Electronic – Cheap easy way to test for RF power output

You arn't going to find any generic device that does everything you want, with an unknown amount of different sensors or modules. The dedicated alarm system devices have custom computer interfaces, and have hardware designed to account for power needs and protection from shorts or esd, considerations for the sensors it is designed for, etc.

What you want to do will require multiple parts. The first is the computer interface. That is easy. You can go the diy route and go for an arduino or launchpad or similar microcontroller board with usb to serial adaptors. You will need to program the microcontroller for your final system. There is dedicated hardware you can use though. What you are looking for would be "USB GPIO Boards" (GPIO = General Purpose Input/Output) with how ever many channels you need. A quick google can show you some, like a 32 channel one for 45 bucks.

The other other option is a gpio port expander. If you are going with a raspberry pi, you can skip the arduino and it's programming, and just go directly with making a linux program for which ever port expander you decide on. With an i2c port expander, you can put multiples on the same raspberry pins, making it as expandable as you need. These can even interrupt so your computer doesn't need to constantly check to see if there has been a change in a sensor. And since it's linux, most port expanders already have drivers for them.

-- The second part is the actual interfacing to the sensors. It will depend on the individual sensors you go with. You will need to power them over the entire length, will need to protect the port expander from over current (transistors or optocouplers) and it's a bit hard to even guess at without knowing what you will be using.

You can use a normal silicon junction diode to measure the temperature. Even inexpensive ones like the proverbial 1N4148 will work. The trick is to bias the diode with a constant current source of one milliampere. You need to make sure that the constant currrent source itself is pretty stable with temperature or it will affect the accuracy of the diode temperature sensor.

Once biased in this manner the sensor diode will produce a forward voltage drop that is amazingly linear over a large range. Applications that I have designed worked over the range of -55C to +135C. The forward voltage drop decreases with increased temperature and so is what we call inversely proportional with temperature. The rate of change of the diode voltage is about 2.2mV per 1C.

It is generally necessary to provide for amplification of the diode voltage drop and provide some offset so that the range of temperatures involved can be read via an MCU's A/D converter. An opamp can work nicely for this part of the circuit. If it is necessary to calibrate the sensor circuit it can be done with two trimpots in the opamp circuit to adjust the gain and offset of the amplifier. Calibration can also be done in software of the MCU as well if you provide a straightforward way to save the scale and offset values of the calibration in something like an EEPROM of part of the MCU Flash memory.

The above approach can be a fun learning experience and should work well for you. The diode is fairly easy to mount at the temperature sensing location requiring just two active wires plus possibly a shield if it is an electrically noisy environment. An alternative you may want to consider instead could be to acquire a low cost IC temperature sensor. The old standard LM75A is available in easy to wire SO-8 package and can be connected on a remote cable of just four wires. You would connect the I²C bus connections of the IC to an MCU control board.

There is a slight challenge that needs to be dealt with for the remote mounting of a temperature sensor. The connecting wires can act as a thermally conductive heat sink on the temp sensor thus offset the temperature reading some. For a simple control system this can usually be "adjusted out" by setting the control setpoint down by an equivalent amount.