Work Around Using a 9V Relay and A 555 Timer?

Discussion in 'General Electronics Chat' started by ajm113, Feb 20, 2011.

  1. ajm113

    Thread Starter Member

    Feb 19, 2011
    Hello all I'm a beginner here and I have basic knowledge of creating circuits a little, anyways I'll cut to the chase, so apologizes if I ask a low level question, I'm pretty much learning as I go about creating my circuit from scratch with no specific guide.

    I'm currently using a 555 timer IC to hold one byte of memory and I want this 555 timer to trigger a 9v Relay, but it doesn't seem the output is high enough to trigger the relay, the Volts I'm getting is 6.8 to the relay and I was wondering if there was a good way to boost up the volts a little.

    One method I was thinking is connecting positive output from the 555 IC to the relay and also connecting a positive line that's using a resistor that will lower the volts 3/4 way to the positive side of the relay as well to add a little boost of power, but I'm not sure if it's the best method...

    Can someone enlighten me and give me a little more information on if maybe this is the best method or simply wait another 4 days to buy a relay that's 3V+ off the internet?

    Thanks, Ajm113.
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Post the schematic.
  3. ajm113

    Thread Starter Member

    Feb 19, 2011
    Okay sorry if it appears confusing, but I don't mind answering any questions.
  4. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    1) You have a resistor labelled "+Adds V to Relay" that shorts the output of the 555 to +9V that probably destroyed the 555.
    2) You have something (a diode drawn wrong?) between pin 8 and +9V. It reduces the voltage by 0.8V.
    3) A very important diode is missing across the relay coil to arrest the hundreds of volts it develops when it turns off.
    4) You have pin 2 shorted to ground.
    5) You do not have a timing capacitor and a timing resistor.

    You should look at the datasheet of the 555 timer. It shows that its output goes only as high as 2.25V less than the positive supply voltage when the load is 100mA.

    I bet your 9V battery is almost dead anyway.
  5. SgtWookie


    Jul 17, 2007
    9v pp3 "transistor" batteries seem to be very popular with beginning hobbyists, but they have very limited capacity and are generally a very expensive way of powering projects.

    A typical 9v pp3 battery might be rated for 500mAH, which means a load of 25mA current over a 20 hour period. Relays are notoriously power-hungry, and just energizing the coil can far exceed that 25mA current.

    You would be better off to use multiple AA rechargeable batteries in (a) battery holder(s), available from many places online, and even your local Radio Shack store if you're in the States. You can connect the batteries in series for higher voltages.

    Besides the output voltage being low, can you explain what you want the 555 timer to do in the circuit? Is it supposed to be switching the relay coil on and off at some kind of interval?
  6. ajm113

    Thread Starter Member

    Feb 19, 2011

    Thanks for the reply, Yeah on the second I have it drawn wrong (This is my second schematic I've ever made), but my 9v and 555 run fine still, 3: I'll add one to keep my relay safe.

    4: Actually pin 2 unless I have it backwards, but I connected a line from my positive through the 10K and connect to trigger and back to ground so it will set a bit to 1 on the 555 timer, I know that connection doesn't seem to have a point, but there is more to it then what I drew. That line will be used to determine if someone triggers a switch or button.

    5: Can explain a bit more how that will help and how to use it? (Will it just save power for my 9V from the 555 and relay?)

    Well the reason why I want to use a 9v is just the tip of the mountain (more like hill), but I plan on using 2 9Vs and I wanted to be save from frying anything with out making a full basic working circuit. I plan on creating a very small and basic control panel those alarm systems use and I plan on using two main devices that more then like use 8v-20v. (Motion Detector, Siren, and maybe plans of using a keypad when I figure out how the data is transfered.)
  7. Commander#1

    New Member

    Jan 22, 2011
    Welcome, Ajm113 -
    It is rather apparent your need for information, so, may I make a suggestion? If there is a Radio Shack near by,
    check out their "Engineer's Mini Notebook #1" by Forrest M. Mims III (about $15) and "Getting Started in Electronics"
    (about $20). They can also be ordered on line from Radio Shack. You've got a good attitude in that you're forging ahead
    with what little you know. Pay attention to what symbols are used and how they are laid out. You can also go on line
    by typing in the words "555 timer" in your favorite search engine. I have found Tony Van Roon's explanations most
    helpful. When you find the relay you want, pay attention to the COIL operating voltage and current. The bypass diode is
    wired in parallel to the coil so that the operating current flows through the coil and not the diode, but when the
    coil current is cut off, the ensuing inductive spike is routed through the diode to ground and not back into the 555's output.
    Now, the 555's output can handle about 200mA - but that's pushing it kind of hard. If your set on 9Vdc, I found a couple of
    9Vdc relays in the Jameco catalog ( that draws only about 40 - 41mA which the 555 can handle quite
    readily without a series resistor. As for that 9V battery - it was originally developed for the early shirt-pocket transistor
    radios - thus the name - 9V TRANSISTOR battery. You want to add a relay and some "bells and whistles" - all of which
    are - as mentioned elsewere - power hungry little critters. Given this entourage, that 9v battery will run out of gas
    rather quickly!! Even if you use 2 such batteries, you will still have longevity problems. May I suggest using a 12Vdc
    lantern battery. It will last a VERY long time - and it has screw terminals for easy use. This will also make available many
    more types of relays - if you happen to use one of lesser voltage, just use enough series resistance to the coil to maintain
    it's rated current - the voltage will take care of itself. If you use a multi-pole relay, one pole can be used to maintain the
    relay's energized state, thus requiring the 555 to just be a monostable one shot.
    I present this as simply some food for thought. I hope it has been of some help. As you learn more about the 555, you will
    begin to see your mistakes and - more importantly - how to correct them. It's one thing to have a bunch of circuits thrown
    at you - yea, you'll get something working; but you'll be a better hobbyist/technician if, in your studies, you learn
    WHY things work the way they do.

    Here's rooting for you!!!

    Phil Potter, Pomona, Calif.
  8. ajm113

    Thread Starter Member

    Feb 19, 2011
    Thanks Commander#1 for your reply, I found it quite helpful! :)

    I have a RadioShack near my house, but they don't sell as much components as they did back then even the books RadioShack sell aren't there anymore.(I guest the demand for circuit parts have went down over the years.) Anyways I'll look into those books and look through more documentation about the 555 so I can get a much better understanding of how it operates.

    If I may ask you a few questions though about your reply:

    Is there a difference if I used a Lantern battery then a Cell Non-Spillable Sealed Secondary 12V 7.5Ah 2-Pin? (I'm guesting how long it will last plays a role too?)

    And ummm, most of your reply pretty much covered everything and I have a small question about Capacitors that kinda fits in this topic. :)

    I have a old wheelock 12v DC Series 7002T-12; I'm looking though it's parts and I've noticed a big red disk capacitor connected from and back to the positive and ground terminal block screws.

    I was wondering if this was used to redirect high voltage back since it appears there is a diode on the positive side.(I'm not sure where you can find the schematic for this loud thing, but I was wondering what the capacitor was for if it just looped right back to ground.)
  9. Commander#1

    New Member

    Jan 22, 2011

    Hi, ajm113 -
    That battery is, in deed, a Sealed Lead Acid (SLA) battery. It is rechargeable where as the lantern battery is not, thus, making
    the SLA battery a little more expensive, but, on the other hand, you only buy it once and then recharge it numerous times
    compared to the lantern battery that gets tossed when it finally dies and you buy another, then another, then . . . ect. (Enter
    the crabgrass of electronics - the trade offs!!!!). By the way, if the place where you are using this circuit is within - say -
    20 feet of a 120Vac mains socket, you might want to consider a small power supply (home built - a nice touch! - or a wall
    wart) instead of batteries. As far as their differences - they both produce 12v at a nice current. Each has their own
    composition/logistics/and costs. it's your call.

    As to the "wheelock 12v DC Series 7002T-12" - I have never heard of this unit before - so - I Google'd that entire
    phrase and - wha law (sp?) - it appears to be a security or fire alarm. In this case, it's operating on 12Vdc and, curiously,
    the spec's for the rechargeable battery involved are quite similar to the SLA battery you mentioned above. Given the
    bells and whistles you want to add on at a later time makes me think you want to make one of these things yourself.
    Excellent! Just don't be in to much of a hurry - you have much to learn along the way. This would make a good learning
    experience for you. Now, what looks like a capacitor (the red disc) very well could be one. It's position would indicate
    that it is being used as a 'decoupling' capacitor. In other words, it will pass any spikes, A.C., etc. to ground that have
    the audacity to wind up on the positive D.C. line. How do they get there? Ha, ha, ha - let me count the ways. No,
    seriously, when that unit turns on, all the D.C. lines become a forest of electronic noise. And if that noise gets into
    the control electronics, all kinds of bad things happen!!! So, you'll probably find more of these type of capacitors strewn
    around the circuits for this one reason. You may even find two or three of them situated side by side (in parallel)
    because capacitors are somewhat frequency sensitive. The larger cap's (more uF's) handle lower frequencies and
    the smaller cap's (less uF's) handle the higher frequencies - nether one can do the other's job very well.

    I hope my ruminations here are adequate answers to your questions. I am only one of a large group of well versed
    individuals here, so, as you read (very important!)/study/and experiment, do bring on your questions and I'm sure
    someone will answer them. Hang in there - we're rooting for you.

    :)eek: Oh my gosh - the world is - - - round :eek:)
  10. ajm113

    Thread Starter Member

    Feb 19, 2011
    Alright thanks! I'll be sure to buy that SLA battery then when my project is ready for prototyping/beta testing and I think my Dad has a power supply I can use, from what it looks like they aren't hard at all to put together. :)

    And thanks for the information about the Wheelock, it's db is like around 115 if fully powered so yea if I ever test this thing I'm going to be inside with a long cord going outside to it. I just hope I don't flip out the neighbors. :D I never knew capacitor could be used like that, thats very interesting. :) This new stuff is really making me want to go into circuit creation then programming software for Windows and Mac. :)

    And thanks again for your help, you rock! It's great to see dedicated users on this forum to help beginners go to the right direction and encourage them. :)