Electronic Thermometer Circuit Description
In the circuit above T2 is a fixed current device that sinks approximately 70 Micro amps. P1 is adjusted so that this current does not vary with temperature by balancing the negative temperature coefficient of T1 with the positive coefficient of T2. R3 reduces the effect that adjusting P1 has on the set current.
T4 provides a current proportional to the absolute temperature and is adjusted to equal the fixed current through T2 at 0ºC. Thus at any temperature other than 0Cº a current must flow into or out of the voltage divider formed by R7 and R6 and provide a voltage for the meter as it flows through P3 and R5 .
P3 is adjusted so that at some standard temperature the 200 milli Volt meter reads correctly. Simply put T4 is a Kelvin thermometer and T2 subtracts 273.15 to convert it to a Centigrade one.
The LM334z (T4 - T2) is a readily available device that provides a current source that is directly proportional to the absolute temperature ( ºK ) over the range 0 to 70°C for larger ranges there are other devices in the same family that can be substituted.
Great care was taken to ensure that temperature changes to the body of the instrument do not cause changes to the readings. T2 has changes nulled by P1 but the resistors themselves change value with temperature and this is also canceled out as follows. Any change to the resistance of R1,P2 and P1,R2,R4, R3 with the temperature of the instrument body will cause changes to the current of T4 and T2 but the same change will occur to P3 and R5 and keep the voltage across them constant.
For example if all the resistors went up 10% there would be 10% less current through T4 and 10% less through T2 and the difference in the currents would be 10% less through P3 and R5 but as they would have 10% more resistance the output voltage would stay the same. Any change in the output of T3 with temperature has no effect.
T3 regulates the voltage to 5 Volt and C1 and C2 provide filtering.
Electronic Thermometer Construction
The circuit was laid out paying attention to keeping T2 and T1 close together and the resistors that could cause temperature drift in close proximity. An Acrobat reader .pdf file of this is provided and can be printed full size and used to hand make a PCB using a etch resist pen or to photo etch a board if required
The sensors T4 is connected by 3 wires. Thin twin shielded wire was used with the shield connected to the + rail (V+). The connection to the multimeter was made with flying leads fitted with pin sockets insulated by heat-shrink. Left shows a bottom view of T4.
Power consumption is a couple of milli-amps so it can be powered by a 9 Volt battery. A panel meter if used can be run from the 5 Volt supply but it must be a meter that can be run with a common power supply and floating inputs.
T1 and T2 should be in close thermal contact. Winding copper wire around them and applying heat-sink thermal grease will aid stability. The red wire with the knot just passes through the board as the switch is on the opposite side of the board to the battery connector. The 4 unused holes (top right) are are for the power leads to an optional internal panel meter.
Source: Electronic Thermometer Circuit