555 timing 1

[neo11]

HEY!!

I am designing a circuit that reads the output from a device and when the output drops below a specified value, I am to output a pulse from a 555 timer chip. The problem is that a 555 timer operates from the positive edge of a pulse and the output from the voltage comparitor will send out a negative pulse as the voltage at the input drops below the reference voltage.

Neo11

 

[nbucska]

invert the output (with e.g. a 74LS04) or switch the + and - inputs of the comparator. <nbucska@pcperipherals.com>

 

[jbartos]

Suggestion: Please, see your posting &quot;Voltage Switching&quot; June 1, 2001 for my suggestion.

 

[bavariamotors]

check out the 555 tutor pages,

http://www.555

tutorial.com

 

[dbest]

hey
put a inverter to your circuit 7404

 

[melone]

Are you sure that you don't mean that you want to trigger off of the rising edge of a signal? The standard National LM555 timer circuit triggers off of the falling edge. However, if you just want to invert the signal, connect the signal to the base of a BJT, and connect the emitter to ground while pulling-up the collector (via a 10K). The collector terminal now contains the inverted signal.

 

[nbucska]

I disagree with melone's solution. It would work only if
the signal source R is small enough for the Ib not to cause
significant error and the amplitude is limited, so it will not blow the transistor.

NEO11 uses a comparator -- why not just switch the inverting and non-inverting inputs ?
<nbucska@pcperipherals.com>

 

[melone]

Nnbucska is correct. There are some assumptions that were made (5V logic, correctly sized transistor, etc.), however, the most basic question has not been answered. Why can't the 555 that Neo11 have negative edge trigger capability? I have just implemented a monostable one-shot using the negative edge as a trigger with the National LM555.

 

[csr]

Hi,

Configure your 555timer ic power supply with a common ground. Say like + or - 5Vdc for 10Vdc supply voltage to the timer so that you can achieve a +5V or -5V voltage swing near saturation.