ELM382
Example Applications
+5V
The following shows two circuits using the ELM382.
In both cases, it is assumed that the 50Hz for the clock
input has been derived from another circuit, with due
regard for the safety of the users. Isolating/stepdown
transformers should be used whenever possible.
1
2
3
4
8
7
6
5
0.01µF
Typically, a sinusoidal AC voltage will be used for
the clock signal, with a peak magnitude that is greater
than VDD (or less than VSS). For these cases, a series
resistor (100KWin the figures) must be added to prevent
the input currents from exceeding the protection diode
capabilities. Another design consideration is the need to
provide a DC path from the Clock input back to either
VDD or VSS at all times (so the CMOS input is not left
floating). As an example, connecting to one side of a
centre-tapped transformer that has its centre connected
to VSS is an excellent way to obtain a signal, while
maintaining a DC path to VSS through the winding. No
extra resistor is needed in this case. Connecting to the
‘DC side’ of a half wave rectifier circuit is likely not
advisable however, without adding an extra pulldown
resistor to ensure that the voltage returns to Vss when
the clock signal is not present. (Recall that a reverse
biased diode is essentially an open circuit.)
100KW
50Hz
Input
start
Control
Output
Figure 2. Retriggerable Control Circuit
The two resistors shown with asterisks (*) may be
required to reduce the effects of induced charges and
currents if the pushbutton is mounted more than a
couple of feet from the IC. In this case, protecting
resistors (typically about 10KW) should be connected as
shown.
The first example (Figure 1) is designed to control a
swimming pool pump. This circuit is useful for stopping
the circulating pump during the night (reducing energy
consumption and heat loss), while providing normal
operation during the day. The interface to the high
voltage pump supply is not shown, but would typically
be a transistor stage driving an electromechanical relay
circuit.
Figure 2 shows another typical circuit. This one is a
retriggerable timer that could be used to control lights,
fans, etc. while providing an ‘auto-off’ feature. For the
circuit shown, the control output remains active for a
minimum of six hours, but the period is extended each
time the start button is pressed.
The pushbutton shown is pressed once on the first
day at about 7pm, resetting the circuit. With D1=H and
D0=L, the circuit will then operate continuously with a
24 hour period, the output (Pump Enable) at a high level
from 7am to 7pm, and at a low level otherwise.
The ELM382 is inherently an astable circuit that
cycles on and off continually, so using it for this
‘retriggerable one-shot’ operation requires the addition
of external latching circuitry as shown. A quad NOR
(CMOS 4001) provides the required logic while leaving
a fourth gate free for other uses (be sure to tie its inputs
to VDD or VSS if unused).
+5V
In operation, a new timing period begins whenever
the pushbutton is pressed, as it is tied to the reset input.
The Control Output latch is also set by this action,
forcing a high output. If a half period is ever reached
the ELM382 Out signal (pin 7) will go high, resetting the
latch and causing a low Control Output. After this time,
the ELM382 will continue to cycle, but the Contol Output
will remain off. As with the circuit of Figure 1,
precautions should be taken if the pushbutton is located
in an electrically noisy environment, or an appreciable
distance from the integrated circuit.
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8
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5
0.01µF
+5V
Pump
Enable
100KW
*
*
50Hz
Input
time zero
As shown by these two examples, the ELM382
simplifies many long term timing applications, allowing
several new possibilities…
Figure 1. Pool Pump Control Circuit
ELM382DSA
Elm Electronics – Circuits for the Hobbyist
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