Help with relays and delays - low voltage

Discussion in 'The Projects Forum' started by billyjoebob, Jul 29, 2009.

  1. billyjoebob

    Thread Starter New Member

    Jul 29, 2009
    4
    0
    I’m working on a project that requires the following:

    A battery powered siren (9 volt) will sound if an input signal is not received from one of two separate input switches.

    Here’s the wrinkle…

    1) The input switches that I’m using are both normally-opened. Unfortunately, that’s the only type that I can find for my needs. If they were made in normally closed, I could just wire them together in a series and the siren would go off when neither switch was sending a signal (but if either was, it would kill the circuit).

    I’m assuming that I can wire a relay to each switch, to individually convert them from N/O to N/C, but not knowing much about relays I could use some help with the best type, etc.

    Now, wrinkle #2

    I need some form of delay (around a second) before the siren activates. Basically, if an input is not received from either switch for around a second, the siren will sound. I’m thinking the easies way would be to feed the incoming load into a capacitor, but once again…not knowing much about capacitors, I don’t know the best type, kind that would provide a one second delay, etc…

    I’m not sure how relevant my end use is, but I need to wire up gym sneakers, so if either isn’t in contact with the ground for over a second, the siren goes off (my goal is a long story…sports use).

    Any help and specific input would be GREATLY appreciated.

    EDIT-

    I've been doing a lot of research on this, and am wondering if a 555 timer would work for this application? What configuration would I be looking at for a 1 to 1.5 second delay?

    Sorry for the basic questions...I'm new to electronics and the learning curve is tough...
    *
     
    Last edited: Jul 29, 2009
  2. billyjoebob

    Thread Starter New Member

    Jul 29, 2009
    4
    0
    Bump - Anyone out there that can help??? :(
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    billyjoebob,

    What's the current draw of the "siren". If you're using a small 9v battery, I assume that the current draw is pretty low. With a 555 you might look at a power-on-delay circuit here: http://home.cogeco.ca/~rpaisley4/LM555.html#29. With a low current siren, it would just might be used in place of the relay coil. The NO switches would be paralleled, and go between the battery + and the circuit +.

    ken
     
  4. rspuzio

    Active Member

    Jan 19, 2009
    77
    0
    > I’m assuming that I can wire a relay to each switch, to individually
    > convert them from N/O to N/C, but not knowing much about relays
    > I could use some help with the best type, etc.

    To work, the relay will, of course, need to have a N/C contact.
    Other than that, it would be most convenient to pick 9v relays
    so that you can run them off the same battery as the buzzer
    without a dropping resistor. In order to save battery life, you
    might want to pick the relay with the largest resistance you can
    find. (More relay resistance will also mean less capacitance
    needed, so you won't need a big honking capacitor the size
    of a beer can to make the gizmo work. :)) Since you will
    be hooking the output to a 9V battery, you are only going
    to draw 9V at significantly less than 1A, so the ratings
    for the output end are not going to be a problem.

    By the way, you can cut down the part count and power
    consumption by placing your switches in parallel, then feeding
    the output to a single N/C relay. This way, when either of the
    shoes is on the ground, current flows through the relay and
    opens the relay contact but, when both shoes are off the ground,
    no current flows through the relay so the contacts close.

    > I’m thinking the easies way would be to feed the incoming load
    > into a capacitor, but once again…not knowing much about
    > capacitors, I don’t know the best type, kind that would provide
    > a one second delay, etc…

    For a delay of a second, you will need a big capacitor, so your
    options are pretty much limited to electrolytics. The capacitor
    needs to have a maximum voltage rating bigger than the voltage
    you are using and a margin of error for safety. So, for instance,
    if you're using the 9V battery to power the relay, you'd want a
    capacitor rated at, say 12V or more. Unless it's all you can find,
    you don't want too large a rating because that would make the
    for a much larger capacitor (perhaps even the size of a beer can).

    As for the value, that will depend on the resistance of your
    relay, so let me illustrate with an example. The simplest way
    to use the capacitor is to connect it in parallel across your
    relay so that, when both switches turn off, the current from
    discharging the capacitor will keep the relay energized for
    another second. Now, the time constant of a circuit is the
    product of resistance times capacitance, so we will want the
    product of the capacitance times the resistance of the relay
    to equal 1 second. For instance, if our relay has a resistance
    of 500 Ohms, (like the one I just pulled from my junkbox) then
    we will want our capacitor to have a capacitance of 1/500 Farad,
    or 2 mF (a.k.a. 2000 μF), which is a fairly typical value for power
    supply capacitors, so it will be easy enough to get one.

    Hope this helps,
    Ray

    P.S. Even though it isn't relevant for designing your circuit,
    I would be curious to hear the sports story behind it.
     
  5. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    You don't need a relay to drive a 9V siren.

    Try the following circuit. The two switches are normally open. In normal use, both are activated so shorting the capacitor and it cannot charge up. The MOSFET N-CH get no gate charge and the siren will not sound.

    When one or both switch is/are de-activated, the capacitor has a chance to charge up. When it reaches about 4V or more, the siren will sound. This charging up takes time and is controlled by the 1MΩ variable resistor.

    [​IMG]
     
  6. billyjoebob

    Thread Starter New Member

    Jul 29, 2009
    4
    0
    I really appreciate all the input guys. I should have provided a bit more information to help you all help me with the design, but I wasn't sure it was relevant.

    I do work with disable children...many with autism spectrum disorders. One common symptom leads children to walk/run with an altered gait, i.e. their heel doesn't always strike the ground and they bear most of their weight on their toes. There is no physical cause for this...it's is mostly out of habit or due to sensory issues.

    As you can imagine, this creates some problems in playing sports.

    My idea was to come up with a simple switch that can be buried in the kid's insoles (under their heel) with wires that run up the leg and plug into the relay unit. The one second delay is to compensate for the brief second that neither heel is on the ground (as with a normal gait). When they are walking with an altered gait, the buzzer creates an audible stimulus that reminds them of this. Not sure how well it will work, but I wanted to give it a try. Unfortunately, I may have to alter the stimulus since some of the kids are very sensitive to sound (also thinking of a cell-phone type vibrator).

    The power supply is flexible. I can get a buzzer that will take any voltage. I was thinking 9v was most common and would give me a lot of unit life between replacement, but I'm flexible (considering the size of the resistors I'll apparently need for a 9v).

    Simplicity is also paramount. I'm a novice at building these things, so I was thinking of assembling a test unit with the various ideas I get and making a "radio shack" special. If it works, I'll go to one of the online PCB vendors to get a more durable unit or two.

    Once again, any help or thoughts you have are greatly appreciated. I like the above designs and thoughts. I'm just trying to get a consensus for the most simplistic unit I can make.

    This may be overly simplistic, but can I run both switches into a joint N/C relay (if such a thing exists), put a resistor on the line (that would generate the necessary delay) and go with something like that? What size resistor would i need? Would it be easier if I downsize the power supply to a 1.5v battery?

    Sorry for such novice questions but as I'm sure everyone knows, this isn't the easiest subject to self educate on...
     
  7. maverich

    New Member

    Aug 16, 2009
    1
    0
    Hi There

    I was reading through the thread and you seem to be in the same boat i was in a few years ago.
    Have you ever looked into some of the more simple Programmable Logic Controllers (PLCs) on the market? I find them really handy for ironing out a lot of the bugs when im doing any sort of circuit design.
    Basically you do all the design on a PC using a drag and drop interface. No ladder logic anywhere in sight, thank goodness! You can then test the circuit in a simulation mode to make sure it works before you ever have to build anything. Timers and delays are all features already built in. Depending on your budget, they can be pretty expensive, about £110 here in the UK, but the cool thing is that you can just buy the programming software for about £10, which allows you to play about forever and a day, learning what everything does and all the little tricks, without ever having to buy the actual PLC. Once you have the circuit designed, you can just go get the PCB made.
    The benefit of using the PLC is that you can make changes easily later should you ever have to say, change the delay, and it kind of comes in its own neat little box.
    The PLC i use in a mitsubishi alpha2. Cool display on it too so you can get it to display text when the alarm goes off, telling you which input has been made for example, not sure if you in the US, but they will definately have tehm over there too, just google it.
    Might be totally irrelevant info for you and if thats the case i apologise, but i know so much more about electronics now because i started out on these things.
    Maverich
     
  8. billyjoebob

    Thread Starter New Member

    Jul 29, 2009
    4
    0
    RSPUZIO - Check your in-box

    And a big bump for anyone else with knowlege to share...
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,173
    397
    Here is a 555 ckt that should do most of what you need. R1, C1 control delay time; R3, C2 control alert on time. At turn on there will be activation untill a switch ia closed.
     
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