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My 555 timer is not working like normal. I don't really know what happened.

I did this on a breadboard and it worked perfectly with a 5s delay at the deck. But now I am building a full circuit were I will need it a lot, I have practically done everything on the diagram but it is not working out, the RC network is correct and properly biase but it is not working well; pin 6 and 2 goes along with the rc network.

How can I fix this?

I am using the following circuit,

enter image description here

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  • \$\begingroup\$ What does it connect to? What pins go to what on the rest of your circuit? What triggers the delay? What is "the deck?" \$\endgroup\$
    – JRE
    Jan 29, 2019 at 12:27
  • \$\begingroup\$ @JRE is an impute signal from an optocoupler powering exactly about 6 to 8 volts to pin 2 of my 555 via the rc network and is to power up a 4081 ic \$\endgroup\$ Jan 29, 2019 at 12:46

2 Answers 2

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You have created what is called a "race condition". You're hoping that pin 2 (trigger) goes high before pin 4 (reset) does, so that the chip stays in reset until the trigger falls below 1/3 Vcc again, 5 seconds later. But they effectively go high at the same time, and there's no guarantee about how the chip will behave in that case.

You need to add an RC circuit to pin 4 so that it definitely rises more slowly than pin 2 does.

Unfortunately, the typical datasheet for the 555 does not specify a "reset recovery" time, but I would assume that something on the order of 1 µs would be more than enough. It also doesn't specify anything about VIH or VIL for the reset pin, which uses a PNP transistor in a configuration that is not typical of most forms of logic. I would probably use a time constant of about 10 µs and see how well it works. You end up with something like this:

schematic

simulate this circuit – Schematic created using CircuitLab


Actually, now that I think about it some more, having pin 6 tied to pin 2 should guarantee the behavior of the circuit anyway, regardless of what happens on the reset pin. I'm not sure why your non-breadboard circuit isn't working. Are you sure you copied the circuit exactly?

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There will be no power-up delay if you switch on immediately after switching off. After switching off wait a few seconds, before switching on again, to allow the capacitor time to discharge.

To get around this problem you could add a diode (preferably schottky because of its lower forward voltage drop) across the resistor with its anode to ground. This will discharge the capacitor more rapidly at power down but the power-on delay, even with the diode in place, will be a little shorter when you power-on immediately after powering off.

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