Game Show Buzzer Power Supply

[bogd]

This question is related to another one . However, rather than resurrecting an almost two year old thread, I preferred to create a new one.

So I have this circuit (the same one as in the previous question, which was taken from here. Basically, a quiz show system, in which only the first contestant to press the button gets his LED to turn on.



The original circuit was designed for a 9V input power supply, and it seems to work fine with it. However, I need to make it work with a 5V one. The 555 (NE555 in my case) supports anything from 4 to 14V, so that should be OK.

However, as soon as I run it on 5V, it becomes... unstable. Meaning that a 555 might trigger if I simply touch one of the trigger contacts with my hand, or a metal object. Even worse, more than one 555 will trigger at the same time.

It looks like the "high" voltage on the trigger line is no longer enough to prevent the line from going low for long enough to trigger a second 555. The actual values look like this: with a 5.7V Vcc, the output (pin 3 of the active 555) goes to 4.1V, and the trigger line to 3.4V.

Unfortunately, I am quite... inexperienced with this, so I am stuck. Can anyone give me some tips on what I could do to make the circuit stable with the new Vcc?

Thank you!

 

[MikeMl]

Try adding the bypass cap from pin 5 to pin1 on each 555. Try putting a 0.01uF bypass cap (very short leads) from pin 4 to pin1 on each 555.

 

[bogd]

First of all, thank you for the quick reply!

Try adding the bypass cap from pin 5 to pin1 on each 555.

As I mentioned, I am quite new at this - could you please give me some more details? What type of cap should I use for this?

Also, I would appreciate if you could tell me in a few words what adding these two capacitors does (I am trying to learn something from this experience, and an explanation from someone more knowledgeable would definitely help ).

Thank you!

 

[MikeMl]

If you read the entire 555 data sheet, both of the capacitors I mentioned are recommended in the applications section.

The TI data sheet says: Adequate power supply bypassing is necessary to protect associated circuitry. Minimum recommended is 0.1μF in parallel with 1μF electrolytic The pin5 bypass is shown in the application diagrams.

If you leave off the pin 5 bypass, noise coupled capacitively to pin 5 can perturb the two comparitors, causing the timer to either trigger or reset spuriously.

The power supply bypass prevents one 555 from triggering its neighbor spuriously due to coupling vial the power supply leads..

What is the source of the 5V DC you are using. How well filtered/regulated is it?

btw-a lot of circuits published on web sites are put there by inexperienced hobbyists that don't read data sheets....

 

[crutschow]

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

btw-a lot of circuits published on web sites are put there by inexperienced hobbyists that don't read data sheets....

Real hobbyists don't read data sheets.