As shown the circuit for the car audio speakers . Dotted line on the left of the figure for the same truck and electric horn circuit , Sl power for the steering wheel horn button switch . S2 for the additional SPDT switch , for the original car horn electric horn and voice switching between . When the switch S2 placed in the ” 2 ” , press the switch Sl, capacitor Cl is charged , transistor VTl, VT2 turn , relay Jl pull its normally open contacts close and J1-1 to the circuit power supply for 15 seconds . ICl is dedicated voice circuit HL-169A, due to their working time is 2.8 seconds, so with the transistor VT3, VT4 composition of self-excited multivibrator , a high output every 3 seconds as a trigger signal , so that every 3 seconds ICl A voice output clock signal into the audio power amplifier IC2 , BL issued by the speaker voice.