556 and Flip Flop

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jrcfg
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556 and Flip Flop

Post by jrcfg » Thu Apr 22, 2004 12:21 pm

Hi folks, here's a question....
I would like to use a 556 timer to flash an LED for 8 sec then stay on. I was thinking I could use the 556, set up one side astable out through a transister and diode to the LED. The other side would be monostable with an 8 second delay to pulse a 4027 flip flop which would take the first transistor low and a second hi. The second transistor would also be attached to the LED (with strategic diodes in place)to supply steady volt. Does this sound reasonable or do I have a cranial rectal inversion? The flip flop would control the transistors.

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Externet
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Re: 556 and Flip Flop

Post by Externet » Thu Apr 22, 2004 3:55 pm

Hi.
I would say you need nothing at all besides the 556 to make it work as you want. One half times for 8 seconds, the other flashes the led directly at the desired rate until times out.
Miguel
- Abolish the deciBel ! -

jrcfg
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Re: 556 and Flip Flop

Post by jrcfg » Thu Apr 22, 2004 6:26 pm

Am I making this more difficult than it needs to be?

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haklesup
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Re: 556 and Flip Flop

Post by haklesup » Fri Apr 23, 2004 12:33 pm

First half of the 556 is running a fast cycle all the time, say 2Hz (16 flashes in 8 seconds). It is in astable mode<p>Second half is configured as a 1 shot with 8 second period triggered by a pushbutton. Output is high for 8 seconds when button is tapped.<p>NAND the output of these two sections. The NAND gate output will be high whenever the one shot is low or the pulse is low. It only goes off when both the timer outputs are high which only happens when you push the button.<p>This design assumes that the LED was on before you push the button and flashes only during the 8 second period.<p>If you want the lamp to be off prior to your cycle, then you should use the additioanl FF to hold that state (also requires a reset button)then toggle it at the same time as you push the button. Finally, AND the state of the FF and the flasher circuit above to get the output you need. (or NAND with Q bar output)<p>To make an AND gate, use two NAND gates leftover in the 74HCT00 package)<p>Another way is to invert the output of the 8 second one shot and apply that to the trigger input of the falsher section. While this pin is forced low, the LED output will be forced high. When the (inverted) one shot goes high, the pulsing timer can run freely and drive the LED.<p>The transistor may not be needed depending on the drive current for your LED and the output drive current of the NAND gate you end up using. For example the 4011 has IOH of only 4.2mA at 5V but the 74HCT00 has 35mA available to drive your load

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