Analogue Thermometer - the Hard Way!

This Instructable describes a simple room temperature indicator using an analogue moving coil Meter.

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

The design aim was to use an existing analogue meter with out any great expense or complication, using cheap common "AA" batteries. As it happens, this design requires a regulated 9 volt supply as I was unable to balance the components sufficiently with out adding further complication!

In the Conclusion Section I offer some ideas to overcome the problems encountered.

5 x Small signal Transistor; any NPN with a gain of 100 or more, at least two of the same type for the differential pair.

2 x 15K ohm Resistor

1 x 12K ohm Resistor

1 x 10K ohm Resistor

1 x 6K8 ohm Resistor

1 x 4K7 ohm Resistor

3 x 1K8 ohm Resistor

1 x 220 ohm Resistor

1 x 2K0 ohm variable Resistor (TrimPot)

1 x 100 ohm variable Resistor (TrimPot) - Optional

1 x 5v6 Zener Diode

1 x circuit board

I decided to use the well known temperature sensitivity of a Silicon Transistor. If you explore other methods then you will soon see that it gets too elaborate or needs amplification etc.

There is an alternative method using the LM35 Precision Centigrade Temperature Sensor giving 10mV/°C and is essentially a better version of my "parts box" proposal here and I give an "Instructable" found here: https://www.instructables.com/Analogue-Thermometer/

A Transistor datasheet will show that a characteristic of a transistor Vbe is about 0.65 volts at room temperature and changes with temperature changes. Higher power devices can have a value as low as 0.5v because a factor is the internal resistance but it is still linear.

So, a forward biased base to emitter junction has a voltage drop of about 2mV per degree Centigrade as can be seen if you divide 0.65v by Absolute Zero 273 plus room temperature, say, 20 degrees i.e 650mV/293 = 2.22mV/°C. It is reported that a simple transistor or diode is apparently very linear and usable to very low temperatures.

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 0-5 degrees then full scale would be 50°C.

The full scale range is about 100mV and my meter is 100mV so that should be easy, just connect them up and job done! Nothing is that easy; firstly there is an offset at room temperature at 0.65 volts and secondly a silicon junction gives a negative 2mV/°C and it is customary for increasing numbers to be shown to be rising or moving to the right!

See: https://en.wikipedia.org/wiki/Silicon_bandgap_temperature_sensor

I decided to make a differential pair transistor amplifier because it will be largely immune to voltage supply change and removes the room temperature offset problem. A third transistor is used as the temperature sensor. A further two transistors are used to create a "constant current source" for the missing "long-tail" due to the low voltage supply.

The differential output taken from the transistor collectors is 100mV full scale.

See: https://en.wikipedia.org/wiki/Differential_amplifier

The idea here is that if the differential pair is as balanced as I can manage then they will also track together with temperature such that only the third, extra, transistor will be responsible for tracking temperature change.

The simple diagram shows the concept but the detailed schematic is what is needed!

Good practise suggests that the differential pair transistors are thermally joined, either mechanically or in the same package.

The inclusion of the Emitter 100 ohm variable Resistor (TrimPot) is optional but ensures good balancing.

With the addition of a 2 way change-over switch and a 10K ohm Resistor the supply voltage can be monitored.

This is in two parts as the differential pair are to be balanced and the absolute value needs setting.

Set the Emitter 100 Ohm trimmer to mid-value 50 Ohms. Connect the 9 volt supply and the Meter should show some value on scale: Adjust the bias trimpot to get Zero reading (the offset has been removed); now adjust the Emitter 100 Ohm trimmer so that the Emitter voltages are the same. Now adjust the bias variable resistor to give room temperature reading (offset has been restored).

The Meter worked as intended but was too sensitive to supply voltage variation at about 2mV/1 Volt change

i.e 1°C after the Zener was introduced to improve the voltage sensitivity; at the 6 volts design it was +/-4°C for +/- 1 volt without the Zener stabilization. The Zener chosen was BZX79 5v6 for low temperature coefficient.

The design voltage had already been increased to 9 volts from the desired 6 volts (I only wanted 100mV full scale!)

To achieve the original aim of 6 volt battery supply it will be necessary to add further amplification and drive the meter as a voltmeter to overcome these problems. So, as I mentioned it was getting too complicated and easier to lean on the experts, hence the LM35 solution.

I do hope that this was interesting as an "Instructable"