So I tried to make a monostable circuit with the 555 timer, following this guide https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/, to turn on a led.

I used a 2200 μf capacitor and a 2.4 kΩ resistor, so the led should stay on for 6 seconds, but when I connect the power (5V), the led is on and never turns off, even if I activate the trigger.

What is wrong?

 

Welcome to AAC!

Post a schematic so we can see how you have things connected.

 

Try 100uF and 56K or 47uF and 120K.
If you check the circuit wiring is correct and it still doesn't works then you have to use the CMOS type as LMC555, TLC555

 

I'm already using the TLC555, I will try with other resistor and capacitor values.
Here's the schematic I'm using:

 

Hello,

What are the specs of the used led?
The TLC555 can source only 15 mA, see the maximum ratings list:



When the led has a voltage drop of 2 Volts and the supply is 5 Volts, you are expecting to have 30 mA in the led.

Bertus

 

The principle is to use the capacitor more less and the resistor more big up to 10MΩ.

 

Caps larger than approximately 100uF aren't recommended because they're too leaky and tolerance is typically 20% or more. This graph shows other combinations you can use to get 6 seconds:

 

Hi

You can use 1.1 x R x C to calculate an approximate delay

1.1 x 820k x 6.8uF = 6.134 seconds

See attached.

eT

 

................
Here's the schematic I'm using:
View attachment 123303

You forgot the pullup resistor for the push-button.

 

I'm already using the TLC555, I will try with other resistor and capacitor values.
Here's the schematic I'm using:
View attachment 123303

For a CMOS 555, Pin 2 should never be allowed to float.

Pull it up to Vcc, quiescently, when its not being pulled low.

 

Thanks all for the suggestions, I don't have time now so I can't test them.
But after a few tests, I dicovered that capacitor 1 never charges, it stays at arround 1.5v. If I disconect pin 7, the capacitor charges to 5v and the led turns off.

 

Yes, that cap is the main problem. I've even had 1 uF electrolytic caps that had such high ESR that they wouldn't charge. High-quality caps help immensely.

Thanks all for the suggestions, I don't have time now so I can't test them.
But after a few tests, I dicovered that capacitor 1 never charges, it stays at around 1.5v. If I disconnect pin 7, the capacitor charges to 5v and the led turns off.

Moderator's Note:
The commercial does not allowed on this forum, please don't do that, otherwise you will get ban as spammer.

 

But after a few tests, I dicovered that capacitor 1 never charges, it stays at arround 1.5v. If I disconect pin 7, the capacitor charges to 5v and the led turns off.

The problem is that you don't have a pull up on the trigger input (as pointed out in post #9) so it can rise above 1/3 VCC, so the output stays high.

 

I added a pull up resistor and tried different combinations of capacitors and resistors and still didn't work.
Is possible that I have a defective TLC555 or I broke it somehow?

Also, I made a astable circuit and didn't work, however I made Schmitt trigger and it worked fine.

 

Hello,

Do you still use the 100 Ohms resistor for the led?
It should be higher than 330 Ohms to avoid overloading the TLC555.

Bertus

 

Hello,

Do you still use the 100 Ohms resistor for the led?
It should be higher than 330 Ohms to avoid overloading the TLC555.

Bertus

No, I changed the resistor.

 

Is possible that I have a defective TLC555 or I broke it somehow?

The device is static sensitive, and you could have broken it. But if it works in some other circuit, damage is unlikely.

Also, I made a astable circuit and didn't work, however I made Schmitt trigger and it worked fine.

Post the complete schematic for a circuit that didn't work and give us the relevant voltages in the circuit.

 

So I tried to make a monostable circuit with the 555 timer, following this guide https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/, to turn on a led.

I used a 2200 μf capacitor and a 2.4 kΩ resistor, so the led should stay on for 6 seconds, but when I connect the power (5V), the led is on and never turns off, even if I activate the trigger.

What is wrong?

Possibly, the leakage current through the huge cap is hogging the current and not allowing the input to rise to a detectable level.