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Temperature intensity meter

  1. Dec 14, 2005 #1
    I've attached a circuit schematic for the temperaure intensity meter...
    R1 is the PTC
    the first 555 works as monostable so it triggers second 555 to produce Astable pulses with given frequency depending on the values(R3 and R3 and C4)

    anyway i want to ask if i have to supply my second 555 with 5v supplier or not... cause in my circuit i've connect pin 3 of first 555 directly to pin 4 of the second 555 and didn't supply second 555...i've shorted pin 4 with pin 8 ( for the second 555)

    want to know your viwpoints, and any suggestion will be appreciated!

    thanks in advance!

    Attached Files:

  2. jcsd
  3. Dec 14, 2005 #2


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    Staff: Mentor

    Each 555 needs its own power supply. Also, I wouldn't mirror the 2nd 555 horizontally on the schematic like that -- it makes it harder to follow.
  4. Dec 14, 2005 #3
    Another question!
    I'm so confused about supplying 7447 in my circuit... I know that i have to power it with Vcc on pin 16 and ground the pin 8 but what about the pins such as RB1 and LT (3,5), please help me!

    what about some pins for other ICs in this circuit such as LS393 and 7408
    any suggestion will be appriciated!




    best regards!
  5. Dec 16, 2005 #4


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    Homework Helper

    The other connections can be left unconnected. They look like test connections from their names (zero blanking and lamp test). Unless a pin suggests some type of ENABLE, you can generally ignore extra connections. They are used for debug purposes, but can be useful to connect to a debug/reset circuit.
  6. Dec 16, 2005 #5
    what do you mean exactly by debug purposes? do you mean that I wouldn't connect pin3,5 of 7447 unless for debugging/resetting conditions?! :rolleyes:

    will the supply of this IC (vCC and GND) be sufficient or i have to connect other pins to have correct output values?

  7. Dec 16, 2005 #6


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    Staff: Mentor

    I disagree. It is very poor design practice to leave unused inputs unconnected. You should instead figure out which direction you want to tie unused inputs to keep them inactive, and either hard tie them straight to that rail, or tie them through a resistor like 10kOhms or so.

    The old TTL parts would self-bias unused inputs, so that they generally did not float (but not always). But for sure with modern CMOS logic, if you leave an input floating, it will buzz and float to mid-supply where it causes excessive current drain. Always tie unused inputs inactive.
  8. Dec 17, 2005 #7
    anyway my IC is TTL not CMOS , you think that I should tie unused inputs inactive to insure that it won't get input float...but what do you mean by input floating , i think you meant that the number displayed on 7-seg is not stable and might disappear due to low voltage consumed by it

    can you clarify your thought more?! with all my respect!

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