Typical Applications
(Continued)
TL/H/7768–14
FIGURE 4. Time Out on Power Up (Relay EnergizedUntil
R
t
C
t
Seconds After V
CC
is Applied)
TL/H/7768–15
FIGURE 5. 5V Logic Supply Driving 28V Relay
30V Supply Interfacing with 5V Logic
Figure 6 indicates the ability of the timer to interface to digi-
tal logic when operating off a high supply voltage. V
OUT
swings between
a
5V and ground with a minimum fanout of
5 for medium speed TTL. If the logic is sensitive to rise/fall
time of the trailing edge of the output pulse, the trigger pin
should be low at that time.
TL/H/7768–16
FIGURE 6. 30V Supply Interfacing with 5V Logic
Astable Operation
The LM122 can be made into a self-starting oscillator by
feeding the output back to the trigger input through a capac-
itor as shown in Figure 7. Operating frequency is 1/(R
t
a
R
1
)(C
t
). The output is a narrow negative pulse whose width
is approximately 2R
2
C
f
. For optimum frequency stability, C
f
should be as small as possible. The minimum value is deter-
mined by the time required to discharge C
t
through the inter-
nal discharge transistor. A conservative value for C
f
can be
chosen from the graph included withFigure 20. For frequen-
cies below 1 kHz, the frequency error introduced by C
f
is a
few tenths of one percent or less for R
t
t
500k.
*
See Chart
TL/H/7768–17
TL/H/7768–18
FIGURE 7. Oscillator
One Hour Timer with Reset and Manual Cycle End
Figure 8 shows the LM122 connected as a one hour timer
with manual controls for start, reset, and cycle end. S1
starts timing, but has no effect after timing has started. S2 is
a center off switch which can either end the cycle prema-
turely with the appropriate change in output state and dis-
charging of C
t
, or cause C
t
to be reset to 0V without a
change in output. In the latter case, a new timing period
starts as soon as S2 is released.
*
Dearborn
Electronics
LP9A1A476K
Polycarbonate
TL/H/7768–19
FIGURE 8. One Hour Timer with Reset
and Manual Cycle End
8