Example Application

4 of 4

Figure 1. Backup Heating Control Thermostat

Figure 1 shows the ELM331 in an example heating

control

circuit.

A

closed

contact

output

occurs

anticipated that this type of circuit could possibly be

used to control temperatures over the range of -40°C

to +40°C.

Power for the circuit is from a 12V supply, that is

reduced to 5V by the 78L05 regulator. This gives a

stable supply voltage for the ELM331, as well as

convenient voltage for use with a standard relay coil.

The type of relay is not important, as long as

consideration is given to its coil requirements, and the

capabilities of the ELM331. In this example, a relay

with a 400

Ω coil resistance was chosen so that a

2N3904 could drive it directly.

which is a negative temperature coefficient type

thermistor. It has a resistance of 10K

Ω at 25°C, and

this value decreases with increasing temperature. This

value was chosen both because it is commonly

available, and because it limits the 0.1µF integrating

capacitor currents to less than 1mA over the typical

range of operation (keeping the thermistor self-heating

to a minimum).

If the thermistor is mounted any appreciable

distance from the ELM331, consideration must be

given to cabling effects such as capacitive and induced

currents. Generally the integrated circuit can be

adequately protected by mounting a small value (220

Ω)

resistor physically close to the ELM331 as shown

below. Take into account it’s value when determining

the setpoint, though.

contact closes for any measured temperatures less

than 10°C. The resistance value was determined from

specs given by the manufacturer, but could have been

determined experimentally as well.

An LED has been provided for visual feedback of

the

circuit

operation.

It

is

connected

to

the

‘measurement in progress’ output, so that it is

energized each time a measurement is being made.

Typically, this would be for about 25mS every 2

seconds.

Variations on this circuit could easily be made…

cooling control thermostat… Rather than a relay output,

the circuit could have been connected directly to other

logic circuits. The measurement in progress pin could

then be used either as an interrupt, or as a busy flag

that can clock in new results on it’s falling edge…

Battery backup is another option that could be

added to this circuit, but then consideration should be

given to using the ELM341 Low Power Thermostat…

ELM331

ELM331DSB

Elm Electronics – Circuits for the Hobbyist

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

2.2K

Ω

560

Ω

10K

Ω

0.1µF

78L05

2N3904

LED

18K

Ω

10K

Ω

@25°C

1N4001

To the

heating

control

0.1µF

+12V

+12V

0.1µF

1

2

3

4

8

7

6

5

12V Relay

see text