component to u1a and u1b of course a similar effect could be accomplished by imposing the square wave output from u1a through c4 to vss since as noted variation of either or both of vdd and vss frequency modulates the oscillator 39 s output the overall frequency and perceived pitch of the circuit 39 s output i e the output of u1b can be increased or decreased by 10 increasing or decreasing respectively the voltage applied at vdd by battery b1 in fig1 this is achieved by r7 and c1 which together form a lag circuit with a time constant of about 0 5 seconds such that when momentary switch s1 is closed and held the voltage vdd increases relatively slowly imparting a vehicle 34 acceleration 34 noise effect in a similar fashion opening s1 results in a vehicle 34 deceleration 34 noise as c1 discharges into the circuit resistor r1 provides a slight 34 step 34 in vdd when the switch s1 is first opened or closed which has been found to enhance the subjective realism of the simulated engine sound at the output of u1b as heard through speaker spkr in a practical application the switch s1 would correspond for example to the accelerator pedal in a toy vehicle due to the self.