Figure 6 below shows how the ELM403 might be

used with a microprocessor. Pullup resistors of 10 K

Ω

are used with the rotary encoder to provide voltages

for the A and B inputs. Power for the ELM403 has

been obtained from the microprocessor circuit, but a

small capacitor has been added to ensure that the

supply is filtered. The output is not inverted (pin 5 is at

0V), so pins 6 and 7 are normally at 0V, providing

positive-going pulses only when the rotary encoder is

turned. To allow the microprocessor plenty of time to

poll the signals, an output period of 2.0 msec has been

with any voltage from 2.0 to 5.5 volts.

The other schematic (Figure 7) shows how the

ELM403 might be used with a digital potentiometer

chip, in this case the Maxim MAX5128. Note that the

ELM403 does not support the write function for chips

such as this, so every time that the circuit is powered

up, the wiper returns to the midpoint position. Since

the Maxim data sheet specifies a rather large bypass

capacitor (0.1 µF), we have shown it here, but you

may not need a capacitor that big if there are no writes

being performed. The ELM403 has been set for

0.2 msec positive going output pulses which are

considerably more than the 80 nsec minimum time. As

the MAX5128 requires positive-going pulses, we have

not put any pullups on the ELM403 outputs, but this

may be a consideration with other potentiometers.

That’s about all there is to using the ELM403.

Connect pullup resistors to your rotary encoder,

connect the encoder signals to the ELM403, then

connect the ELM403 to the controlled circuit. Use a

common supply for all the devices, and add a bypass

capacitor across the supply line. The controlled

devices can be digital pots, microprocessors, or almost

any circuit that will accept CMOS signals. Enjoy!

Example Applications

Figure 6. Connecting the ELM403 to a microprocessor

Figure 7. Connecting the ELM403 to a MAX5128

9 of 9

ELM403DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

ELM403

0.01µF

your microprocessor

port pins

1

2

3

4

8

7

6

5

10 K

Ω

10 K

Ω

rotary

encoder

A

B

C

1

2

3

4

8

7

6

5

MAX5128

0.1µF

1

2

3

4

8

7

6

5

+5V

+5V

10 K

Ω

+5V

rotary

encoder

A

B

C

H

W

L