(e) Meanwhile, C3 charges. When the voltage across C3 (applied to
pin 2) exceeds the voltage at pin 3, the output at pin 1 goes low, de-energizing K1 and
the seat tilt motor.
(f) Changing the setting of R36 does not change the time constant, but
varies the length of time that K1 and the seat tilt motor are energized. Moving the
center tap up makes the control voltage more positive. C3 takes longer to charge up
enough to exceed the control voltage. The motor runs longer and the seat tilts farther
(g) R37 and R38 preset the control voltages for A1.2 and A1.3,
controlling the back tilt and base (raising and lower) motors. The A1.2 and A1.3 circuits
function exactly like the A1.1 circuit.
(h) As long as one or more of the A1 outputs is high, one or more of
the diodes, CR14, CR15, and CR16 will be forward biased holding Q2 on. The last
motor stops running when all the A1 outputs go low. Q2 will turn off at this time, turning
off Q14 and Q1 and removing V2. C3 will discharge through CR4, R11, and R12 to
ensure that the chair moves to the same position every time.
(i) While this circuit may seem very complex, it is much more reliable
than a mechanical servo system.
(j) While K1, K2, and K3 are energized, their N.O. points close,
applying 120vac to the motors allowing the chair to go to the operate position. 120vac
is also applied from pins 10, 8, and 6 of P1 to CR7, CR8, and CR9. This is rectified and
filtered to +135vdc. R15, R17, and R35 form a voltage divider network, Q7's collector
goes low and is applied at the base of Q4. The low from the collector of Q2 is also
applied at the base of Q4 through CR21. Q9 will also be on at this time. Its collector
will also be low. These three lows ensure that Q4 stays off. Q4's collector goes high to
the base of Q5. Q5 is off. Its collector goes low to the base of Q8. Q8 is off and its
collector goes high. This high turns on Q9. The low on the collector of Q9 latches Q4
off. The low from the collector of Q5 is also applied to the base of Q13, keeping Q13
and K4 de-energized.
(k) Pushing the stop operate button before the chair has reached the
preset position will apply a high to the base of Q3. Q3 turns on and its collector goes
low. This low turns off Q2, Q14, and Q1 taking away V2. K1, K2, and K3 will de-