D1 is a generic diode - Radio Shack 1N4005 - It makes the waveform unipolar
R1 is a 47 ohm resistor - This makes the circuit draw the same current as the Timer Speaker
C1 is a 10 uFarad capacitor - This removes the buzzer waveform
Opto-Isolator - Any Opto-Isolator will work (a generic NPN transistor may be substituted)

Note: Disconnect the speaker in the Talking Timer.
Set Timer for 'Repeat'
Set Timer for 'Countdown' Mode
Turn off Voice Mode
Set Buzzing Mode