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Topic: How to tring IC 555 with positiove voltage ? (Read 1 time) previous topic - next topic

mmc01

I want to connect PIR Sensor to ic 555 which output of PIR Sensor is logic "1" . But ic 555 will work when trigger pin change from "1" to "0". I want to use output of ic 555 to 12V. How to change input of 555 to work when input is logic "1" ?

Grumpy_Mike

What do you want the NE555 to do when it gets a logic one?

If the input is outside the voltage of the 555 then use two resistors as a potential divider to cut it down.

mmc01

I want to use NE555 for delay. when NE555 get logic 1 then delay LED 30 Sec.

Grumpy_Mike

So you want to use the 555 as a monostable?
The input will immediately change the output but will not change back for 30 seconds. Right?

That is a long time and using passive components it will not be very accurate or repeatable.

mmc01

Yes, I want to use 555 as a monostable. when somebody move pass PIR Sensor then turn on LED about 30 Sec. I'm not want accurate . The output of PIR when somebody pass is "1" . I can't change output from PIR sensor logic 1 to 0 . If I connect output from PIR Sensor to input of NE555  then LED at output of NE555 will turn on when have no somebody and turn off when somebody pass PIR .

Grumpy_Mike


Pelleplutt

Use an inverter from 4000 or 74HC series
Or an NPN transistor.
Or use another monostabile ex 74HC123 (both pos and neg trigger)

30 seconds are not long time.

AND this is an uC forum, use an arduino (nano)

Pelle

MarkT


So you want to use the 555 as a monostable?
The input will immediately change the output but will not change back for 30 seconds. Right?

That is a long time and using passive components it will not be very accurate or repeatable.


Thats where the CMOS 555, the 7555, is a winner - the resistor value can be much higher
allowing large time constants without having to use electrolytic caps.  Input current 20pA...

To get a +ve edge triggered monostable you inject the pulse to pin 6, and the output
pulse is negative going (pins 2 and 7 wired to two resistors and a cap as usual).

Note that if the 7555 is run from 12V then a minimum voltage of 8V is needed to trigger
on pin 6...

If your PIR sensor only works at 5V then you'll have to use the normal monostable circuit
and add an inverter between PIR and pin 2 of the 7555.  pin 2's threshold is 1/3 supply,
ie 4V (for an accurate 12V supply).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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