Need Help With Signal Amplitude Modulation

I want to change a constant-amplitude sine wave(or any periodic wave) to variable-amplitude sine wave. For example a 1 Khz sine wave must have 4 V amplitude for 10ms duration and 8 V amplitude for other 10 ms duration like in the attached figure. I think this is called signal modulation. But I dont know how to setup a circuit that enables me to do it. I am not allowed to use IC's except 555 and 741.
Thank you for your answers..

If you "aren't allowed" to use voltage-controlled amplifier ICs then make one.

Make an opamp amplifier with two feedback resistors in series to ground. Then use a transistor across the grounded resistor to short it which will increase the gain of the opamp.The input modulation turns the transistor on and off.

I would sketch it if it wasn't homework at school.

thank you.. I am going to try..