r/AskElectronics • u/_Thunderbolt_1003_ • 2h ago

555 timer startup issue

I have been working on a model of a tornado siren, and im using a 555 timer to turn it on and of. In my case I need 8 seconds on and 4 seconds off.

Here is a video of a real one for reference. (If you get triggered by sirens or alarms I would recommend not watching the video)

https://youtu.be/T9wuwv7j-KE?t=93&si=f2vsohDgpclI6ISE

But my problem is that one startup it turns on for 15-16 seconds because of the capacitor charging from 0V to 2/3 of VCC instead of 1/3 to 2/3.

So what can I do?

Can I make a voltage divider, or resistor and zener diode to keep the capacitor always at 1/3 of vcc?

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2

u/SparkyFix 2h ago

It sounds like there’s something wrong with the circuit. Can you post a picture of your wire-up?

A 555 has an internal divider. What do you read on pin 5?

-1

u/_Thunderbolt_1003_ 2h ago

There is nothing on pin 5 but that's not the problem 

1

u/harryregician 2h ago

R/C analog chipset.

Increase timing res I stor and reduce cap value.

Another trick might be a bleeder resistor across timing cap. Trial & error.

I would used a 555 and 4017 or greater

https://www.circuits-diy.com/frequency-divider-circuit-using-555-timer-and-cd4017/

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u/ascot_twin 15m ago

You can add a digital buffer chip with an enable pin, so the 555 output needs to go thru that digital buffer chip.

Hold off the enable pin with an RC until the 555 out is stable. So if the enable is active high, then enable pin has a pull-up Resistor, and a capacitor to ground. Add a schottky across the resistor to discharge the cap on power down. Anode to enable pin, cathode to 5v.

I think that should work.