# What am I doing wrong (555 timer as a one-shot)?

• Number2Pencil
In summary, A averagesupernova is trying to use a 555 timer to produce a quick pulse whenever a switch is switched a certain way, but is having difficulty doing so. He is using the correct connections, but the switch is not working properly. He believes that connecting the switch to Vcc instead of 5V may have solved the problem.
Number2Pencil
What am I doing wrong (555 timer as a one-shot)??

I'm trying to use a 555 timer to produce a quick pulse whenever a switch is switched a certain way. I have it set up exactly as the data sheet describes (attached picture) and am using a 47 micro Farad Cap as C1 and a 1k ohm resistor. I have the switch connected to "Trigger" (pin 2)...but when I test it out...The pulse is lasting indefinently...and i do mean a long time. it should only be lasting a fraction of a second.

I thought it might be a bad IC so I used the other one I had...got the same result. So I got desperate and tied the switch to where Vcc is instead of 5V...it actually worked like a one shot! I know this isn't the correct way to handle an IC...so WHAT could be going on?

#### Attachments

• 555 dataA.JPG
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no...i'm not using any transistors...i simply have a switch pumping out 5 volts when connected

Your connection of the 555 timer is for a simple monostable operation. If you have it connected as you show it in the diagram then the output from pin 3 should go high (to Vcc) when pin 2 is grounded or made to be less than 1/3 of whatever Vcc is. The time your output from pin 3 is high should be equal to 1.1 X 1000 x 0.000047 seconds, approximately 53 milliseconds.

Are you sure you are triggering the number 2 pin to go low, less than 1/3 Vcc?

It would help me to see how you have the rest of the circuit connected.

well I have a switch connected to pin 2, and pin 3 is connected to an active HIGH clear on a counter. everything else is connected as shown...I even checked and rechecked a few more times to make sure of it.

flickering the switch should get it to pulse going one direction or the other...but I'm getting a constant high on the output, i only want a small pulse long enough to trigger a reset on the counter ( a 1 shot).

I'm serious...connecting the switch to Vcc actually made it work...so I'm going to stick with that and hope my teacher doesn't notice...if he does i'll probably be back here asking for more help

Pin 2 should not be left floating if I recall the inner workings of the 555 correctly. Use a pull-up resistor to Vcc on pin 2 and switch it to ground when you want to start a cycle.

Averagesupernova said:
Pin 2 should not be left floating if I recall the inner workings of the 555 correctly. Use a pull-up resistor to Vcc on pin 2 and switch it to ground when you want to start a cycle.

is this what you're describing?

#### Attachments

• 555 data redone.JPG
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I believe Averagesupernova wants you to connect the resistor to Vcc and then to pin 2. The switch then is connected between pin 2 and ground. When the switch is open, pin 2 will have a voltage on it that is close to Vcc and definitely higher than 1/3 Vcc. When you close the switch, this will cause pin two to become grounded and thus less than 1/3 Vcc and the output of Pin 3 will then go high.

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## 1. Why is my 555 timer not working as a one-shot?

There could be several reasons why your 555 timer is not working as a one-shot. Some common reasons include incorrect wiring, incorrect component values, or a faulty timer chip. Double check your circuit and make sure all connections are correct. Also, make sure you are using the correct resistor and capacitor values for the desired time delay. If all else fails, try using a different timer chip to see if that solves the issue.

## 2. How do I calculate the time delay for my 555 timer one-shot circuit?

The time delay for a 555 timer one-shot circuit can be calculated using the formula t = 1.1 x R x C, where t is the time delay in seconds, R is the resistance in ohms, and C is the capacitance in farads. Keep in mind that this formula assumes that the trigger and threshold pins are connected to ground and the discharge pin is connected to Vcc.

## 3. Why is my 555 timer one-shot not triggering properly?

If your 555 timer one-shot is not triggering properly, there may be an issue with the trigger signal. Check to make sure the trigger pin is connected to ground and that the trigger signal is within the recommended voltage range. You may also need to adjust the trigger voltage level using a voltage divider circuit if the trigger signal is too high. Additionally, make sure the trigger pin is not accidentally connected to the threshold or discharge pins.

## 4. Can I use a variable resistor for the timing in my 555 timer one-shot circuit?

Yes, a variable resistor (potentiometer) can be used to adjust the timing in a 555 timer one-shot circuit. However, keep in mind that the timing will not be precise as it would be with a fixed resistor and capacitor. Also, make sure to use a logarithmic potentiometer to get a more linear change in timing.

## 5. How do I reset my 555 timer one-shot circuit?

To reset your 555 timer one-shot circuit, you can use a switch or connect the reset pin to ground. This will discharge the capacitor and reset the timer. Keep in mind that the reset pin should always be kept low (connected to ground) unless you want to reset the timer.

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