Connect with us

555 sequential timer circuit

Discussion in 'General Electronics Discussion' started by RobSmith, Dec 20, 2011.

  1. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Hi All,
    I have a 1940 Lister diesel charging set which uses various large solenoids and oil filled dash pots to control the startup sequence.

    I was wondering if I could make a small circuit that could perform the startup and stop sequences. The start up sequence is something like.... Turn on fuel / engage starter contactor (approx 6 seconds) / switch over compression changeover valve / switch in generator contactor etc...

    I found various 555 sequencer circuits on the internet, the 555 data and other bits of information and developed this circuit....

    [​IMG]
    [​IMG]

    I started with one 555 timing an interval and just switching two LED's on and off.
    I chose 12v as my voltage to work with as I can then switch relay coils on the output.
    Then I added a second timer section and linked them to alternate between section 1 and section 2. It could have been more than 2 sections but all I wanted to do was prove it worked.
    I then rummaged around to find a chunky transistor to switch the bigger loads. I found I had some TIP31A transistors and some TIP122 transistors. Looking at the data sheets the TIP122 seemed to have a much bigger gain so a small input and handling a big output.
    That has a maximum 5v Base/Emitter voltage so I decided to add a potential divider from the output of the 555 to 0v to give about 4v for the TIP122 as the 555 output is 10.6v.
    With the TIP122 connected Base / Emmiter voltage comes down to 1.4v over the transistor so maybe the potential divider is not needed but I left it in there to ensure the transistor did not see any more than 4v.
    This is working well and I have been triggering a relay and another solenoid with it. This load is only about 0.5A so have not tried out getting the full 5A out of it yet.
    The next problem I had with it was when I was using a piece of wire to cause a trigger on the first section, where my start switch now is, it would also trigger the second section. I tried a 100R resistor and that stopped it triggering both. I tried a 1.5K resistor and that also worked so I settled on a 1K resistor as I have more of those than any others.
    I then tidied up my scrawled mess into the image above.
    It would be great if some of the more knowledgeable folk on here could have a scan over the circuit to see if there is anything to comment on.

    Rob
     
    Last edited: Dec 20, 2011
  2. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    I am just trying to figure out if I can get photobucket to give me a good image.
    This has come out rather fuzzy.
    Edit: I can't figure out how to get higher resolution images uploaded. I can email it to anyone who want to have a look whicle I try to figure it out.

    Edit: I just noticed I managed to post this thread twice. Can an Admin person delete the other one?

    Rob
     
    Last edited: Dec 20, 2011
  3. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    save your images as .gif's, this will allow for larger resolution photos with less overall size. If not post two pictures.
     
  4. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    I will try it.

    Edit: TA-DA see above.

    Rob
     
    Last edited: Dec 20, 2011
  5. duke37

    duke37

    5,227
    718
    Jan 9, 2011
    Comments

    1. The 4V maximum on the transistor I think is the maximum b-e voltage with the base negative. This will not apply here.

    2. The transistor needs a specific input current, this can be obtained with a single resistor from the 555.

    3. A junction transistor will drop a volt or two, so dissipating heat. A fet can be turned on to provide a very low resistance so will need a very little heat sink.

    4. You should have catch diodes across the solenoids to suppress high inductive voltages.
     
  6. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Hi.
    Thats great. Regarding No4 in the data sheet for the TIP122 there is an 'alternative circuit'.
    See: http://www.fairchildsemi.com/ds/TI/TIP120.pdf
    This shows an internal diode.
    Should I place an additional one across the output in a similar manner?

    Edit: Regarding No1. Could I just run the output of 10.6v through a current limiting resistor into the TIP122? I think that is what you mean with No 2.I don't understand everything in the data sheet so how much current should I feed it with. In this first attempt went for about 10mA as the gain of 1000 would provide more than the 5A max output so didn't see any reason to go for more current. There are 'switching off' limits of 2mA or so so went above these. Does that sound sensible or not?

    Rob
     
    Last edited: Dec 20, 2011
  7. duke37

    duke37

    5,227
    718
    Jan 9, 2011
    The data sheet says drive with 20mA to provide 5A. The drop across the transistor will be 4V so there will be 20W dissipated - a lot.
    An IRF540 fet will have resistance of less than 0.1 ohm when turned fully on, so loss = 5 * 5 * 0.1 = 2.5W

    The catching diode must be connected across the solenoid, not the transistor to stop the collector/drain rising above the supply voltage on turn off. The diode will need to pass 5A for a very short time so a 1A diode (1N400x) should be adequate.
     
  8. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Hi,
    That is great. My computer just had a freeze up monet so needed a restart..hence going offline for a bit.

    I spotted where you get the 20mA to provide 5A in the data sheet so that makes sense.
    For the last little while I have been looking at 'catch / flyback' diodes on the internet and realised they should be across the solenoid coil. I have quite a few (1N400x) diodes of all different sizes I will see what I have.

    With the TIP122 I can see in the data the Collector/ Emitter saturation voltage is 4v. Does that mean across my output I will be only seeing 8v of my 12v supply?

    I will have a look at what an IRF540 is.

    Rob
     
  9. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Hi,
    The IRF540 looks like it might be a much better option if the voltage drop over it is very small and the energy wasted as heat is far less.
    I found the data sheet.. http://www.vishay.com/docs/91021/91021.pdf
    I have never used a MOSFET before so unsure how it is used.

    Can I just replace my TIP122 for one of these?
    And are the 'equivalent' connections Base - Gate, Collector - Drain, Emitter - Source.

    Looking at the data and grasping the odd numbers it looks like I need to feed it at least 4v but less than 20v to get over the Gate-Source threshhold and with a current of more than 0.250mA.

    Is that right?

    Rob
     
  10. duke37

    duke37

    5,227
    718
    Jan 9, 2011
    Yes, if you lose 4V in the switch, you will only get 8V in your load.

    The equivalent connections as you say are:-
    Base = Gate
    Source = Emitter
    Drain = Collector

    12v is good to turn the fet fully on so connect it to the output of the 555 without a voltage divider. A gate resistor of about 100 ohm is recommended to inhibit oscillation during the transition. The current going into the gate in the steady state will be so low as to be unmeasurable.
     
  11. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    That is great.
    We went out Christmas shopping today :eek:(
    I did a search on the maplin website before we went and found there was a Maplin in Peterborough and they had two IRF540 in stock.
    I now have those here and will have a play later.

    Rob
     
  12. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    Must be nice to live close to an electronics supply house. I am seriously jealous.
     
  13. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    It is the first time I have been in there.
    Rob
     
  14. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Here is the latest incarnation.
    I now have the IRF540 installed.
    There is very little voltage drop over it so the relay coil sees very nearly the full 12v which is good.

    [​IMG]

    [​IMG]

    Before I removed the TIP122 I had in there before I measured the voltage drop across that.
    That was only about 0.6v so the relay coil was seeing around 11.4v. I could not find the 4v voltage drop mentioned.

    Rob
     
    Last edited: Dec 22, 2011
  15. duke37

    duke37

    5,227
    718
    Jan 9, 2011
    The 4V drop on the TIP122 will be there if supplying 4A, your relays presumably take much less than this.

    Do you need a relay? I was under the impression that you would be driving the solenoids directly.

    A little simplication can be made by using a single 470R resistor to feed the common led connection.
     
  16. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Hi,
    The relay I was playing about with was only drawing about 0.5A. It just gave me a bit of a load to try it out with.
    The solenoids are very heavy and covered in oily filth so not realy suitable for the dining room table.
    Edit 2: I may well have some thumping great relays in there too as an intermediate point. It is this that will be being started:
    [​IMG]

    Yes... now that you mention the 470R resistor I can see how that would work and save having both.

    Edit: Had a thought about just using the one 470R resistor and thought that it would just create a path across the two LEDs from 12v to 0v.

    Edit 3: Just realised I should have the catch diodes on the circuit too...... back to the diagram again.

    Edit 4: Updated with catch diodes in:
    [​IMG]

    [​IMG]
    Rob
     
    Last edited: Dec 22, 2011
  17. duke37

    duke37

    5,227
    718
    Jan 9, 2011
    You are right about the resistors, my thought was a good way of zapping the leds!
     
  18. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Fairly happy with the circuit now.

    I am going to figure out a little veroboard layout allowing me to make up single stages that can be 'stacked' one after the other to make a chain of these as long as I require.
    I can then also use them for alsorts of other little circuits with my two small boys.

    Is etching a proper circuit difficult? and costly?
    That would be a nicer way of doing things.

    Rob
     
  19. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    I have updated my layout, made up a vero board layout.....

    [​IMG]

    and made up a couple of circuits.
    My camera is useless under artificial light so will take a picture tomorrow.

    Rob
     
  20. RobSmith

    RobSmith

    56
    0
    Dec 16, 2011
    Something seems to be cacheing an old version of the image.
    Hopefully it will sort itself out.
    The only difference is I added an extra linking wire across two tracks on the 12v line so the vero board can take more current.
    Rob
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-