Simple & Cheap Analog to Digital converter

From the times were ADC were expensive and rare, comes a hardware-software solution for data acquisition for PCs. Based on the old Joystick port from the IBM-compatible, a technique of triggering a monostable multivibrator a resistive transducer (thermistor, photocell, strain gauge, etc) and then counting how much cycles it passes before the output of the multivibrator goes high is quite simple to implement on most PC platforms and on most languages. A VisualBasic and Qbasic sample files are shown.

Step 1 The joystick port

For years many home IBM compatible PCs came with a Joystick port that had X axis, Y axis, A & B buttons signal pins. The small DB-15 connector can be accessed at address 200h & 201h. The joystick itself was just two potenciometers and two buttons. Inside the motherboard of the PC, two retriggerable monostable multivibrators (RMM for short) with fixed capacitors and IN/OUT pins were located. The RC time constant was set by the potenciometers and the capacitors. To start the signal conversion, you would make a WRITE to port address 200h and then start counting until you get a bit HIGH by doing a READ to port address 200h. This could be implemented in any language (basic, pascal, c).

Comments

[Biotele]

I enjoy finding simple circuits. Nowadays, you can find a DAC in about any microprocessor. But back in the '90s, this would have been an elegant solution. Try measuring skin resistance and repackage this instructable as a computer based lie detector. You will get huge hits. Very few people can understand analogue engineering these days.