# Modifying a digital thermometer to beep at specific temp

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#### Anonymous

I'm trying to make a digital thermometer beep at a specific temp so I don't have to sit and watch it. Any help on how to build a cheap one would be great too. All the thermistors I see advertised have leads that need to be hooked up to a computer. Any ideas on how to wire some Radio Shack parts up to do it myself? Thanks in advance to any electrical engineers or smart poeple!!

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#### Anonymous

Dear all, The basic circuitry would be a voltage comparator driving a small 'beeper'. If the thermistor operating point(resistance at the point of interest) is available then it would be trivial to compare the voltage across the thermistor and a known voltage(for example set up by a potentiometer.) An opamp and a 555 would be sufficient along with a couple of other passive components. But be careful about where this homebrewed circuitry is going to be used. Have fun.

Tomy

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You didn't specify your desired temperature range, But if you go to EXTECH.COM your heart will go pitter-patter. Item # 401012 is good for a range of -58 to 158 F. and sells for a mere $19.00 + S/H. They also carry a multitude of pyrometers and controllers. T #### thomas Ng Just buy an LM34(Fahrenheit) or LM35(Celcius). They cost about$2 each. They give an analog
voltage whose value increase with the termperature.
You can then simply use an ordinary op-amp (I prefer the general-purpose 351) to trigger the buzzer.