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555 timer

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directbuyers

New Member
on the #3 out pin of a 555 timer chip, if I attach an LED & resistor with the other end to ground I get a response like "on and off" but if I attach the LED to the #3 pin and the other end to the positive instead of the ground I get the opposite response like "off and on". Why? What is the chip sending out of that #3 pin?
 

MrDEB

Well-Known Member
pin 3 is going high and low

so an LED resistor connected from V+ to pin 3 AND another resistor LED connected from pin 3 o ground you will get alternating LEDs.
Remember the 555 will only sink or source about 20-25ma max if im not mistaken.
 

MikeMl

Well-Known Member
Most Helpful Member
The 555 has a "push-pull" output, which lets it either "source" current from its supply pin to a load, or "sink" current through a load to ground.
 
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mneary

New Member
so an LED resistor connected from V+ to pin 3 AND another resistor LED connected from pin 3 o ground you will get alternating LEDs.
Remember the 555 will only sink or source about 20-25ma max if im not mistaken.
There are many types of "555". For example the National LM555 will source and sink 200mA.
 

Karkas

Member
I guess you turned the LED backwards, I don't think it will do that with the anode to pin 3 and the cathode to Vcc, if you turned it backwards of course it will do the opposite, your on time will be when the 555 sinks current, instead of when it is supplying. The experts, correct me if I'm wrong.
 

audioguru

Well-Known Member
Most Helpful Member
so an LED resistor connected from V+ to pin 3 AND another resistor LED connected from pin 3 o ground you will get alternating LEDs.
Remember the 555 will only sink or source about 20-25ma max if im not mistaken.
The normal 555 will sink or source 100mA to 200mA depending on its supply voltage.
The Cmos 555 will sink 50mA and will source 10mA if its supply is 12V to 15V. Its output current is very low at low supply voltages.
 
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