Astable Multivibrator

Discussion in 'Supporting Physics' started by Technician Q, Nov 6, 2019.

  1. Hi,

    My fellow technician wanted me to do some Christmas decorating so bought some of these:

    https://www.rapidonline.com/rapid-led-christmas-tree-project-kit-70-1000

    It's two multivibrator circuits that are supposed to flash at different rates. Problem is I've soldered it up and one circuit is stuck on. I've inspected solder joints and swapped components (the transistors and capacitors) but I'm now a bit stumped. Been quite a few years since I did electronics so bit rusty on troubleshooting techniques :(
     
  2. Its a long time since I played with mutivibrators, so I'm fishing a little.
    Are both strings of LEDs on the "broken" circuit on? if not are any of the LEDs the wrong way around?

    T
     
  3. Yes. Both arms of the circuit are lit. One mistake I did make was to get the capacitors mixed up. One of the two multivibrators has 22µF capacitor and the other 10µF. But the other circuit works with the same mistake so I think it's probably something else. I last did serious electronics in 1996/97 :D
     
  4. PhysicsSimon

    PhysicsSimon COMMITTEE

    Have made this myself (and copied the circuit too) - if I were you I'd check the capacitors and/or transistors are the right way round & there's no tiny link of solder joining 2 of the transistor legs....(I need a magnifying glass to properly check these days)
     
  5. Yeah it was an absolutely minuscule thread of solder between the legs of C2.
     
  6. PhysicsSimon

    PhysicsSimon COMMITTEE

    Woop - feel dead clever now! :).....and a teensy bit smug:cool: (and yes, technically it was between the legs of your capacitor instead of a transistor like wot I said but I'm claiming it anyway!)
     
    Tonytech and chris vaudin like this.
  7. Would not have been able to help but I have added one to our Rapid order. It's research of course not just a prep room christmas decoration. I will offer it up as a suggestion to the electronics teacher as a bit of fun for the kids, think this will put me in the DT techs bad books as I'm sure he'll be called on to make an in-house version.
     
  8. They're pretty easy to make - as long as your solder joints aren't messy! Different resistors and capacitors create different speeds of flashing. They only work because real life isn't perfect.
     
  9. PhysicsSimon

    PhysicsSimon COMMITTEE

    I cut out a cardboard tree adding LED's & circuit on the reverse, sized to just fit in the window panels we have in our prep room door - quite nice in that the 4 strings of LED's are slightly out of phase so it looks like the timing is slightly changing

    On the other hand no-one seemed particularly impressed (except me) - I'll dig out again soon
     
    Technician Q likes this.