In your
previous thread (now closed) on this subject, you stated you would feed a low-voltage signal into a transformer and step that up to the 115 V AC required to operate the solenoid in the reed frequency meter. So far, so good. A 6.3 V center-tapped filament transformer with a 115 V AC primary should do nicely. Although most of these are designed for 50 Hz or 60 Hz primary excitation, they will have enough iron to serve at 350 to 450 Hz.
If you use a 555 to make a variable frequency oscillator, you should use the output to clock a D-type flip-flop to create complementary square wave output at half the 555 frequency on the Q and /Q outputs of the flip-flop. So the 555 would oscillate between 700 and 900 Hz. Use the two flip-flop output signals to drive a pair of transistors (2N7000 MOSFETs are easy, but NPN power transistors will also work) connected as switches between the two secondary terminals of the transformer and circuit common. Connect the center tap of the secondary to a +5 V DC supply. You should insert a current-limiting resistor in series between the center-tap and the +5 V DC supply. Use diodes across the transistor switches to protect them from "flyback" voltage generated by the on/off switching. The square waves from the flip-flop will alternately turn the transistors on and produce a high voltage in the transformer primary. You can play around with how much current gets switched by appropriate sizing of the current-limiting resistor in series with the center-tap and the +5 V DC supply. You can even try adding a resonating capacitor to the load side (the primary of the filament transformer) to see what effect this has.
This isn't "rocket science" and should be well within your capabilities. BTW, pin-outs on a 741 op-amp were shown in your first post on
this thread. Most folks would just Google "
741 datasheet" to get pin-outs. The pin-out sometimes varies with the particular package, which you failed to specify. Do you need help building a 555 variable-frequency oscillator or choosing a flip-flop?