For example, suppose you have two 7555 timer chips wired in astable mode, and one of them modulates the signal from the other. This can be done by taking the fluctuating voltage on the Threshold pin of the first chip and connecting it with the Control pin of the second chip. In Figure 3, the control voltage from the first chip is the blue triangular waveform while the square wave from the output of the second chip is red. You can see that when the control voltage increases, the frequency of the square wave decreases.