(7) Off-time monostable circuit. Refer to figures 2-2 and 2-3. The purpose
of this circuit is to control the time period the on-time monostable output remains low.
The off-time monostable consists of 1C1 pins 1 through 3, IC2, C4, C6, C9, R7, R11
through 22, R35, and R37. 1C1 pins 1 through 3 comprise a NOR gate of which pin 3 is
its output. This output is an inversion of the clock oscillator when the on-time
monostable is low. Pin 3 is held in low when the on-time monostable is high. R7 and
C4 shape the NOR gate (not OR gate) output into positive and negative impulses,
biased at Vcc to prevent mistriggering of the monostable and are applied to IC2 pin 2.
C6 and R11 through 22 determine the time periods for which the off-time monostable is
high. Components C9, R35, and R37 are used to adjust the delay multiplier in the time-
period equation as follows:
T = (.975.125) RC.
Where R equals the value of the resistor expressed in Ohms times C, the
(8) On-time monostable. Refer to figure 2-3. The on-time monostable
controls the relay system and determines how long the off-time monostable remains
low. Components IC3, C5, C7, C8, R8, R23 through 34, R36, and R38 comprise this
circuit. Essentially, this monostable and its corresponding components act in the same
manner as the off-time monostable. The on-time output triggers the off-time
monostable via 1C1 pin 3 and the relay system, both directly and through 1C1 pin 4. All
components are mounted on the printed circuit board as seen in figure 2-4.
(9) Relay system. Refer to figure 2-2. The relay system provides two
functions; first, it interfaces the on-time monostable with the motor-speed-control circuit
and, as a secondary function, dumps the vacuum-collection system to atmospheric
pressure during off cycles. The relay system consists of K1, K2, Q4, Q5, R9, and the
solenoid. During on-time cycles, relay K1 is closed and provides base bias to Q1 from
the R2, R3 series combination. The motor speed is then fixed by the R2, R3
combination during each on cycle unless changed by the operator. Relay K2 is not
energized during the "on" cycle, thus allowing the vacuum to exist. During off cycles, K1
is normally open and stops M1. Relay K2 is energized as 1C1 pin 4 goes high and
turns Q4 on. When K2 closes, Q5 turns on and energizes the solenoid to a normally
open state. When the solenoid opens, the vacuum collection system is dumped to
atmospheric pressure and remains so until the next on cycle. Switch S3 must be closed
to operate the motor-speed-control circuit and solenoid. Components K1, K2, Q4, and
R9 are mounted to the printed circuit board. Component Q5 and the solenoid are
mounted to heat sink #2.