555 timer

Hi Roger

Inside the 555 you have 3 resistors from Vcc to Ground, all of equal value (5k) giving thresholds of 0.66Vcc and 0.33 Vcc.

In astable mode, the capacitor charges until it reaches 0.66Vcc (at this time the output is high) the output toggles low and the capacitor discharges until it reaches 0.33Vcc. The output then goes high again...

Pin 5 is connected to the 0.66Vcc point, so increasing the voltage on this pin increases the 0.66Vcc threshold thus making the output stay on for longer (capacitor needs longer to charge before the output trips). Vice verca with a decreasing voltage on pin 5.

Without this pin, if you want to change the frequency of operation, you would have to insert a potentiometer in the place of one of the frequency control resistors. If this is remote from the 555 timer (mounted on the side panel), noise can get coupled into the connecting wires and accidentally trip the internal comparators, giving spurious pulses.

If you add a control pin, you can decouple this point to remove noise pickup.

Hope this helps

Rgds

--
Bill Naylor
www.electronworks.co.uk
Electronic Kits for Education and Fun

e.

e

ges

s

ide

If Roger takes a look at the Fairchild data sheet for the 555

he will see how to vary the 555 output for both astable (one-shot) and multistable (oscillator) operation like Bill says. See "Pulse Width Modulation" and "Pulse Position Modulation" on pages 8 and 9.

You also might add a few precautions to Bill's response here. The 555 was crafted back when the earth's crust was cooling, and "rail-to- rail" inputs were no more than a twinkle in the eye of aspiring engineers. The differential pair comparator input of the standard 555 does not work properly if it's more than about 1.5V below Vcc. Also, the comparator offset and other imperfections of the circuit become more pronounced as the upper and lower limits (set by the three 5K resistors) become smaller and closer together. Lowering the control voltage on pin 5 below about 1/4 * Vcc means less control.

So for a Vcc of 5V, the control pin can range between about 1.25V and

3.5V. For a Vcc of 12V, control coltage can range between 3V and 10.5V. This will provide a good range of oscillator frequencies.

Cheers Chris