Resistor vlaues

Discussion in 'The Projects Forum' started by allenpitts, Mar 11, 2015.

  1. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello ACC forum,

    Planning to build an LED display called LED Dice with Slow Down.
    http://www.555-timer-circuits.com/led-dice.html
    1. I get the pin 3 from the 555 timer going to the clock, pin 14,
    on 4017 decade counter. But what makes the clock slow down?

    2. In the upper left hand section of the schematic, in the components
    going into the 555 there are three components that look like
    resistors. Not sure I have got them right
    10 M = 10 mega ohms
    3M3 = 3,3 mega ohms
    4M7 = 4.7 mega ohms
    Is that right. I could find resistors with these values
    at DigiKey but I have never used resistors with such
    high resistance.
    Just want to make sure I have it right before I start building the circuit.

    Thanks.

    Allen in Dallas
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    The high R values are needed for the touch switch. If a pusch button SW were used, R values could come down.
     
  3. allenpitts

    Thread Starter Active Member

    Feb 26, 2011
    77
    7
    Hello Bernard and Forum,

    Sorry about the thread title misspelling.

    Actually had planned on closing the circuit with either a relay or a transistor
    operated by a PIR sensor.
    The PRI sensor, (HC-SR501) outputs a s.s volt signal for about 5 seconds.

    Would the value come down half or three quarters?

    Thanks

    Allen in Dallas
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    1. The frequency of the 555 depends on the 100nF , 10k, & conduction state of BC557. With 1uF discharged, 555 stopped or verry slow. Shorting touch pins, charges 1 uF, turning on BC; 555 @ about 10 k Hz. As 1 uF discharges, 555 slows down over about 10 sec.
    2. Decay rate depends on 1 uF & 10M. If C is increased to 10 uF then 10M is 1M.
     
  5. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    This is a better link to your circuit. Note the modification at the bottom of the link, your link above does not seem to reflect the modification. The circuit is based on a very old circuit. The idea was the 555 would free run in an astable mode so all the LEDs were On/Off so fast they appeared as a blur and when a button was released the oscillator simply stopped wherever it was incrementing the 4017. This newer approach is pretty cool. When the wires are bridged with a finger the action of the top left transistor and the capacitor up there cause the oscillator (555) frequency to sweep down to a slow stop. Nice touch.

    <EDIT> I see as I was slowly typing away at my snail pace Bernard answered the question. :) </EDIT>

    Ron
     
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