Trigger higher voltage from a parallel port ?

Thread Starter


Joined Jun 30, 2007
My understanding is pretty basic.

I am hooking relays to a parallel port which only has 3.5 volts which is not enough to trigger relay. I need some help please to wire up a 9volt battery in a circuit.
[EDIT -> Actually I think I have this working , I have just just used the 3.5 volts from a parallel port to the emitter and base of a transistor, to switch on 6 volts going across the emitter and collector.)/edit]

Also I wish to use the port to receive analogue input from a potentiometer. I can read bits, bytes or words from the port with the software. My understanding is that a pot will change the voltage from about 0 to 5 volts but to the bare port this will mean nothing excepting it is still receiving the same signal.
So I need a way to convert the voltage change into something else,
like an on -off pulse, or I maybe could use a picaxe chip(when I get one, which is a scary thought when you look at my first question) to send something more detailed if I say got a chip kit and programmed it. Im just thinking out loud not sure here, but if anyone could help even saying these ideas are off track or on . It will be to use the potentiometer for position sensing.



Joined Jul 1, 2007
Hi Mike,
I'm in the process of building a PC interface for a generic data aquisition system using the parallel port. Still plenty of niggles to still sort out as I am a relative novice at all this! I have got a basic system working though and am able to output up to 8 voltages and input 11 voltages and control this through software ( VB6 ). Forget using a PIC or other controller it isn't necessary. To input voltages you need an Analogue to Digital Converter (ADC). I use the TLC2543 ADC ( popular ). The voltage is extracted from the ADC using the Serial Peripheral Interface (SPI) protocol. The parallel port is used to serially feed the ADC an instruction and serially return the result. The Digital to Analogue Converter (DAC) I use is the TLC5628. Again this uses SPI and is controlled in the same way as the ADC. You only need to use 5 parallel port pins to fully control both the ADC and the DAC and they can be connected straight onto the parallel port without practically no intervening components. To input the voltage from your pot, you would just connect it to one of the 11 inputs of the ADC and you can read and display it in software. This may sound like a tough technical challenge but in hindsight it's really not, and I can say this because I've been where you are now.. ;o) Having said that you need to have at least some programming skills because you will obviously be controlling the ADC and DAC using software.


Joined Jun 30, 2006
Has a schematic. ;)

For one relay you need a +12VDC source, A diode, transistor and two resistors.
The schematic they have has a nice "It's on" LED that isn't really needed but only adds an LED and resistor to the circuit.

You want a relay that will work on +12VDC or a solid state relay.

With such a setup you could control any voltage the relay contacts and any contacts connected to them are rated for.

You can use their windows software or there's probably a dozen variants out there that can work. Linux probably has a library and IDE but I'm unfamiliar with it though I will eventually need to use it.

Oh, this seems to allow just the +5 from the port to control a solid state relay