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 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 and R23. 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.

CT255 • 9

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Ramsey Electronics CT255 manual
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CT255 specifications

The Ramsey Electronics CT255 is a versatile and advanced communication tool designed for radio enthusiasts and professionals alike. As a 2-meter FM transceiver, it is known for its robust performance and extensive feature set, making it ideal for both casual users and serious operators.

One of the standout features of the CT255 is its compact design, allowing for easy integration into various environments such as home stations or portable setups. The unit is built with user-friendliness in mind, featuring an intuitive interface with a clear display and easily accessible controls. This ensures that users can quickly tune into their desired frequencies and modify settings without extensive technical knowledge.

The CT255 operates on a frequency range from 144 to 148 MHz, providing access to the popular 2-meter amateur radio band. This frequency capability is complemented by an impressive output power of up to 25 watts, ensuring robust communication over extended distances. It also includes a built-in automatic repeater shift, which simplifies contacts through repeaters, making it an excellent choice for those looking to enhance their communication range.

In terms of technology, the CT255 employs advanced digital signal processing (DSP), which significantly improves audio clarity and reduces background noise. This makes communication clearer, even in less-than-ideal conditions. Additionally, the transceiver features an array of scanning functions, including memory scan and priority scan, allowing users to monitor multiple channels effortlessly.

The CT255 comes equipped with multiple memory channels, enabling easy access to frequently used frequencies. This feature is particularly useful for users who participate in various nets or have multiple repeaters they commonly contact. Moreover, the unit supports CTCSS and DCS tones, which adds an extra layer of privacy and reduces interference from unwanted transmissions.

Safety and durability are also key characteristics of the CT255. The robust construction ensures it can withstand the rigors of regular use, making it suitable for field operations. It is designed with heat dissipation features to prevent overheating during prolonged use, ensuring reliability during critical communications.

In conclusion, the Ramsey Electronics CT255 is a well-rounded transceiver that brings together advanced technology, user-centric design, and excellent performance. Its range of features makes it an appealing choice for radio amateurs seeking a dependable and efficient communication solution. Whether for casual chatting or emergency communications, the CT255 stands out as a reliable tool that meets the demands of modern radio users.
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