555 Toggle switch problem.

I recently built a Toggle switch board based on 555 chip, for those who are interested to build one, i have attached some schematics and PCB design.

The circuit works fine, except "2" little problems which i might rectify with some of your help.

Problem No.1: When I connect power to the circuit, for some reason the relay is engaged instantly (without pressing the toggle switch). This doesnot happen every time i connect the power supply but most of the time.

Problem No 2: In a different scenario when the circuit is connected to the Power and the Relay is disengaged and i start some other electical appliance for e.g. A Vacuum Cleaner or a room heater or my Power amp, starting these appliances somehow engages the Relay automatically (without pressing the Toggle switch). just as above this problem also doesn't occur every time.


Any help or recommendation will be deeply appreciated. Thanks...

Best Regards,

Some questions and observations...
What are you using for a power supply?
I don't see any decoupling capacitors around the 555...

djsfantasi said:

Some questions and observations...
What are you using for a power supply?
I don't see any decoupling capacitors around the 555...

Click to expand...

Yes i am using it for a a 220v power supply (On/Off Purpose) in my Pre-amp, Attached are the pictures of the board i built in my way.

The 555 will come up in a random state at power on (as you found). Add a power-on reset to start the 555 in the OFF state. Add a resistor (say 10kΩ) from the Reset pin 4 to +5V and a capacitor (say 10μF) from pin 4 to ground.

Add a 0.1μF decoupling capacitor directly from pin 8 to pin 1 and a 10μF capacitor across R3 to minimize false triggering.

Where does the +5V come from?

crutschow said:

The 555 will come up in a random state at power on (as you found). Add a power-on reset to start the 555 in the OFF state. Add a resistor (say 10kΩ) from the Reset pin 4 to +5V and a capacitor (say 10μF) from pin 4 to ground.

Add a 0.1μF decoupling capacitor directly from pin 8 to pin 1 and a 10μF capacitor across R3 to minimize false triggering.

Where does the +5V come from?

Click to expand...

Hello,
Thank you for the useful quote, i will try this technique and will update. As a matter of fact i did try a resistor 1K from the Reset pin 4 to +5V and a capacitor 100uf from pin 4 to ground, this did not help though but now i will try with a 10K resistor and a 10uf cap, i hope that will change something.

As for false triggering i will try your method, thank you.

About the 5volt supply to the circuit, Plz pay attention to the picture i have attached with this quote, on the right side of the board you can see a Transformer (Primary 220v - Secondary 12v), a Bridge rectifier, Filter Cap, Regulator 7805 and hence 5v Dc routed to the circuit.

Best Regards,

crutschow said:

10μF capacitor across R3 to minimize false triggering.

Click to expand...

Question: Do you mean in series after R3?

lubnaan90 said:

Question: Do you mean in series after R3?

Click to expand...

I meant in parallel across R3.

lubnaan90 said:

.....................
As a matter of fact i did try a resistor 1K from the Reset pin 4 to +5V and a capacitor 100uf from pin 4 to ground, this did not help though but now i will try with a 10K resistor and a 10uf cap, i hope that will change something.

...................

Click to expand...

If 1k and 100μF did not work then 10k and 10μF likely won't either. Keep the 100μF and increase the resistor value to 10k or more (up to a 100k) and see if that works. That value needed depends upon how fast the power supply voltage comes up when you turn on the power.

crutschow said:

I meant in parallel across R3.

Click to expand...

OK, GOt it, Thanks.

crutschow said:

If 1k and 100μF did not work then 10k and 10μF likely won't either. Keep the 100μF and increase the resistor value to 10k or more (up to a 100k) and see if that works. That value depends upon how fast the power supply voltage comes up when you turn on the power.

Click to expand...

Just Tried with 10K & 100uf, seems to work fine.
As for False trigger, will try tomorrow and Update, Thanks.

Oh, by the way, there is this new problem i just noticed, Every time when i disconnect the Mains "220v" (when the Relay is not engaged) from the circuit, after disconnection the relay engages for about half second and disengages. Any way to overcome this problem also?

crutschow said:

10μF capacitor across R3 to minimize false triggering.

Click to expand...

Tried this But unfortunately the relays fails to engage with the cap across R3.

Just put 10nf cap from gnd to pin 6&2.
This will resolve false trigger if home appliance or fast connect/disconnect from mains occur.
I had a same problem, and I resolved it in this way.

Regards