voltage controlled oscillators

19.FM - VCO - 555Timer II Ham lesson o' de day

Since the outputs of these comparators at 2 and 6 (those triangles) control the internal flip-flop (box with Rst/Set) that toggles the output of the 555 (pin 7), adjusting the pin 5 control voltage also adjusts the frequency at which the 555 toggles its output. 

So if we think about how this circuit worked in a “basic” manner, and add in Pin 5’s control voltage…

  • it would seem that if the capacitor was charging, and had reached the threshold for stimulating Pin 2, you could stimulate Pin 6 with Pin 5 instead of waiting for the capacitor to finish charging to Pin 6.  The crossed out is me not looking at the schematic carefully enough since the control voltage from Pin 5 connects to BOTH Pin 2 and 6. 
  • So eureka!  Pin 5’s control voltage affects both comparators at Pin 2 and 6 therefore when the voltage from Pin 5 comes through, (assuming enough voltage to at least be 2/3 supply), both comparators are stimulated.  This would offer a flip-flop response directly related to the frequency of Pin 5’s input signal.  The output pin (3) would then effect its load based on that frequency. 

Increasing the input voltage at pin 5 increases the output oscillation frequency while decreasing the input voltage decreases the output oscillation frequency.

Sounds like frequency modulation doesn’t it?