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COMPUTEMP 255
BINARY THERMOMETER
KIT
Ramsey Electronics Model No. CT255
A really neat educational kit to learn the principles of binary
math, how simple digital to analog converters work, and how
instrumentation amplifiers may be used to measure the
environment.
• Visibly counts up to the current temperature in binary numbers !
• Eye catching display, great for home or office
• Super accurate readout of +-1 degree Celsius, no calibration
required !
• Precision sensor, desi
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Ramsey Publication No. MCT255 Price $5.00 KIT ASSEMBLY AND INSTRUCTION MANUAL FOR COMPUTEMP 255 BINARY THERMOMETER KIT TABLE OF CONTENTS Introduction ...................................... 4 Circuit Description ............................ 5 Schematic Diagram .......................... 13 Parts Layout Diagram ...................... 14 Parts List .......................................... 15 Assembly Instructions ...................... 17 Case Assembly ...........................
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INTRODUCTION Years ago Ramsey Electronics had a really neat little kit called the CompuTemp 127. This was a favorite kit purchased by many who were into the electronics hobby in the ‘70s and ‘80s. It was unfortunately retired to make space for a growing list of other popular kits being added to the catalog at the time. This is an original drawing (touched up) from the original 6 page manual showing the stylish kit case that John Ramsey had spent his hard-earned money on the die for when
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CIRCUIT DESCRIPTION There’s a lot of circuitry in the CT255 that is in use to this day in a variety of circuits. We will begin with the temperature sensor itself, detailing how we did this 20 years ago and how we do it now. The actual temperature sensor is the LM35DZ. It is a specialized part that is pre-calibrated at the factory to output 10mV per degree Celsius. For example if we connected the part to power and measured the output at 0 degrees Celsius, the output would be 0mV. If the te
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The DAC (Digital to Analog Converter) The resistor values in this ladder are chosen to give us nice even steps in the voltage range we are able to work with. The outputs of the counter can’t actually achieve 5 volts with the LEDs on the output, but really close at 4.67V. We then figure what the voltage will be across R38 when all outputs of the counter are at 4.67V and we will have the maximum output of our DAC. In this case it is 3.10V. Of course the minimum works out to be 0V. To find ou
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in series with the input to reduce any noise that may be present on the temperature sensor to increase accuracy of your reading. C3 in the feedback branch is also used to reduce noise in the reading. In the Celsius jumper setting, R37 and R27 are used to adjust the gain of our temperature sensor for a Celsius reading. The gain of a non-inverting amplifier is calculated by the formula : A = 1 + Rf / Ri. Where Rf if the feedback resistor R27, and Ri is the input resistor R37. (Even though t
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How the counter oscillator works U1:D is set up as a simple oscillator to generate our clock pulses that drive the ripple counter. When the comparator U1:B output is low, this circuit will run normally, generating pulses for our counter. When the U1:B comparator output is high, the oscillator stops. This is how the count is “held” for display. This oscillator works through the same principle as the comparator, by comparing the charge voltage across C1 charged through R1 to the voltage at
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to add 32. We do this by switching in R22 into the circuit, which combined with R23 adds in the 32 offset that we need. The problem however is this acts like a voltage divider on the output of U1:A our scaling amplifier, so now we have to adjust the non-inverting amplifier’s gain to compensate. First off we need to set the zero point of the sensor to a count of 32. This requires us to have a voltage on the opamp pin 6 of 12.16mV * 32 * 2. (Don’t forget to scale by 2 for our 1/2 degree ste
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At 100.0 degrees Fahrenheit the temperature sensor will have an output of a value related to Celsius, so to make life easier, we will convert to Celsius first. So: C = 5/9(F-32) or C = 5/9(100—32) which is 37.78 degrees Celsius. This means the output of the sensor should be 377.8mV at 100.0 F. So to convert 377.8mV to 1.965 volts we need to multiply by: 1.965 / 0.3778 or 5.201. This will be the gain we need from the non-inverting amplifier. We go back to our old non-inverting amplifier f
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128 times to get the most significant LED to change state just once. Normally an 8 bit number has these significant values: Bit #: 7 6 5 4 3 2 1 0 Value: 128 64 32 16 8 4 2 1 Say bits 7, 3, and 0 are on, all others are off. The value would then be 128 + 8 + 1 or 137. Say instead bits 6, 5, 4 and 1 are on, all others are off. Our value would then be 64 + 32 + 16 + 2 or 114. Our display is set up to read from 0.0 degrees to 127.5 degrees in 1/2 degree steps. What we do is stick in an imag
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You may notice though that the smallest step in this case is 1/256. This 8 bit DSP is unable to accurately represent this number smaller than 3.906E-3. Most fixed points DSPs however are 16 bit, 24 and 32 bits. 24 bits of resolution means the smallest number we can represent becomes 1/(2^24) which is 5.96E-8, which is considerably smaller and more accurate. Still the math isn’t perfect, and these “steps” will accumulate over time and cause errors. A 24 bit DSP number is usually in a 0.24 f
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CT255 SCHEMATIC DIAGRAM CT255 • 13
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CT255 PARTS LAYOUT DIAGRAM CT255 • 14
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PARTS SUPPLIED WITH YOUR CT255 KIT Capacitors 4 .1 µF disc capacitor (marked .1 or 104 or 100 nF) [C3,4,5,6] 1 1 µF electrolytic capacitors [C1] 2 10 µF electrolytic capacitors [C2,7] 1 470 µF electrolytic capacitor [C8] Resistors 8 390 ohms (orange-white-brown) [R5,6,7,8,9,10,11,12] 1 1K ohms (brown-black-red) [R39] 13 10K ohms (brown-black-orange) [R2,3,4,17,19,21,26,29,30,33, 34,36,40] 10 20K ohms (red-black-orange) [R13,14,16,18,20,25,28,32,35
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RAMSEY "LEARN-AS-YOU-BUILD KIT ASSEMBLY There are numerous solder connections on the CT255 printed circuit board. Therefore, PLEASE take us seriously when we say that good soldering is essential to the proper operation of your binary thermometer! • Use a 25-watt soldering pencil with a clean, sharp tip. • Use only rosin-core solder intended for electronics use. • Use bright lighting; a magnifying lamp or bench-style magnifier may be helpful. Do your work in stages, taking breaks to
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CT255 THERMOMETER ASSEMBLY INSTRUCTIONS Although we know that you are anxious to complete the assembly of your binary thermometer kit, it is best to follow the numbered assembly steps when building. Try to avoid the urge to jump ahead installing components. Since you may appreciate some warm-up soldering practice as well as a chance to put some landmarks on the PC board, we’ll first install some of the larger components. This will also help us to get acquainted with the up-down, lef
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installed). Since a diode will only conduct when it is forward biased, this diode acts as a “reverse voltage” protection circuit. Diodes are often used as “one-way switches” in this manner to protect internal circuitry. 3. Identify C8, the 470 uF electrolytic capacitor (small cylindrical component coated with plastic and marked 470). Electrolytic capacitors are polarized with a (+) and (-) lead and must be installed in the correct orientation. Ordinarily, only the negative side is marke
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Next we will install some of the output voltage divider resistor networks. The circuit board has been laid out to avoid confusion with these multiple rows, and we will try to avoid sounding too redundant when installing them, Remember to save those leads as we will be installing some jumper wires very soon. Pay particular attention when installing so as not to mount the resistors in the improper locations. 7. Install R36, R33, R29, R26, R21, R19, and R17, all 10K ohm (brown- black-o
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16. Install C2, 10 uF electrolytic capacitor. Remember to observe polarity with electrolytic capacitors! Check the parts placement diagram for correct orientation. 17. Install R34, 10K ohm (brown-black-orange). 18. Install R30, 10K ohm (brown-black-orange). 19. Install R4, 10K ohm (brown-black-orange). 20. Install R2, 10K ohm (brown-black-orange). 21. Install C1, 1 uF electrolytic capacitor. Remember to observe correct polarity! 22. Install R1, 47K ohm (yellow-violet-orange).
