Example Application

6 of 6

Figure 1. RS232 Control Examples

Figure 1 shows how the ELM621 would typically be

connected in a circuit. The three port pins have been wired

to control an LED and a relay, as well as monitor the state of

a pushbutton.

Since the relay requires 12V for its operation, the main

supply for this circuit would likely be 12V, with the +5V

derived from this using a regulator (typically a 78L05).

Alternatively, many computers have 5V and 12V available

that may be used. The actual source is not critical for this

application, so is not discussed further here.

The RS232 interface circuitry shown is very basic, but

completely adequate. Rather than use a dual supply and

RS232 drivers, the circuit ‘steals’ negative voltage from the

controlling device’s TxD line, and uses it to provide the

negative bias for the ELM621’s output via the PNP

transistor. The PC connected to this port should be set for

9600 baud, 8 data bits, no parity, and either 1 or 2 stop bits.

The relay and LED drive circuits shown are conventional

types. An NPN transistor is used to increase current drive

while interfacing to the relay’s 12V supply, while the LED is

simply driven directly by the B1 port pin.

The pushbutton is wired to B3 with a 1K

Ω resistor in

series to provide some protection against electrostatic

damage. This resistor is not essential if the switch is

ELM621

ELM621DSA

Elm Electronics – Circuits for the Hobbyist

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

mounted nearby, but is good practice. The switch uses the

B3 internal pullup resistance to provide ‘wetting’ voltage.

Assuming the chip has just been reset, this resistance is

enabled in the initiating statement:

AT O1 O2 PN

The above statement sets both B1 and B2 to outputs,

while leaving B3 as an input with the pullup resistance on.

To turn the relay (B2) on, one then needs only issue:

AT S2

while it will be turned off with:

AT C2

Alternatively, AT T2 could have been used, followed by

a single carriage return character each time that a state

change is required. Control of the LED is similar, requiring

an S1 to turn it on, and a C1 to turn it off.

The pushbutton switch state can be determined by

issuing a read command of bit 3:

AT R3

When the switch is pressed, the response will be B3=0,

otherwise it will be B3=1 (due to the internal pullup).

That should get you started. Perhaps with a some

programming,

the interface could be used to monitor a

thermostat and turn the heat on, or control a robot, or write

data to an EEPROM …

10K

Ω

4.7K

Ω

2N3906

1N4148 or

1N4001

+5V

3 (RxD)

7 (SG)

2 (TxD)

RS232

DB25F

Connector

0.1µF

+5V

1

2

3

4

8

7

6

5

47K

Ω

100K

Ω

0.1µF

1K

Ω

220

Ω

2N3904

Control

Output

1N4001

+12V

12V Relay

2.2K

Ω