Interface

Open Datalogging Project

to convert the resistance or voltage into a pulse train that can be passed to the line input of a PC sound card.

The circuit is designed to be build on either a small strip board or on a breadboard, using standard and cheap components.  There are two circuits, one has a complex non-linear response, and a slightly more complex, but much more linear version.
circuit diagram for non-linear interface board This circuit uses the 555 timer as an astable by varying the voltage at terminal 5, it is possible to modulate the signal.  If RA is considerably larger than RB then the change of length of the 'space' is small compared to the change in the length of the 'mark'.

Possible values...

VCC = 9 V
RA = 220 k Ohm
RB = 10 k Ohm
C = 220 pF

This will have a resting frequency of about 27 Hz and a duty cycle of about 4%.  This means that as sampling at 2,700 Hz should be sufficient to determine the modulation with a reasonable degree of accuracy.

Why the Pulse Position Modulator?

There are several different configurations of the 555 that will allow voltage or resistance to frequency conversions.  This one is selected for the following reasons...
  • as the mark is a spike, it is possible to detect it audibly with a crystal ear piece.
  • the moduation input is used as this part of the 555 circuit is realtively immune to noise.


This material is published under an appropriate free and open licence - circuits and worksheets under Creative Commons and the software under the GPL.  

If you wish to help with this project please contact Richard Rothwell - richard@indca.co.uk