frankly i think he is at least a little bit confused else I am realy lost. why is 12 KHz necesary ? what has it got to do with a current source. if precision is not necesary he can just use a transistor with a suitable resistor on the base and measure the current with a tester and ajust until correct then just feed the base via the resistor with the 555 output. or make a precision current source like someone else will have to suggest and power it with the output of the 555 or even gate the current with another transistor.