Help with a project - controlling a relay remotely by SMS

[rengler]

Hi everyone,

I'm new to this forum and I'm looking for some advice. I'm a journeyman electrician, so I am familiar with electricity and circuits, and I have taken some electronics training during my technical schooling, but that was almost 20 years ago, and I don't have much practical experience with electronics since then. I don't really have any programming experience.

I have an idea for a project, but I need some advice on how to proceed. I am looking for a method of turning on and turning off a 1200W, 120V AC electric heater from a location that is about 20km away. My idea is to use a GMS modem, connected to a microcontroller, connected to relay that has a contact rating that is adequate for switching the load of the heater. I would like to be able to send an SMS message to the GSM modem, which would communicate with the microcontroller, which would send an output to energize the relay, which would switch on the heater. I could the send a second SMS message which would turn off the heater. The heater would be used to preheat an aircraft engine without the need to actually drive to the hangar the night before a flight to turn on the heater.

I'm wondering what type of microcontroller would be best suited to this application, and my experience level. Ideally it would be reasonably priced and relatively easy to program. I'm also wondering which GSM modem would be best for this application, or is there another type of technology that would be better for this application?

Thanks in advance for you suggestions.

 

[Sceadwian]

Sparkfun sells cell phone chips that will do just what you're asking.
ADH8066 GSM Module - SparkFun Electronics
50 dollars. Just about any general purpose micro controller will communicate with those boards, though it would be recommended to find one that had an onboard buffered UART.
It's not the greatest size of connector to chose for a starting project but if you're dedicated it should be manageable, if you can find a compareable GSM module elsewhere that has standard .1" headers on it it would make prototyping easier.

For the distances you're asking a cell phone SMS is a very good choice, keep in mind however the reliability of SMS messaging isn't the greatest so you'll probably want to have the device send an acknowledgment back when it turns on. SMS messages can get lost or be delayed by minutes or hours. No wireless sollution is going to be 100% failsafe so make sure whatever heater you're toggling can't damage something if it doesn't turn on or off when you send the message.

 

[rengler]

Thanks for the link, looks interesting. The heater won't damage anything if it stays on, but I agree it would be great to get some feedback to know if the heater was operating. Maybe I could connect a temperature sensor of some kind to the microcontroller to monitor the temperature in the engine compartment.

 

[rengler]

I was doing a little more research on the ADH8066 module you posted above, and it looks like they make a breakout board for it that would solve the connector issue.
ADH8066 GSM Module - SparkFun Electronics

 

[KeepItSimpleStupid]

There was a Circuit Cellar article similar to this. It used email messages sent from a phone to an email gateway.
The program basically checked the email periodically for a specifically formatted message and the controller executed the proper commands.

Check the ads in current issues of Nuts & Volts and Circuit Cellar for ideas. There may be commercial products that will work for you.

Minimizing the amount of data might be an issue. I did some playing around with a phone with a GSM modem built into it (Motorola Razr V3xx). Retreiving TXT messages probably isn't hard. I know expensive ways to do it, but not simple ones. LabView, a PC, the right phone, and a couple of USB devices.

Labview does the PC, does embedded devices and does Linix, but I have no experience with the latter two.

 

[MikeMl]

Some of my pilot buddies use a commercial system that does this. They use it to remotely switch on an engine preheater prior to going flying. Seems that they but the system, but then have to pay for cell SMS service for the box.

ps,**broken link removed** being used by the aircraft owners...

 

[rengler]

Some of my pilot buddies use a commercial system that does this. They use it to remotely switch on an engine preheater prior to going flying. Seems that they but the system, but then have to pay for cell SMS service for the box.

ps,**broken link removed** being used by the aircraft owners...

That's exactly what I'm looking for, but I'd like to build it myself... I'm probably looking for the educational experience of building it more than I'm looking for the finished product.

 

[KeepItSimpleStupid]

Here is an SMS tutorial: Short Message Service / SMS Tutorial

 

[Diver300]

If you are using such a system to mainly receive SMSs, and you are not sending SMSs, or only sending a few, it may be a lot cheaper to use a European prepay SIM. They are called PAYG (Pay As You Go) in England.

The credit on Canadian and US SIMs time-expires, so you have to add somewhere around $100 per year.

That doesn't happen on European SIMs. The cellular companies typically demand that you make at least one call or send one text message every 3 months, and add credit every 3 years. That would be £10.00 or €10.00

Texts are free to receive on European SIMs wherever they are. The minimum charge for GPRS access is 1p, wherever the SIM is used. So if all you are doing is receiving texts or sending small amounts of GPRS data, then the costs can be far lower using a European SIM in the US or Canada compared to using a local SIM. Also, a European SIM can usually roam onto any network in the US or Canada, rather than being confined to one.

If you are making calls, sending many text messages or using a lot of data, the roaming charges on a European SIM used in the US or Canada make it far more expensive than a local SIM.