Connect with us

Kids Bike Timer circuit not working, Why?

Discussion in 'Electronic Basics' started by Tristar, Aug 24, 2005.

Scroll to continue with content
  1. Tristar

    Tristar Guest

    Kids Bike Timer circuit not working, Why?

    OK, I checked and double checked all my solder joints and can't find
    anything out of place or poorly soldered. I ran a check on the 555
    timers before adding them to the board. All IC's are soldered directly to
    the board. This is a two sided board and I found it about impossible to
    solder a IC socket on both sides of the board.

    Here's what I'm doing and here's what is happening/not happening.

    Depress SW2 once, LED lights up. (this is an indication that the missing
    pulse detector is working) As long as SW2 is pressed once every few
    seconds the LED will remain lit. (SW2 will later be replaced by a reed
    switch triggered by a magnet attached to the wheel of the child's
    exercise bike.)

    OK, I press SW2 and then SW1 (SW1 should start the countdown) and then
    keep pressing SW2 every few seconds to keep it active.

    What is supposed to happen at 30 seconds is that contacts at CN1 are to
    close triggering a small prerecorded voice chip announce the successful
    completion of 30 seconds of pedaling. 30 seconds later CN2 should go off

    I can successfully set the length of time the missing pulse detector will
    tolerate with the pot at R12.

    Nothing else seems to happen. I've got power and ground to the IC's. I
    did notice the two 555's get warm but not real hot.

    Any ideas? I'm not sure where to start troubleshooting this.

    I'm posted a capture of the board and the schematic John Fields and the
    rest of you have helped me put together
    at alt.binaries.schematics.electronic

    Thanks for your patience here!

  2. John Fields

    John Fields Guest

  3. Tristar

    Tristar Guest

    The two 555's are cmos IC's. They are both mouser part number 511-TS555CN.

    I'm running 6 VDC as input voltage from a gel cell battery.

  4. Tristar

    Tristar Guest

    Sorry, no scope. Just a pair of multimeters, digital and analog.

  5. John Fields

    John Fields Guest

    You need to leave a little bit of the message you replied to in your
    reply so we can see what you're replying to.

    OK, first we need to find out why the 7555's are running warm.

    Set all the pots at their midpoints and measure all the voltages on
    the 7555's, relative to ground. With a 6V supply you should get, on
    both chips:

    1 0
    2 6
    3 0
    4 <0.4
    5 4
    6 6
    7 6
    8 6

    The voltage on pin 4 will be greater than 0.4V, and that's a design
    error, but it won't affect whether the chips run hot or not, so
    we'll deal with that later. Also, the RC network on U1 has a long
    time constant, so if you measure the voltage on pins 2 and 6
    (they're supposed to be in parallel) soon after you power up the
    circuit, the voltage may not have risen to 6V by the time you get
    there, and if it hasn't you should see it rising slowly. Be sure to
    use your digital meter to make the measurement, since the current
    source for the cap is the 2 meg pot cranked down to 1 meg and if you
    use your analog meter (which I assume has an input impedance of
    something like 20000 ohms per volt) It'll just swamp the capacitor
    and cause the voltage to start falling as soon as you put the probe
    on pin2.

    Hmmm... It just occurred to me that the reason for the 7555's
    running warm might be that if you haven't adjusted the pots to half
    scale then they could be sitting anywhere, and if they're sitting at
    minimum resistance then current will just be _pouring_ into the poor
    little 7555's. Ouch!!! You might want to adjust the pots for
    half-scale and try to run the thing again. It might just work if the
    7555's haven't been toasted.

    Post back with what you find.
  6. Tristar

    Tristar Guest

    OK, here's my readings.

    They may have been getting hot due to the position of the pots. with the
    pots centered they seem to stay cool.

  7. John Fields

    John Fields Guest

    That's what it was, then.

    There's still a problem, though. Pins 6 and 7 should be at 3mV and
    pin 5 should be at about 4V. It looks like pins 6 and 7 are
    parallelled in your board capture jpeg so I suspect you made a
    transcription error. Why don't you go check again and see what you
    get, since everything else looks OK . The 6V on pins 6 and 7 that I
    put on my chart was wrong. I forgot that with the output (pin 3)
    low the discharge MOSFET is turned on so it can discharge the cap,
    so what you were reading (3mV) is good.

    Post back with what you find, and if everything is OK we'll go on to
    the next step.
  8. Tristar

    Tristar Guest

    John, you are right, Transcription error.

    This is what I should have posted.

  9. John Fields

    John Fields Guest

    I'm getting confused.

    From your last post, if it was a transcription error, this is what
    you should have:

    1 0
    2 6
    3 0
    4 ~ 0.6
    5 4
    6 3.0 mv
    7 3.0 mv
    8 6

    No need to post another chart, if that's what you have just say so
    and we can continue.

    And _please_ bottom post so we can keep track of the chronology.
  10. Tristar

    Tristar Guest

    Sorry, Yes, I have the values you charted.
  11. John Fields

    John Fields Guest

    Please bottom post.

    OK. Since U1A seems to be OK, The next thing to do is to check out
    U5A and the wiring to U1 and U2. First, with power down, make sure
    you have continuity between U1-4 and all the other points shown
    connected to it on the schematic. Then, with R12 adjusted to
    mid-scale, apply power, connect your voltmeter to U1-4 and press and
    release S2. The LED should light and the voltmeter reading should
    go high immediately (rise to 6V) and stay there for about seven
    seconds, then the LED should go off and the reading should go low
    (fall to ~0V (0.6V is OK)) and stay there.

    It that works, the next step is to test the timer.

    Connect your voltmeter to U1-3 and press S2. The LED should come on
    and the voltmeter should read 0V. While making sure that The LED
    stays lit by pressing S2 every five seconds or so, press and release
    S1. U1-3 should go high immediately and stay there as long as you
    keep pressing and releasing S2. If it does, after 20 seconds or so,
    stop pressing and releasing S2. The LED should go out in about
    seven seconds and when that happens U1-3 should go low and stay low.

    Try all that and please bottom-post pack with what you find.

    P.S... If you want to make things a little easier, you could build
    an astable multivibrator with the other 7555 you've been playing
    with and connect its output to the junction of R11 and S2. That
    would simulate the button pressings and relieve you of the task of
    having to continuously press and release S2 while doing other things
    at the same time. It's a very easy circuit, and you can find it in
    Figure 1-A of the 7555 data sheet, located at:

    Use a 1 megohm resistor and a 1µF capacitor an you'll get about 1.5s
    out which will be fine.
  12. ehsjr

    ehsjr Guest

    Tristar wrote:

    You may have missed my reply elsewhere. I think you have a
    problem where the output transistors do not appear to have a
    + source connected to the collectors, if I recall the post.

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