Example Application

4 of 4

The ELM440 can be easily configured to

provide a 60Hz output as shown in Figure 1 below.

A 3.579545MHz crystal is connected to the

integrated circuit, along with suitable loading

capacitors, and the 60Hz output is then taken from

pin 7. Pin 5 is grounded, as no reference signal is

supplied. As usual, it is also good practise to place

a bypass capacitor across the power supply as

well.

Figure 2 shows how easily this circuit can be

modified to operate in synchronism with the AC

power line. A reference signal is taken from one

half of a 16Vrms CT transformer winding through a

47K

The resistor was chosen to limit peak currents in

this circuit to about 0.24 mA, which is well within

the IC’s capabilities. The ELM440 will continue to

use this signal as it’s reference, as long as it is

available. Upon loss of the AC, the circuit will

automatically switch over to using the crystal as a

reference.

Using a rectifier circuit for the reference signal

can pose some problems if one is not careful. Full-

wave circuits can be particularly troublesome as

all of the diodes may cease to conduct after the

peak of the waveform, and the input voltage level

is then essentially floating. In this case, at

minimum, a large value (470K

Ω) resistor should

also be connected between the sync input and

times.

Both circuits shown are simple, leaving little

room for change. One modification that might be

considered for either is to replace one of the fixed

value capacitors with a variable one, in order to be

able to trim the oscillator frequency for greater

accuracy.

ELM440

ELM440DSB

Elm Electronics – Circuits for the Hobbyist

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

60Hz Out

0.1µF

+5V

1

2

3

4

8

7

6

5

27pF

3.58MHz

27pF

Figure 1. Typical Connections

Figure 2. Using the Sync Input

0.1µF

+5V

1

2

3

4

8

7

6

5

60Hz Out

27pF

3.58MHz

27pF

47K

Ω

8V

8V

+