The speaker emits a distinctive warble tone when ring pulses are applied to the phone line. Use this circuit as a remote bell or disconnect the phone`s ringer for direct use. Rl adjusts the duration of the output; R2 and R3 control the tone`s duty cycle and frequency. The transistor is a general-purpose NPN photodevice
The neon bulb and transistor are coupled with the heat-shrink tubing to form an optoisolator.