Example Applications

4 of 4

The following shows two circuits using the ELM381.

In both cases, it is assumed that the 60Hz 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.

Typically, a sinusoidal AC voltage will be used for

the clock signal, with a peak magnitude that is greater

resistor (100K

Ω in 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

floating). As an example, connecting to one side of a

centre-tapped transformer that has its centre connected

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.)

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.

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 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 10K

Ω) should be connected as

shown.

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 ELM381 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

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 ELM381 Out signal (pin 7) will go high, resetting the

latch and causing a low Control Output. After this time,

the ELM381 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.

As shown by these two examples, the ELM381

simplifies many long term timing applications, allowing

several new possibilities…

ELM381

ELM381DSA

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

Figure 1. Pool Pump Control Circuit

1

2

3

4

8

7

6

5

time zero

60Hz

Input

*

Pump

Enable

100K

Ω

+5V

+5V

*

0.01µF

1

2

3

4

8

7

6

5

start

60Hz

Input

Control

Output

100K

Ω

+5V

0.01µF

Figure 2. Retriggerable Control Circuit