Author Topic: Active Low  (Read 1752 times)

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Offline TravelingPencilTopic starter

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Active Low
« on: May 02, 2007, 04:45:35 PM »
What is active low and how do I get/use it? I know it is used with 555 timers and is 0 volts but what is it? ???

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Re: Active Low
« Reply #1 on: May 02, 2007, 05:23:24 PM »
from first link in a google search:  :P
http://www.kpsec.freeuk.com/555timer.htm

Quote
Trigger input: when < 1/3 Vs ('active low') this makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M.
Threshold input: when > 2/3 Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance > 10M.
* providing the trigger input is > 1/3 Vs, otherwise the trigger input will override the threshold input and hold the output high (+Vs).

Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k.

Control input: this can be used to adjust the threshold voltage which is set internally to be 2/3 Vs. Usually this function is not required and the control input is connected to 0V with a 0.01F capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.

The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V when the timer output is low and is used to discharge the timing capacitor in astable and monostable circuits.


and the 3rd result:
http://www.iguanalabs.com/555kit.htm

Quote
Pin 2 (Trigger) is the 'on' switch for the pulse.  The line over the word Trigger tells us that the voltage levels are the opposite of what you would normally expect.  To turn the switch on you apply 0 volts to pin 2.  The technical term for this opposite behavior is 'Active Low'.  It is common to see this 'Active Low' behavior for IC inputs because of the inverting nature of transistor circuits like we saw in the LED and Transistor Tutorial.

Offline TravelingPencilTopic starter

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Re: Active Low
« Reply #2 on: May 10, 2007, 05:27:18 PM »
Srry, forgot to google it, i have seen that third link before but thanks

 


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