View Full Version : Need help with a variety of issues

Kurt Finnie
02-08-2007, 10:03 PM
I'm very new to this but I have put together a basic application of timed light and horn sequences.
I'd like to get this from the development board to 12 volt, 100 watt halogen lights and auto horns.
What relays would be appropriate?

My pub performs a variety of light functions based on the value of a variable which is currently written into the pub.
Next, I'd like to press a keyboard key to select the function.
Later, I'd like to press a key on a radio frequency transmitter to select the function.

I'd also like to write the contents of an ascii file, letter by letter for a timed period, to a single led display.
Eventually, that led will be several feet tall.

I'm sure most of these items are real simple but I just need some guidance to get me headed in the right direction.


Mike Green
02-08-2007, 10:41 PM
Look at the keyboard driver and matching demo that's installed with the Propeller Tool.

Look at various on-line electronics catalogs (like Jameco, Digikey, or Mouser) for relays that can handle the voltage and current you want to use.
Read the "What's a Microcontroller?" tutorial downloadable from the Parallax website for information on controlling relays. This is written
for a Stamp, but the same things apply to the Propeller, particularly regarding controlling relays. Jameco may have some little RF
transmitter/receivers for remote control that may be applicable.

There have been some threads here on controlling LED displays with a Propeller. Have a look at what's been discussed before. Use
the Google search engine specific to Parallax (search.parallax.com (http://search.parallax.com)).

02-08-2007, 10:47 PM
For the relays, I would use a Transistor switch to drive each Relay.
www.wam.umd.edu/~toh/ElectroSim/relay.html (http://www.wam.umd.edu/~toh/ElectroSim/relay.html) Here is a good example.

This should help you interface a high current/voltage device to your propeller.
Just be sure to include that diode across the relay coil to protect your transistor and Propeller from the back EMF.

I wouldn't try connecting a relay to the Propeller directly, I don't know if it has the current output needed to drive such a device. (Plus not many relays are 3 volts)
Besides any old transistor should work in a design like this since it's just a switch. Even those cheap "switching" transistors you can buy at radio shack.

--Andrew Arsenault.

02-09-2007, 07:07 PM
If you want to drive multiple relays then a chip like the ULN2003 (or ULN2007) is a nice choice, it incorporates multiple darlington transistors, with base resistors and the protection diodes in one package.

Most relays will work, but it sounds you are trying to use car parts (horn, lights) so the natural choice would be the relais used in cars, these are specially designed for low voltage high current applications. The ULN2003 would be quite capable of driving these.

For the radio connection, you need a "radio modem" or if its just one, or a few, single functions you want, try a A.M./F.M. Remote control system such as a "keyfob" or a "keeloq" transmitter/receiver system.

If you want an interface over which you want to send complicated commands, or for example text strings, then the best bet is a "radio modem". Its like having a wireless RS232 connection.

For the led display many commercial "led bars" exist that can be driven (programmed) with an RS232 signal, but they generally are small, and are hard to modify to drive a display that is several feet high.
The prop would be suitable to drive such a display, but you need "matrix scan" software. And, for the hardware, you will need a large number of chained shift registers with output latches (74HC595) for the column drivers (with output driver transistors for the high currents that are needed). For the line drivers you could use the same ULN2003's I recommended for the relays.


02-10-2007, 01:45 AM
For an example of someone who built a giant numeric LED display (similar to what I think you want to do), go here (http://www.sparkfun.com/commerce/present.php?p=GPSClock-1)