Thread: control radio antenna power to go up via usb

I'm eventually going to do this with a relay connected to my comm port. Supposedly a simple VB program can be written to control this. My plan was to have it put up the antenna when a label or some control that I can watch for from the outside of the program is visible.

can you get specific as to what voltage you want to output from the comm ports?


btw, i love the kmfdm pic

I don't know what a comm port puts out. I haven't done it yet, but others have. Search for "relay" & "comm port".

any ideas?

um, yeah, so can you convert it up to 12v? you should be able to, but i don't remember enough from my electronics class to do it.

um, yeah, so can you convert it up to 12v? you should be able to, but i don't remember enough from my electronics class to do it.

This post may help you design a relay board, so if you are into it then read on

First thing first the parallel or com port cannot supply enough current to drive a relay directly. Youll need to use a transistor to boost it up first.

The problem with connecting directly a relay card to a port is that the BIOS/windows may play with it. Youll see your antenna go up and down before it even boot up to windows. Then some program may even take over it before your application, a printer driver or a hyperterminal perhaps...very unlikley but it can happen.




If you want it secured then the way to get around it is to assign an ID to the relay board so it only react to a particular software. The software must initialise/send an ID/issue a command to the relay controller before the relay is activated.

If you dont care about relay comming on/off as the PC boots up then go for the simple design. Not exactly good for controlling the central lock or boot lock for example.

Otherwise you may need to use a microcontoller or some logic circuitry to ensure the relay only react to a particular application and not to some any software/glitch that put a high or a low signal into the port. You did say you dont know much about electronics but if you know someone who know about MCU programming its not so hard to implement it...a PIC 12C508 can easily intelligently control a bank of relays.

The PIC can be programmed with a bit-banged UART, set to a particular baudrate, 9600-8-N-1 perhaps. Only the RX/TX lines are needed. The remaining output of the PIC can be multiplexed to control multilple relays.

A command is sent to a comport...lets say $00-RELAY1-ON

00 = the ID of the relay board
RELAY1 = the relay number to be controlled
ON = turn the relay on

$00-RELAY1-STATUS
This return the status of the RELAY1 if its currently ON or OFF


In event of a power failure the last STATUS of the relay can even be stored in the EEPROM (no EEPROM on 12C508 but many other have)

Just an example but I think you get the idea.


Ill be in needing a relay board too but not now so I may look into designing one in the future. I still have the vehicle tracking device to finish off, got abit complicated with voice synthesiser so thats how busy I am. But if you can wait longer Ill publish a decent relay board for all to use. I just need someone to do the window software for me.

Anyway the idea is there about controlling the relay in a proper way.

Hope that helps

The COM port has two controllable 5V outputs. All you need is a 5V relay. Connect the contacts to the car's 12V and the antenna remote line and the coil to GND and DTR or RTS.

EDIT: Damn Ricky, you're too quick!

That was not quick at all, I have to be careful with my spelling...mr spell check is around

EDIT :

Im not really sure if its safe to directly connect a relay to the port. I know there are some sensitive relays. But a typical car relay have a coil resistance of 80ohms or so. Use a wrong relay and that can damage the port, the back EMF need to be taken into consideration too.