Data Acquisition

Figure 5.30Switch position sensing circuitsWhere it is likely that the signal source will be a button, switch or contact that bounces orglitches, or the signal may be a voltage higher than TTL levels, additional de-bounce and/orvoltage divider circuitry is required.5.10.2 AC/DC voltage sensingIn industrial monitoring and control, the throwing of a switch is used to begin or end anaction, such as switching power to a motor or other machinery. In critical processes, theaction of turning the switch is not necessarily enough to confirm that the motor has receivedpower. This would require the sensing of the AC/DC voltages present at the motor inputs. Asthe AC/DC voltages involved could be quite high, any sensing circuitry directly connected tothe digital I/O interface would need to provide high isolation, in addition to compatibilitywith the TTL digital I/O interface. A very simple AC/DC voltage sensing circuit thatperforms these tasks is shown in Figure 5.31.+5V10K470 Ohm47KR XFilter switch0.1µFIsolated inputnot polarizedCircuitry sharingPC groundFigure 5.31AC/DC voltage sensing circuitThere are several advantages of this low cost circuit. It is polarity insensitive and can bedriven from 12 or 24 volt AC control transformers, the input voltage range can be extended