1. Overclocked2300

    Thread Starter Senior Member

    Apr 24, 2005
    124
    0
    So ive built this following circuit, but with out the 555 timer and such. When I first started it up it started going on its own (the leds were moving sequently like they are supposed too, but fast. I have pin 14 floating) But when I go to put pin 14 to ground it stops. If I put a clock signal on pin 14, will it slow it down so I can see it move ( a slow clock signal)?

    [​IMG]

    I do relize your not supposed to leave pins floating on a CMOS chip, but its the clock.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Hi overclocked2300,

    The 4017 is behaving just as I would expect it to with the input left floating. Any time you are designing with CMOS IC's it is a good practice to tie all unused inputs to the appropriate logic level.

    Once you connect the 555 timer up to the 4017 it should operate just fine.

    hgmjr
     
  3. Overclocked2300

    Thread Starter Senior Member

    Apr 24, 2005
    124
    0
    So even if I get a slower clock it will work fine?

    Wait a second though, isnt the 555 timer TTL logic? I thought CMOS and TTL dont get along quite nicely.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You are correct in your statement that CMOS and TTL are not intrinsically compatible. There are however fairly straightforward measures one can take to work around the incompatibilities. There is an entire series of logic devices with the 74HCT prefix that was specifically designed to bridge the transistion from TTL to CMOS based designs. Generally TTL and CMOS devices can be used together as long as you are not pushing the circuit to work at the high end of their intended operating frequency, temperature, and device fan-out.

    As for the 555 timer, it is available in both a bi-polar and a CMOS version. LM555 is a bi-polar version and the LMC555 is the CMOS version.

    Actually the bi-polar version of the 555 timer device is TTL-compatible rather than being TTL. One of the characteristics of TTL is that it is designed to run on +5V power only. The 555 timer is designed to run at voltages as high as +15V.

    The reason that you are seeing the 4017 run when you leave its clock input flapping in the breeze is that its very high input impedance makes it act like a tiny antenna picking up anything that looks like a signal. Once you connect it up to a low impedance signal source such as your 555 timer it should settle down nicely.

    The schematic you have in your posting does not indicate the use of any power supply bypass capacitors so I would recommend that you get at least a couple of 0.1 uFd caps that have a voltage rating of 25 volts or higher and connect them to power and ground as physically close to the each of your IC's as is practical. This will help eliminate the potetntial for supply noise to cause a false triggering of either device.

    A larger valued electrolytic or tantalum cap across the power supply (say 10 uFd or more) would also be a good idea. It will help keep the power supply bus a bit cleaner.

    Hope this is helpful.

    Good Luck,
    hgmjr
     
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