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Making My First Digital Thermometer for Less Than $30

  • So the summer holidays are here, and I am not the kind of person to just lay around and do nothing so I thought I would share with you all my first project called:

    "Making My First Digital Thermometer for Less Than $30"

    Ok so doing a quick Google search they are pretty cheap, but from what you will get out of it will be the sweet taste of satisfaction of creating something. ;)

    These directions will show you how to make a simple digital thermometer for less than $30 using a few electronics and three (3) ICs.
    Digital Thermometer could be used:

    • It can be used for measuring child’s food temperature.
    • It can be used for measuring temperature in laboratory experiments.
    • You can use it in measuring body temperature.

    The following are the parts required:

    • ICs: CA3162, CA3161 & LM35.
    • 10k resistor(potentiometer)
    • 50k resistor (potentiometer)
    • 0.27µF capacitor
    • 2N3906 transistor
    • 2N3906 transistor
    • 2N3906 transistor
    • Seven segment LED display - I liked to buy mine online as there were a wider range of choice

    Places to buy this stuff from:

    Newark - http://www.newark.com/
    Premier Farnell element 14 - http://uk.farnell.com/

    I found that a lot of these tools were also available at home too, so the price would probably be a lot cheaper in the end.

    Tools that you will need:

    • Soldering iron - http://www.homebase.co.uk/webapp/wc...ay?langId=110&storeId=10151&partNumber=337237
    • Solder
    • Side cutters (you can use anything to cut the remaining capacitor and resistors legs)
    • Solder sucker (to remove excess solder in case of a mistake )
    • Screwdriver (for adjusting the potentiometers)
    • DIL sockets

    Step 2: The Circuit

    Circuit diagram

    Circuit Diagram - Electronic Components.png

    Mount ICs: CA3162, CA3161 and LM35 on DIL socket and connect the other components in their right positions. You can etch your own PCB or build it on a breadboard using connecting wires. Everything in the circuit diagram is in their exact positions, so you can build it into a circuit board straight away.

    Step 3: Soldering the components


    This is a straight forward step of soldering the components together. Start with the resistors and the capacitor, then the transistors followed power terminals and potentiometers then the IC and seven segment LEDs display last. Keep in mind to use DIL sockets to mount the actual IC, and if you don't use DIL sockets, take care when soldering the ICs pins. The ICs are very sensitive to heat and over exposure can destroy them so you need to wait for each IC pin to cool before soldering the next pin. Switches should be placed on wires for mounting onto the enclosure onto the seven segment LED display, just leave an opening on the enclosure to match the LED display positioning. You can place the whole PCB at the top of the enclosure, unless you actually want to solder wires.

    Step 4: Making the enclosure

    This step is optional because it doesn’t affect its operation, but for the purpose of neatness and being pleasant, a plastic enclosure will be appropriate. I would recommend switches mounting on the outside, drilling a hole for the seven segment LED display and sticking the whole circuit board to the top of the enclosure, with some small holes for ventilation.

    Step 5: Testing

    • Use POT R2 for Zero adjustment.
    • Check whether IC2 and IC1 are properly mounted on holders.
    • Check the capacitor C1 is well placed as close as possible to the power and ground pins of the CA3162.
    • Capacitor C2 could be a polyester type while C1 can be a ceramic capacitor.
    • The DC power supply used for powering this circuit must be well regulated and free from any unwanted noise.
    • Check whether power source is 5v dc for powering the circuit.
    • Power the circuit of the digital thermometer after all of the above testing.

    When on, the seven segment LED display should shows temperature values in degree Celsius.

    Other Sources for Products

    I found using a range of connectors found helped as they had a wide range of sources and suppliers.

    In terms of testing I found using a multimeter was a good choice too!
Harald Kapp likes this.
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