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 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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Ramsey Electronics CT255 manual Circuit Description, Binary counter
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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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