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555 timer output hight and low? Answered

i just soldered up a 555 monostable circuit following an internet tutorial and for some reason the output seems to be both low and high.
it can power a LED connected to the 0V & pin 3 and the 9V & pin 3.
any ideas what might have happened?




Best Answer 7 years ago

Maybe the monostable circuit is not monostable but oscillating, so the output will be alternating between high and low.

Measure the voltage at the output. With an oscilloscope if you can access one or with a voltmeter/multimeter in AC mode.

how would i use a multimeter in AC to show if it was oscillating?
i'm pretty sure i haven't made it's astable, but it'd be good to check.

Edit: i just looked at my circuit again, and the trigger pin (2) should have been wired between a resistor from the positive and a capacitor to the negative.
instead i just wired it straight to the negative, could this be the cause of my problem?

Sorry, I have neither a data sheet of the 555 nor some circuit diagrams to analyse around here.
Just correct it and see what happens ;-)

Edit: Had a quick check on the interwebs: A low voltage (< 1/3 U[Pin8]) on Trigger will set the output immediately. So depending on what's on the other pins, that might force some oscillation.

Switch your multimeter to AV voltage mode, connect one terminal to GND (0V) of the circuit and the other terminal of the multimeter to the output of the circuit. If the output is stable at a voltage (either high, low or somewhere in between) the multimeter will show 0V (or a very small value). If the output is oscillating, it might show anything between 0V and the supply voltage (depending on what happens at the output and the internals of the multimeter - this is a crude test).
If you can measure an AC voltage and your multimeter has a frequency range, try that to measure the frequency.

When the circuit is triggered, pin 3 should go high, which would turn on an LED connected between pin 3 and ground. When the time delay of the monostable has ended, the output on pin 3 will go low, alowing an LED connected between teh 9V power supply and pin 3 to turn on. When the output of the 555 is low, it will sink current, so it will be almost like a direct connectin to ground.

If you have two LEDs connected, one between pin 3 and ground and the other between 9V and pin 3, one should always be on when the other is off, but never both at the same time.

If the LED is blinking on and off repeatedly, then you may have set up your circuit as an oscillator by mistake.