Looking for a cheap way to measure temperature

[Mo_AB_4EVER 0]

Hi everyone! My friend wants to control a roof door for regulating temperature in a greenhouse. His PLC is a TI305, with only discrete IO. I have no analog IO on the PLC. I tought of measuring the temperature with a cheap temperature sensor, rig it up to a PC that he already has, and transmit the temperature to the PLC via a DDE/OPC server. This is the ideal way because I would like to log the temperature to the hard disk also. In other words, in this project, bringing the temperature into DDE/OPC is easier than adding a thermocouple card on the PLC. Or maybe I could rig a thermocouple to a cheap omron controller that has RS-232 communication. Or maybe there are standard kits with sensor and software for DDE access that can be plugged directly into the computer via RS-232 or USB... Anyway, we need to get the temperature into the PLC, with idealy a resolution of 0.1 degres, but probably 1 degree is okay. Anyone has ideas, other solutions and/or experience regarding this issue?

[Beuwolf_1 0]

Have you thought about a cheap temperature controller to xmit the temps to the plc via RS 232? I think if you just used the controller as a form of temp sensor you shouldn't have a problem. Or you could just use the dry contact of the alarm of the temp controller to tell the PLC to run what ever heat functions you needed, and you gain the fun of PID control as well, which would help prevent overshooting and temp "sag" Hope this helps, Beuwolf

[Mo_AB_4EVER 0]

It is a viable solution, but this implies some programming on the PC side, if drivers don't exist for the said controller. I was thinking of an out-of-the-box solution, containing a temp sensor and a software CD that puts the temperature available via DDE or OPC.... Thanks anyways

[Pierre 1]

One of the cheapest way is through 1-wire devices. Many can be networked to a PC and the protocol is simple. They also are dirt cheap. Pennies or a few bucks for a mounted board Here are some links: http://www.hobby-boards.com/catalog/index.php?cPath=24 http://www.maxim-ic.com/products/ibutton/s...e/1wire_api.cfm http://www.i2cchip.com/

[steve_33 0]

You could use a basic mercury-switch thermostat from your local hardware store for about $20. Connect it to a discrete input on the PLC. You did say 'cheap'. At least it will keep the plants from getting toasted until you work out the OPC, data logger, thermocouple, RS-232 comms, and USB issues.

[Mo_AB_4EVER 0]

Yeah, the initial setup will be just like that. The real issue here is logging the actual temp, as well as using it for control. I've seen the 1-wire sensors on the web, I think they are worth giving a shot. Thanks for the info, Pierre!

[Big Country 3]

Automation Direct sells a very nice process controller, that is around 150 bucks. It takes rtd or regular thermocouples. It has 2 selectable alarms that turn on a isolated relay for an input or whatever.

[Mo_AB_4EVER 0]

Souds good, never thought that automation direct made such things. I'll take a look, thanks!

[AutoCraft 0]

Sparky, I'm wondering what you finally decided to go with? Measuring temp. with a PLC cheaply is an issue that is hard to solve. It would be nice if a single wire transmitter could be read through the PLCs serial port. These devices can be purchased for around 20 bucks.

[BobB 58]

I am using a PLC to measure temperature, humidity etc in 5 shadehouses. I grow orchids. I bougfht some temperature/humidity sensors that are made in Checoslovakia. Unfortunately they are not cheap but they are good. Analogue output 4-20ma into the PLC. Using the humidity readings to run a fogger to keep humidity at a premium for the little dears. If the temperature gets really high I set off misters (as opposed to foggers) to reduce the temperature. Also use the PLC to automatically water - it has a wireless connection to my computer. Running Citect SCADA on the computer to log temperature and humidity. A mate of mine also developed some devices for measuring Ph and moisture level in pots. Having a play with those at the moment. Some things wrong with them but looking promising. I cannot help you with cheap as it seldoms relates to good. Depends what you are looking for I guess.

[DanW 41]

1) How do you propose to control the roof door vent(s) ? Some sort of actuator is needed to position the vent/dampers/whatever. Is the door either fully open or fully closed so that a relay output either drives the door open or drives the door closed? 2) Cal Controls sells single loop on/off-PID temperature controllers. Cal had a Windows program some years ago, WIN 98 era, that would datalog the data from the controller (via RS-232, I think), show data as a trend and display a controller faceplate to show the current temperature. I don't know if CAL still offers the program. They also offered separately an OPC server, but OPC servers are not cheap (~$400 at the time) and there are no free ones. But an OPC server is worthless if you aren't running an OPC client. I am unaware of commercial DDE communications packages in the single loop world. The single loop controller world is huge, probably 50-100 vendors worldwide. I suspect other brands offer a low cost Windows data logging program. You're wrapped in data logging, but you might want to think about what the final control element action is going to look like. Maybe misting (turn a valve on) makes more sense?