Analog output from a computers RS232 serial port

[David Hulbert]

What's the easiest way to get analog voltages from a computer? The
current shouldn't be a problem as the signal could be amplified.

Can you just connect the data and ground pin from a serial port and
send all 0s for 0V, 1010101010 for 1/2 voltage and all 1s for full
voltage (with differnt combinations in between)? It's to connect to a
motor, which I doubt would notice anything happening over 1kHz.

Would I have to smooth out the output some how? If so, how can I do
this? (I have a bit of elecronics skill but not lots)

Can you program the data lines on a USB cable to do the same? This
would be more practical but I don't know if it's even possible.

What about parallel ports?
The diagram at http://www.epanorama.net/documents/pc/parallel_dac.html
looks like it may work but I don't know. Might this be easier than a
serial port?

Also, once I've got the voltage, it will probably be from 0V to 5V or
something. What would the easiest way to get that from 0V to around
12V?

Cheers
Dave

 

[Robert Baer]

What's the easiest way to get analog voltages from a computer? The
current shouldn't be a problem as the signal could be amplified.

Can you just connect the data and ground pin from a serial port and
send all 0s for 0V, 1010101010 for 1/2 voltage and all 1s for full
voltage (with differnt combinations in between)? It's to connect to a
motor, which I doubt would notice anything happening over 1kHz.

Would I have to smooth out the output some how? If so, how can I do
this? (I have a bit of elecronics skill but not lots)

Can you program the data lines on a USB cable to do the same? This
would be more practical but I don't know if it's even possible.

What about parallel ports?
The diagram at http://www.epanorama.net/documents/pc/parallel_dac.html
looks like it may work but I don't know. Might this be easier than a
serial port?

Also, once I've got the voltage, it will probably be from 0V to 5V or
something. What would the easiest way to get that from 0V to around
12V?

Cheers
Dave

You would need to convert the serial data blocks to parallel data
bytes, latch themas they arrive, and use the parallel data to drive an
A/D converter.
Just get a plugin D/A card...

 

[John Schuch]

--<snip>--
The question is whether you are looking to get a stable-state voltage,
or some analog signal (like audio). The circuit you linked is
capacitively coupled so it would pass audio (or some fairly fast moving
analog signal) but not DC voltage.

Here's a link to a much better D/A design based on a resistor network
like the one you included:

http://www.eecs.wsu.edu/~ee314/documents/old.docs/lab7.html

This is just a thought; the sound system on a PC is essentially a D/A
converter. Maybe they can be driven to a stable-state DC output. If not
the newer multi-channel systems, perhaps a really old add-on sound board
that you can pick up for a couple bucks.

John

 

[SolarWind]

What's the easiest way to get analog voltages from a computer? The
current shouldn't be a problem as the signal could be amplified.

Can you just connect the data and ground pin from a serial port and
send all 0s for 0V, 1010101010 for 1/2 voltage and all 1s for full
voltage (with differnt combinations in between)? It's to connect to a
motor, which I doubt would notice anything happening over 1kHz.

Would I have to smooth out the output some how? If so, how can I do
this? (I have a bit of elecronics skill but not lots)

Can you program the data lines on a USB cable to do the same? This
would be more practical but I don't know if it's even possible.

What about parallel ports?
The diagram at http://www.epanorama.net/documents/pc/parallel_dac.html
looks like it may work but I don't know. Might this be easier than a
serial port?

Also, once I've got the voltage, it will probably be from 0V to 5V or
something. What would the easiest way to get that from 0V to around
12V?

Cheers
Dave

Dear Dave,

If you just want to control the speed of a DC motor(0 - 5V) by
parallel port, you have done your own design in half way, use your own
design ckt:
http://www.epanorama.net/documents/pc/parallel_dac.html
and remove the output capacitor and connect the output to the base of
a power transistor, and a 1K resistor "R" to be connected to the
transistor emitter and ground, connect the transistor collector to 5V
or 12V directly, you can get 5V/1A DC source from key board connector,
or DC 12V from PC internal connector. This is an emitter follow
circuitry, the DC output voltage is from transistor emitter, the base
impedance is very large = Hfe(amplify of transistor) x R, so the
signal on the base will be not pulled down by the load, now you can
control the pins state of parallel port and you will get different
voltage output in DC, you can connect a 0.2uf/10v capactor between
transistor base and ground to smooth the control voltage, but that
will cause some time delay after changing the parallel port pins
state, so you need to choice the time constant of capacitor and
resistor between the capactor to meet your need.

Note: You should connect a 47uf/10v capacitor to emitter output and
ground to eliminate the spark pulse from motor, if without this
capacitor, the transistor may be punched through by the high voltage
spark that is created by your DC motor brush contacts and the motor
inductive coil.

Now you got a very simple and cheap but effective circuit.

Good luck!

 

[David Hulbert]

[email protected] (David Hulbert) wrote in message news:<[email protected]>...

Dear Dave,

If you just want to control the speed of a DC motor(0 - 5V) by
parallel port, you have done your own design in half way, use your own
design ckt:
http://www.epanorama.net/documents/pc/parallel_dac.html
and remove the output capacitor and connect the output to the base of
a power transistor, and a 1K resistor "R" to be connected to the
transistor emitter and ground, connect the transistor collector to 5V
or 12V directly, you can get 5V/1A DC source from key board connector,
or DC 12V from PC internal connector. This is an emitter follow
circuitry, the DC output voltage is from transistor emitter, the base
impedance is very large = Hfe(amplify of transistor) x R, so the
signal on the base will be not pulled down by the load, now you can
control the pins state of parallel port and you will get different
voltage output in DC, you can connect a 0.2uf/10v capactor between
transistor base and ground to smooth the control voltage, but that
will cause some time delay after changing the parallel port pins
state, so you need to choice the time constant of capacitor and
resistor between the capactor to meet your need.

I think I understand that. Thanks for your reply. I'll try doing that
but might buy a pre-built board if it gets too complicated!

 

[BobGardner]

Check out the ads in the back of Circuit Cellar... lots of smal a/d and d/a
boards available... all kinds 8 bit, 12 bit, 0-5V, +-10V... its out there......

 

[Watson A.Name]

--<snip>--
The question is whether you are looking to get a stable-state voltage,
or some analog signal (like audio). The circuit you linked is
capacitively coupled so it would pass audio (or some fairly fast moving
analog signal) but not DC voltage.

Here's a link to a much better D/A design based on a resistor network
like the one you included:

http://www.eecs.wsu.edu/~ee314/documents/old.docs/lab7.html

This is just a thought; the sound system on a PC is essentially a D/A
converter. Maybe they can be driven to a stable-state DC output. If not
the newer multi-channel systems, perhaps a really old add-on sound board
that you can pick up for a couple bucks.

John

All you need is a R-2R ladder DAC. You use the parallel port and a
25-pin connecotr, and some resistors, I think 27 in all. See URL
http://www.see.ed.ac.uk/~gjrp/EE3/Comms/Lecture16/sld016.htm

Instead of being only four bits high, you would make it 8 bits high.
The resistors would be a convenient value, I used 10k and 20k 5%. But
1% would be best. Instead of the B1, B2, etc., switches, you would
connect the resistors to the 8 bit pins of the parallel port. It's
highly advisable to do this to a parallel adapter card, because if you
mess up the motherboard's parallel port, it's a Real Bad Thing. The
opamp would have a gain of a bit less than 2.5 (2.5 would give you an
output of 12.5VDC assuming each bit has an output of 5VDC.) But since
the resistors load the output pins a bit, the voltage might be less
than 5V.