555 astable

[Audioguru]

try it without attaching pin7, its internally grounded. Just going from what I read at talking electronics

Pin 7 of a 555 is not internally grounded. It actively goes to ground to discharge the timing capacitor when the output pin3 goes low.

 

[plutes]

Pin 7 of a 555 is not internally grounded. It actively goes to ground to discharge the timing capacitor when the output pin3 goes low.

maybe thats his problem, no cap from pin 6 to 0v. I only see a transistor internally and pin 7 being the collector path. I don't see the path from pin 3. Do you know why his 555s are burning up?

 

[Nanren888]

The first figure by the OP shows using pin 7 to discharge. In this configuation you need pin 7.
Another configuration is to charge the capacitor towards the output, both high & low, as you've shown. In this confdiguration, no pin 7.

 

[Bluejets]

equal mark space

will reqire a slightly different circuit with diodes added to even get close if my memory serves me correctly.

 

[Audioguru]

A normal 555 astable oscillator circuit charges the timing capacitor through two series resistors but uses pin 7 to to discharge the capacitor though only one of those resistors making the output not to be symmetrical.

The 555 uses an old TTL design so then the output high voltage is far from the positive supply but the output low goes almost to the negative supply (ground) causing the output not to be a symmetrical squarewave if the output is used to charge and discharge the timing capacitor.

The Cmos version of the 555 has an output that goes "rail-to-rail" so it produces a perfectly symmetrical squarewave when its output charges and discharges the timing capacitor..

 

[colum]

Sorry, don't understand. Pin 7 is usually the internal controlled discharge path, so the capacitor will not discharge if not connected to pin 7.

Coincidentally, I got handed the other day some old throw away boards labelled "RS 434-071" which are pre-printed (not populated) boards for 555 modules. These boards just allow you to build basically the same circuit on a PCB.
Not everyone's perfect option, but one way to have these useful little circuits always at hand.
.
Rather a slow option, but can send you a couple if you want.

Thanks much for the kind offer but I used all my 555s up so I will try to get a different brand and I'm determined to do all hard wirin g and fix this board.. Thanks colum