This is an interface circuit for digital data input. This circuit consist of touch button, op-amp, diode, capacitor and resistors. Any conductor surface can be used as the touch probe, such as a machine-screw head having comparable area around 0.5-inch-square or similar area of any pattern on printed-circuit board. This circuit need a DC power supply that is produced by AC-DC adapter, so an oscillation will occur on the first op-amp when our body extend reception of AC line voltage field. This circuit provide both CMOS output and TTL output. To generate TTL output, a additional op-amp must be added but fortunately it’ uses the same type with the touch probe op-amp. Here is the schematic diagram of the circuit:

Data Entry circuit schematic