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auto on and of to over-ride a manual switch

Discussion in 'General Electronics Discussion' started by oliver, Jan 8, 2014.

  1. oliver

    oliver

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    Jan 8, 2014
    Greetings to all,
    I have a machine that has a push to make on and off switch.
    I also have a digital timer that will switch on and off at what ever time I decide.
    I would like to integrate the two so that it will switch the machine on when I wont it and off when I wont it
    I cannot just cut the power to the machine as this would cause a safety issue it needs to spin down when told to stop and visa versa

    the idea I have is that when the times is activated for power on I need to send a single dead short across the on switch and that's it
    when the timer switches off I need to do the same but across the off switch.

    can anyone please assist with this....


    Oliver
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi Oliver and welcome to Electronics Point :)

    That's a good description of the problem.

    It's possible to make a circuit that will take the output from your timer and pulse one relay when the output goes ON, and another relay when the output goes OFF. The contacts of those relays can then be connected across the pushbuttons of the machine. This circuit will need a low-voltage DC supply, e.g. 12V.

    Another option would be to buy a timer with two separate outputs, which can be programmed to turn on briefly at different times of day.

    If you want to go the first route, can you give me a link to some details about the timer you have, and let me know what DC supply you have. I can draw up a circuit which you can build on stripboard. It will have a few transistors, resistors, and capacitors, and two relays.

    Is there a safety issue with starting the machine at a certain time of day without warning?
     
  3. oliver

    oliver

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    0
    Jan 8, 2014
    Hello KrisBlueNZ.
    thank you for your reply.

    there is no safety issue with switching on, only off

    the current timer I wish to use is a standard digital time that cuts the power as set
    Tedelex bnd-50/sas36

    http://bainian9908.en.made-in-china.com/product/cqvEPxnZbHkX/China-Timer-Socker-BND-50-SAS36-.html

    I can provide any voltage required by you to make it work

    I am open to make anything you suggest, this has become an absolute nightmare for me
    but all in the interest of saving power

    thank you again for your reply

    Oliver
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Have you bought that timer? If not, can you find one that has a contact closure output instead of a switched mains socket on the back?

    You can use that one, but you'll need some way to convert the switched mains voltage on the socket into a signal that can be used by the switching circuit.

    There are many ways to do this, but in the interest of simplicity and safety it might be best to use a second power supply, plugged into the back of the timer, to provide a DC voltage into the circuit which is present when the timer is ON, and disappears when the timer is OFF.

    Actually I think it might be easy enough to use a single power supply plugged into the timer. I'll draw something up soon.
     
  5. oliver

    oliver

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    Jan 8, 2014
    I had the timer unit on a light. maybe you could suggest a better one or two that I can locate for the job

    I had intended on dismantling the unit for the project and would have disconnected the 15 amp plug completely.

    but I am open to your advice here..

    I will build whatever we create into its own box and can supply any number of transformers inside to supply whatever we need space is not a problem and our supply will be 220 v ac so any standard transformer can be used..

    thank you again
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK, I'll stick with the design I've been working on. It uses a 24V DC power supply that's plugged into the timer. I'm still in the process of drawing it up, then I will simulate it because it's a bit of an unusual design. Then I'll post it. It should be up within 24 hours.
     
  7. oliver

    oliver

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    Jan 8, 2014
    thank you, I look forward to getting your design
    you have no idea how much I appreciate your assistance

    thank you
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Sorry for the delay. I'm still trying to figure out a good way to design this circuit. I'm with my partner for the next three days so I can't spend much time on it, but I'll get there, don't worry!
     
  9. oliver

    oliver

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    Jan 8, 2014
    I am at your mercy
    thank you
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK Oliver, here's the design I've come up with.

    It uses quite a few more components than I had hoped, but I can't see any way to simplify it, and it does seem to work pretty well (in simulation, at least).

    [​IMG]

    The circuit requires a 24V DC power supply that is powered via your timer. When the timer turns ON and the 24V DC appears on the circuit's input, it pulses relay K1 ON for around 0.4 seconds. When the timer turns OFF and the 24V DC drops away to zero, the circuit pulses relay K2 ON for around 0.3 seconds. I'm assuming the pulse durations aren't critical.

    The circuit uses four transistors, five diodes, one IC (U1), four capacitors, 14 resistors, and two relays. You can build it up on stripboard - Google stripboard construction to learn more about this. You will need all your components to be THT (through-hole technology) - i.e. with wire leads.

    The three components at the left side of the diagram represent parts inside the 24V DC power supply and are not part of the circuit design. They are needed for simulation.

    The circuit accepts 24V DC from the power supply on the two rails marked +24V (connect to power supply positive) and 0V (connect to negative).

    The blue trace on the graph, marked V(n001), is actually the voltage at DP's anode. It represents the mains supply (from the timer) turning ON and OFF.

    Here are some notes on the components in the design. I will include information on suitable components available by mail order from Digikey (http://www.digikey.com) but you can also use locally available equivalent parts.

    If you need more specific information please let me know.

    Here's a complete components list:

    2x 1k5 (1.5 kilohm) 1W resistor (R1,2)
    1x 180R (180 ohm) resistor (R3)
    4x 5k6 (5.6 kilohm) resistor (R4,9,11,14)
    5x 22k resistor (R5,8,10,12,13)
    2x 3k3 (3.3 kilohm) resistor (R6,7)

    1x 1000 µF, 35V electrolytic capacitor (C1)
    3x 10 µF, 35V electrolytic capacitor (C2,3,4)

    1x 1N5404 diode (D1)
    4x 1N914 or 1N4148 diode (D2,3,4,5)

    2x 2N3906 or BC557B transistor (Q1,3)
    2x 2N3904 or BC547B transistor (Q2,4)

    1x TL431LP shunt regulator IC (U1)

    2x relays, 24V DC coil, coil resistance 2400 ohms or higher, SPST (aka SPNO) or SPDT (aka SPCO) or DPDT (aka DPCO) contact arrangement.


    R1 and R2 must be rated for 1W (one watt) power dissipation and should be mounted raised off the board slightly (e.g. 1/4 inch). Something like http://www.digikey.com/product-detail/en/FMP100JR-52-1K5/1.5KWCT-ND/2058904

    All other resistors should be 1/3W or 1/4W rated. Digikey have the Stackpole RNMF series available ex stock in all the required values.

    Resistors are non-polarised and can be inserted either way round. All the other components must be connected the right way. Google electronic component polarity markings to find out how they're marked.

    The capacitors are all standard electrolytic types rated for 35V or more. The hollow plate on the diagram (the top plate on C1, C2 and C3, and the left plate on C4) is the positive terminal.

    D1 is shown as an MURS320 or 1N5404, but the MURS320 marking is only there because the simulation software doesn't have the 1N5404, which is the part I recommend. D2,3,45 are 1N914 or 1N4148 glass diodes.

    There are four transistors. Q1 and Q3 are 2N3906 types (PNP) and Q2 and Q4 are 2N3904 types (NPN). These should be readily available anywhere but you can also use BC557B for the PNPs and BC547B for the NPNs.

    U1 is a TL431LP device. It looks exactly like a transistor but it isn't one.

    You will need to download the data sheets for the transistors and U1 to figure out which wire is what. These data sheets are available through Digikey. Do a search for the part number, and you'll see a column of PDF symbols near the left side of the results table. These are links to the data sheets. These components are available from several manfacturers; they are interchangeable.

    The resistors marked K1 and K2 on the diagram with resistances of 3840 ohms actually represent the coils of the two relays, K1 and K2. These relays must have a 24V DC coil and a rated coil current no higher than 10 mA (milliamps), which corresponds to a coil resistance of 2400 ohms or more.

    Two suitable relays available from Digikey are the Omron G5V-1-D and the TE Connectivity V23026A1004B201 but you can use a local alternative as long as its coil is rated for 24V DC with a resistance of 2400 ohms or higher. You'll need the data sheet too. The marked resistance, 3840 ohms, matches the Omron relay.

    The relays only need to have an SPST (aka SPNO) contact arrangement. The Omron and TE Connectivity relays both have an SPDT (aka SPCO) contact arrangement, so you need to connect to the COM (common) and NO (normally open) terminals. You can also use DPDT (aka DPCO) relays; use the COM and NO connections on one side.

    The wires from the relay contacts connect in parallel with the START and STOP buttons on the machine to be controlled.


    That's all the special information on the components.

    Here's a circuit description. Don't worry if you don't understand all of it.

    The graph shows the circuit's operation over a six second period. Incoming power is applied for the first second, and removed for the next second, and the cycle repeats three times.

    The blue trace, V(n001), represents the mains supply. It is ON for one second, then OFF for one second, and the cycle repeats two more times. This demonstrates the generation of the relay pulses three times each.

    R1 provides a load to discharge the output capacitor(s) in the power supply, so that when the power supply loses its AC power source, the output voltage will drop within a reasonable amount of time. This is needed because the circuit monitors this voltage to detect loss of power so it can pulse the STOP relay. R1 ensures that the voltage falls reasonably quickly.

    R2, R3 and U1 are the voltage monitor that detects falling input voltage. R2 and R3 form a voltage divider that provides a fixed proportion of the input (power supply) voltage into U1, which has an input voltage threshold of 2.5V. This translates into a power supply voltage threshold of around 18V.

    When the power supply voltage is greater than the threshold, U1 pulls its output (the K node) down to 0V, which turns Q1 ON. R4 provides positive feedback (hysteresis) to the measurement node so that the circuit switches quickly and cleanly.

    The green graph trace, V(k), shows the voltage at node K relative to the 0V rail.

    The voltage at noce C swings up to +24V when the input voltage is greater than around 18V, and down to close to 0V when the input voltage is less than around 18V.

    The red trace on the graph, V(c), shows the voltage at node C relative to the 0V rail.

    The 0V rail is interrupted by D1. The 0V rail right of D1 is named 0VB. C1 provides smoothing between +24V and 0VB. This allows the relay coil drive circuit for K2 to start with about 24V even when the input voltage has dropped significantly, so K2 will close reliably.

    When power is applied and node C goes positive, current through C2 and R8 turns Q2 ON, applying about 24V across the K1 coil. When C2 has charged up, Q2 turns OFF, and K1 drops out. This generates the START pulse from K1.

    The pink trace on the graph, I(K1), shows the current pulse through K1 when power is applied.

    C3 and associated components delay detection of low voltage. This is needed because when the circuit is initially powered up, as the input supply voltage rises, U1 will briefly detect that the voltage is below the threshold. Once the incoming voltage has passed the threshold, U1 will pull node K down, and Q1 will pull node C high, but node C will be low briefly during power-up. R10 and C3 form an RC delay circuit to prevent Q3 from turning ON during this brief power-up pulse.

    When the incoming power supply voltage has dropped below the 18V (approx) threshold and node C has gone low long enough for C3 to charge to Q3's base-emitter threshold voltage, Q3 turns on, and node D goes high.

    The cyan trace on the graph, V(d)-V(0vb), shows the voltage at node D relative to 0VB, i.e. the voltage across R12.

    This rising voltage is coupled through C4 into Q4 in the same way that node C is coupled into Q2. This causes Q4 to turn ON briefly and produce an activation on K2, operating the STOP button. The supply for K2 is provided by C1, which discharges somewhat during the pulse.

    D2 and D4 protect Q2 and Q4 from negative base voltages, and D3 and D5 protect Q2 and Q4 from back EMF from the relay coils.

    Feedback, opinions or suggestions, anyone?
     

    Attached Files:

  11. oliver

    oliver

    11
    0
    Jan 8, 2014
    Thank you KrisBlueNZ,
    I must apologize for not responding sooner but did not get a email from the forum
    It was only by chance that I decided to check that I found your reply

    I will order the parts today and will start building the circuit.

    Thank you again for your assistance, will give you feed back as soon as its done

    Oliver
     
  12. oliver

    oliver

    11
    0
    Jan 8, 2014
    I got all the parts today and after some checking and re-checking I turn it on and it worked like a charm

    I was so exited I can hardly wait to go and install it tomorrow

    Thank you again

    Oliver
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That's great!

    You're welcome :)
     
  14. oliver

    oliver

    11
    0
    Jan 8, 2014
    Hello Krisbluenz, All is working great, just a small question can i eliminate the transformer and use a 7824 with a rectifier strait off the 220 volt ac supply to save space???
     
  15. davenn

    davenn Moderator

    13,348
    1,774
    Sep 5, 2009
    ahhh, no

    what do you think will happen to a 24V regulator getting 220V instead of a max of ~ 35V ?

    Dave
     
  16. oliver

    oliver

    11
    0
    Jan 8, 2014
    Hello Dave,

    BANG lol

    any ideas what i can use...
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    You should be able to find a small switching converter that will produce 24V from your mains supply. It doesn't need to be very big. A rated output current of 100 mA would be enough, which is 2.4 watts. You should be able to find something small and relatively cheap.
     
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