sunnysky


    Jul 15, 2016
    It seems your smoke gen, already uses dual momentary coil latching inputs.

    But to make this reliable each stroke must produce only 1 pulse, although bounce is expected for ~1 ms, a carefully chosen cap that wont burn out the contacts yet discharge the cap to sustain the pulse and then charge up with a pull up or down R.

    I outlined the easiest method using the trailing edge of S1 reed contact closure for Start, gated with D FF which is triggered on the upstroke (memory)

    Stop does not need to be edge sensitive so S2 goes direct to Stop smoke.
    Good luck.
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