Ramsey Electronics CT255 User Manual

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, designed with remote sensing option

• Resurrected Ramsey kit from the past; build something your Dad

built 20 years ago!

• Jumper settings for Fahrenheit or Celsi u s te mp era tu r e sc a l es

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

• Runs from 7-15VDC.

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Many other kits are available for hobby, school, Scouts and ju st plain FUN. New kits are always under development. Write or call for our free Ramsey catalog.

COMPUTEMP 255 KIT INSTRUCTION MANUAL

Ramsey Electronics publication No. MCT255 Rev 1.1a

First printing: November 2001

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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 ......... ... .................... 23

Troubleshooting Guide ..................... 25

Conclusion ....................................... 26

Warranty .......................................... 27

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RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone (585) 924-4560

Fax (585) 924-4555

www.ramseykits.com

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 he was fresh out of college. We have come a long way since then!

LENS

CR4

FILTER

CR10

P.C. BOARD

FRONT VIEW OF

PCB BOARD IN CASE

At the time, this case was one of the only good looking extrusion cases that a hobbyist could use for his projects. Now there are a huge variety to choose from. Hobbyists have no idea how good they have it!

A very popular kit at the time, we have brought it back for your enjoyment. It makes a great conversational piece too!

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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 degre es Ce lsius, the output would be 0mV. If the temperature were then raised to 50 degrees Celsius, the output would become 500mV, which is 10mV * 50C. Really simple isn’t it?

In the past we used a diode junction to do the sensing, because a diode junction has a very predictable change of voltage drop versus temperature. In the case of silicon, this drop works out to be 2.1mV for every degree Celsius. Unfortunately using diodes caused us to use calibration to compensate for offset and gain, which was a real pain.

Now it comes down to needing to display this output in a form our eyes can understand, and this means driving a display. In our case we will be displaying the value in a way that both humans and computers can understand: binary. How do we do this easily? Hang on, here we go…

The binary counter

U2:A and U2:B form a cascading ripple counter which is simply designed to count pulses on the clock pin (pin 1), and output the count as binary. The high order bit of U2:A (pin 6) is then tied to the clock input of U2:B to make an 8 bit counter in total.

A binary counter is a very simple set of flip-flops that are fed from one to another in sequence. The lowest order (bit 0) is tied to the input of the next (bit

1) and the output of bit 1 is tied to the input of bit 2 and so on. For every two pulses on the input to the flip flop, the output switches once. When the flip flops are cascaded, they make a “ripple” counter, meaning all outputs are effected by the single input, but all in succession, not simultaneously. When we put the kit together, this count sequence will make much more sense since you can see it occur!

The binary count is then tied to a resistor “ladder”. Believe it or not, this makes a very simple digital to analog converter! As the count increases on the outputs of U2, the voltage increases in a linear step across R38 and C5. This is set up here to be an eight bit digital to analog converter. Many digital to analog converters use a ladder style resistor network just like this. To increase number of bits, they just use more precision on the resistors, and increase the count.

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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 our minimum resolution we find the number of steps the counter has, which is in this case 2^8 which is 256 possibilities. 0 is one of them, so take 3.10V and divide by 255 to come up with 12.16mV per step or count. This means this is the smallest change in voltage that we can measure with our DAC.

When the counter is counting up, you can take a volt meter, probe R38 and see the voltage “ramp” up with the count. On an oscilloscope, you can not only see the voltage ramp up, but also take 255 steps on the way up to 3.10V, each step being 12.16mV! It’s worth it just to check this.

The ADC (Analog to Digital Converter? In a sense…)

Well, you may wonder how this ramp from our counters and resistor ladder can be used to display temperature. This will be done in a way many analog to digital converters used to make conversions. ADCs use much more complicated methods now to increase conversion speed, but that is beyond this manual. Now you know that each count is equal to 12.16mV, so if you have a count of 100, you should have an output of 1.216V from our DAC. Well this means that the display should also be indicating 100 degrees Celsius, right? This also means that our temperature sensor should be putting out 1.216V at 100 degrees Celsius to get this count of 100, but it doesn’t since it only outputs at 10mV per degree Celsius, which would only be 1.00V. This means we need to scale the output of the temperature sensor to meet the needs of our DAC so the count display and the voltages match .

Scaling to Celsius

To scale the output, we just have to figure out what it will take to make 1.00V become 1.216V. Not too tough here, we just multiply our 1.00V by 1.216. But wait, we’re going to make things complicated. We want to display temperature in 1/2 degree increments. This means that we have to multiply the sensor output by 2 to multiply our reading by 2. In this case we need 1.216 x 2 or a gain of 2.422. Since the display has a potential count of 255, and we will never find an air temperature of 255 degrees Celsius (and like it), we can divide the display by 2 to scale it to the range of 0-127.5 and use the last LED for 1/2 degree steps.

U1:A is set up as a non-inverting amplifier, and the output of the temperature sensor is sent to the non-inverting input (pin 3) to be amplified. R39 and C6 are

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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 th e 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 the input isn’t at R37, it is considered that for mathematical purposes).

So looking at the values in the circuit: A = 1 + 1430 / 1000 or A = 2.43 As you can see this is very close to our requirement of 2.422, which is more

than close enough for +-1 degree C resolution.

More on the ADC

To stop the count when the output voltage of our temperature scaling amplifier matches the count of our ripple counter DAC, we use a circuit called a comparator. A simple op-amp makes an excellent comparator, as shown by U1:B. When the voltage at pin 5 is above the voltage at pin 6, the output at pin 7 goes as high as possible, in this case about 3.9V. When the voltage at pin 5 is below the voltage at pin 6, the output goes as low as possible, or zero volts. It is called a comparator because it is in reality comparing two voltages, and giving you an output based on the difference. We employ this circuit to stop the count in our ripple counter as soon as its output voltage surpasses the output of our temperature sensor circuit. This allows us to take a reading from the display, and this is what a computer would see as a value for a complete Analog to Digital conversion!

Oh that extra count!

Ok, maybe you’re smart and figured out that for the count to stop, the output voltage of the DAC has to surpass the output of the temperature sensing circuit. This means that if your temperature is zero, and the count is at zero, the output of the DAC does not quite surpass the output of the temperature sensor. This means that the temperature reading will have to increment at least once to have a reading. This m e a ns th at your temperature readings will at the most be high 0.5 degrees F or 0.5 degrees C depending the range you are in. However, on average it will only be 0.25 degrees high. All ADCs have this problem, and it is related to the bit resolution (in our case 8 bits). This is still within +-1C accuracy.

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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 pin 12. When the output of U1:D is our max of 5V and the output of U1:B is 0V (running mode), the voltage at pin 12 should be about 3V (R4, R2 and R3 act as a voltage divider). At this point C1 is charging through R1 over a period of time. Eventually its charge will surpass our 3V at pin 12, where the output of the opamp comparator will switch to low. This makes our voltage at pin 12 about 1.6V. Now the capacitor C1 discharges until it surpasses 1.6V in the opposite direction, and the output switches back to high. This repeats over and over and is the basis of our oscillation. The reason why the oscillator stops when the output of U1:B goes high is the voltage at pin 12 becomes a level that the charge on C1 can never reach. It is greater than 5.0V due to R15 limiting the voltage on C1 to 3.4V, so the comparator function is disabled.

Restarting the conversion cycle

Now our count stops at a certain voltage, but how do we make the count reset every so often so we can get a new reading? Simple. We have another opamp stage that is set to be yet another comparator. In this case we are comparing the voltage between R30 and R34, in this case 2.5V, to the charge on C2. C2 charges through R24 in about 5-6 seconds. This capacitor charges while the output of U1:B is in the disable count mode with the output being high. Once the charge on the capacitor surpasses the 2.5V threshold, the output goes high, clearing the count on the ripple DAC.

This in turn re-enables the oscillator of U1:D since U1:B compare values are reset, and C2 then discharges quickly through D9 to get ready for the next round starting with the first clock pulse in U1:D. C2 does not charge until the count is complete, so the reset cycle will hold the finished count long enough to be seen.

For the complicated Fahrenheit design

Skip this section if you don’t want a bad headache. Now how do we switch to Fahrenheit? Not a simple as it sounds. For one our

Celsius to Fahrenheit conversion is listed as: F = 9/5 * C + 32 We see by this formula that not only do we have to scale by 9/5 we also have

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to add 32. We do this by switching in R22 into the circui t, 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 steps!) This means we need pin 6 to be 778mV when the output of U1:A is 0.0V.

We do this with our voltage divider of R22 an d R2 3. We know our supply voltage is close to 5.0V from our regulator VR1, so we choose a random resistor for R22 within the 1K to 10K range. In this case we choose 10K. Now we can find R23 using the formula:

Vout = Vin * R23 / (R22 + R23) or 0.778 = 5.0 * R23 / (10000 + R23) Rearranging we get: R23 = Vout * R22 / (Vin—Vout) or R23 = 0.778 * 10000 / (5.0—0.778) So R23 = 1.84K. The closest 1% tolerant resistor is then 1.82K. Now we have to find out what it takes for 100.0 degrees Fahrenheit to have a

proper count of 100.0 on the display. For this we need a voltage of 12.16mV * 100 * 2 or 2.432 volts. This will have to be at pin 6 of U1:B, not pin 1 of U1:A. So how do we get that?

First we have to work backwards from pin 6 of U1:B. Here we will insert our

2.432 volts. This means the voltage at pin 1 of U1:A has to be found by taking the 5.0 volts on R22 on the header side and the 2.432 volts at the pin 6 side into account to find what the voltage needs to be at the pin 1 of U1:A side needs to be to make the numbers work. This can be found by looking at the current through R22, which is found through Ohms law by:

I(R22) = (5.0V—2.432V) / 10000 = So I(R22) = 0.256mA Then using this current to find the voltage across R23. So V(R23) = 1.82K * 0.256mA or 0.467 volts across R23. This does not take into account current into pin 6 of U1:B, which could effect

these levels considerably if it is large enough. We have designed in a nice rail to rail opamp to reduce this to a minimum. This means the output of U1:A needs to be 2.432—0.467 or 1.965 volts at 100.0 degrees Fahrenheit.

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+ 19 hidden pages