Ramsey Electronics COMPUTEMP CT255 Manual

Binary thermometer kit

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COMPUTEMP 255

BINARY THERMOMETER

KIT

Ramsey Electronics Model No.

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, designed with remote sensing option

•

Resurrected Ramsey kit from the past; build something your Dad

built 20 years ago!

•

Jumper settings for Fahrenheit or Celsius temperature scales

•

Range of 0-127.5 degrees Celsius in 1/2 degree steps. (32—261.5 F)

•

Runs from 7-15VDC.

CT255 • 1

CT255

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Page 1 COMPUTEMP 255 BINARY THERMOMETER 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. •...
Page 2 Ramsey Electronics publication No. MCT255 Rev 1.1a First printing: November 2001 COPYRIGHT 2001 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York 14564. All rights reserved. No portion of this publication may be copied or duplicated without the written permission of Ramsey Electronics, Inc.
Page 3: Table Of Contents
Parts List .......... 15 Assembly Instructions ...... 17 Case Assembly ........ 23 Troubleshooting Guide ..... 25 Conclusion ........26 Warranty .......... 27 RAMSEY ELECTRONICS, INC. 590 Fishers Station Drive Victor, New York 14564 Phone (585) 924-4560 Fax (585) 924-4555 www.ramseykits.com...
Page 4: Introduction
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.
Page 5: Circuit Description
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.
Page 6 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.
Page 7 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.
Page 8 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.
Page 9 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.
Page 10 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. 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.
Page 11 128 times to get the most significant LED to change state just once. Normally an 8 bit number has these significant values: Bit #: Value: Say bits 7, 3, and 0 are on, all others are off. The value would then be 128 + 8 + 1 or 137.
Page 12 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.
Page 13: Schematic Diagram
CT255 SCHEMATIC DIAGRAM CT255 • 13...
Page 14: Parts Layout Diagram
CT255 PARTS LAYOUT DIAGRAM CT255 • 14...
Page 15 PARTS SUPPLIED WITH YOUR CT255 KIT Capacitors .1 µF disc capacitor (marked .1 or 104 or 100 nF) [C3,4,5,6] 1 µF electrolytic capacitors [C1] 10 µF electrolytic capacitors [C2,7] 470 µF electrolytic capacitor [C8] Resistors 390 ohms (orange-white-brown) [R5,6,7,8,9,10,11,12] 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) [R13,14,16,18,20,25,28,32,35,38]...
Page 16 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. •...
Page 17: Parts List
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.
Page 18 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).
Page 19 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.
Page 20 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.
Page 21: Front View
install all eight so that they are in line. A trick may be to fashion a cardboard “standoff “ and pre-bend the LEDs before installing 35. Install D1, one of the LEDs. The flat side is oriented as shown in the diagram.
Page 22 40. FOR REMOTE SENSOR OPERATION. If you wish to run the sensor to a remote location, you will need to obtain a 3.5 mm stereo plug and wire as shown in the following figure. Flat Side LM35DZ RING 3.5 mm Stereo Plug (Not Included) SHIELD Remember that the temperature sensing circuitry is relying on a very small...
Page 23: Assembly Instructions
CONGRATULATIONS ! Your binary thermometer is now complete! Have a final look over your work, paying particular attention to the orientation of diodes, capacitors, and IC’s. Remember that any problems you find now can save time and effort after the unit has been cased up and final assembled.
Page 24 USE OF YOUR COMPUTEMP 255 Now we are ready for the moment of truth, the initial power up and running of your binary thermometer. Energize the unit with a suitable 7.5 to 12DC power supply (noting that the center pin of the power supply is the positive connection) and watch your thermometer count up the first of many times.
Page 25: Troubleshooting Guide
TROUBLESHOOTING GUIDE If your CT255 does not work at all, recheck the following: • correct orientation of VR1, U1, and U2 (see PC board layout diagram) • You should be able to measure the voltage on the input and output termi- nals of voltage regulator VR1.
Page 26: Conclusion
As our customers, we value your opinions, comments, and additions that you would like to see in future publications. Please submit comments or ideas to: Ramsey Electronics Inc. Attn. Hobby Kit Department 590 Fishers Station Drive Victor, NY 14564 And once again, thanks from the folks at Ramsey! CT255 •...
Page 27: Warranty
But on rare occasions a sour component can slip through. All our kit parts carry the Ramsey Electronics Warranty that they are free from defects for a full ninety (90) days from the date of purchase. Defective parts will be replaced promptly at our expense. If you suspect any part to be defective, please mail it to our factory for testing and replacement.
Page 28: Victor, New York
Assembly and Instruction manual for: RAMSEY MODEL NO. CT-255 ESTIMATED ASSEMBLY TIME Beginner ... 3.5 hrs Intermediate ..2 hrs Advanced .....1.5 hrs RAMSEY ELECTRONICS, INC. 590 Fishers Station Drive Victor, New York 14564 Phone (585) 924-4560 Fax (585) 924-4555 CT255 • 28 www.ramseyelectronics.com...