have its mode of operation controlled by a programming or learning switch 300 positioned on the outside of the head unit 12 and coupled via a line 302 to the p 26 pin of the microcontroller 282 the wired control panel 43 is connected via the lead 43 a to input pins p 06 and p 07 the microcontroller 282 has a 4 mhz crystal 328 connected to it to provide clock signals a force sensor 330 includes a bridge circuit having a potentiometer 332 for setting the up force and a potentiometer 334 for setting the down force respectively connected to inverting terminals of a first comparator 336 and a second comparator 338 the comparator 336 sends an up force signal over a line 339 a the comparator 338 sends a down force signal over the line 339 b respectively to pins p 04 and p 05 of the 8 bit microcontroller 282 although details of the operation of the microcontroller in conjunction with other portions of the circuit will be discussed hereinafter it should be appreciated that the p 01 pin of the microcontroller is connected via a resistor 350 to a line 352 which is coupled to an npn transistor 354 that controls a light relay 356 which may supply current via a lead 358 to a fight in the head unit or the like similarly the pin p 000 feeds an output signal on a line 360 to a resistor 362 which biases a base of an npn.