Analogue Thermometer

I used a spare analogue voltmeter as a room temperature indicator. The Meter acts as a showpiece as it has a large movement which shows the d'Arsonval construction.

https://en.wikipedia.org/wiki/Jacques-Ars%C3%A8ne_d%27Arsonval

This Instructable describes a simple room temperature indicator using an analogue moving coil Meter powered by four "AA" batteries.

The Meter can be switched to check the Battery voltage level.

This is an easy project using a LM35 Precision Centigrade Temperature Sensor as it operates from a voltage as low as 4 Volts meaning a 6 volt battery pile of 4 x 1.5 volts will give a long life of many months.

1 x LM35C or LM35D Precision Centigrade Temperature Sensor TO-92 package.

1 x Battery holder (4 cell)

1 x Battery connector

4 x 1.5 volt "AA" Batteries.

1 x 2 pole change-over switch

1 x 6K8 ohm Resistor

1 x 1K ohm linear variable Resistor (TrimPot)

1 x circuit board

I have a 100mV/1mA meter movement and I want to display room temperature so, if 25°C is centre scale and say the lowest temperature would be 0°C then full scale would be 50°C.

The LM35 gives 250mV at 25°C; the Meter requires 1mA for 100mV full scale meaning half that for half scale.

So, the difference is 250mV - 50mV at 0.5mA gives a series resistor value of 200/0.5 = 400ohms so a 1Kohm variable resistor puts that nicely in the middle-ish.

To measure the Battery voltage of a 6 volt battery pack then a resistor is needed in series with the meter so,

6v/1mA Full scale is 6Kohms meaning a 6K8ohm resistor will give a reading safely under full scale and gives an indication just above centre-scale for a depleted battery voltage of 4 volts.

In a convenient box/enclosure mount the switch and battery holder.

Connect the Meter terminals to the change-over switch common contacts.

Connect the Battery Negative to both switch terminals that are switched to the negative pole of the Meter.

Connect one end of the Variable resistor to one switch terminal on the positive pole of the Meter and the fixed Resistor to the other switch terminal.

Connect the Variable resistor Wiper connection to the spare Variable resistor terminal (prevents open-circuit over-run) and then connect to the LM35 Output pin2 (middle).

Connect the negative battery connector wire to the switch Negative and to the LM35 GND pin3

Connect the positive battery connector wire to the Meter series resistor and to the LM35 +Vs pin1

The addition of a 2 way change-over switch and a 6K8 ohm Resistor the battery voltage can be checked.

Connect one end of the 6K8 ohm Resistor to positive battery connector wire and the other end to the change-over switch terminal.

Caution: Do not connect the Battery until the variable resistor is set to mid value resistance to ensure the Meter is not over-loaded.

Set the switch to measure battery voltage.

Connect the Battery and check the Battery voltage is as expected.

Set the switch to measure sensor voltage.

Adjust the variable resistor to give the correct reading on the Meter: The LM35 specification says that the output voltage will be within 0.5°C. This can be compared to a known temperature or the mV output value can be measured (i.e 10mV/°C so room temperature will be some 200mV) then the variable resistor can be adjusted.

Caution: If the sensor is wired remotely then the manufacturer's application notes should be examined for correct usage.