> From:         Bala.Chandar@AVENTIS.COM[SMTP:Bala.Chandar@AVENTIS.COM]
> Sent:         Wednesday, February 18, 2004 4:54 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE:] Sensor for Thermometer

> I have a digital thermometer that also shows humidity and time (TempTec, made
> in Taiwan - Photograph attached). It displays indoor as well as outdoor
> temperatures. For the purpose of measuring outdoor temperature, a sensor is
> connected through a long thin wire. This wire gradually got worn out at a
> point (by getting caught between the window frame and the shutter) and the
> sensor is now lost. Now the outdoor temperature is displayed as "LO" all the
> time.

> The sensor had a plastic case and was circular in shape (around 10mm dia. 3mm
> thick). What can be used in its place now to measure outdoor temperature?

> Would appreciate suggestions / ideas / pointers.

> Regards,
> Bala

An inexpensive temperature sensor can be made from a bipolar transistor connected
as a diode. Tie its collector to its base; determine the polarity of the connection by
measuring the polarity of the open circuit voltage on the wire leads. If the transistor
is an NPN type, connect the positive wire to the collector/base terminal. If the transistor
is a PNP type, connect the positive wire to the emitter.
This will only work if the circuit (in the case) was made for this type of sensor, but
many simple, inexpensive temperature measuring circuits have been made this way,
so your chances are good. The transistor must have good thermal contact with the
air being measured, so a metal case type (2N2222, e.g.) might be preferable.

John Power

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