The hardware can be broadly divided into the following parts:
1) Sensors used: Our system is capable of measuring 16 analog voltage inputs with a range of 0 to 3.33V and a resolution of 13mV. At the moment, two temperature sensors made using AD590 ICs are connected to channels 0 and 1. An LDR is connected to channel 2 to measure the ambient lighting conditions.
2) Signal conditioning circuits:
The op-amps used were from LM324 quad op-amp ICs, which are specifically designed to operate from a single rail supply with a high degree of linearity even near saturation.
3) Analog multiplexers: For recording 16 channels, we need a 16x1 analog multiplexer which is not available readily as a single IC. Thus two 8x1 analog multiplexers (CD4051 or 74HC4051) were connected in parallel to produce the desired effect. The output-inhibit inputs were connected to complimentary outputs, so that the outputs of the multiplexers could be connected together.
4) Channel selection logic: Since the parallel port has a limited number of I/O pins for direct access, it was required to establish a 16 channel selection sequence using only two lines. Since in our project, all the 16 channels were accessed sequentially, a four-bit counter (74LS93) was employed with the two lines controlling its count and reset inputs.
The lower three bits were connected directly to the selection inputs, while the highest order bit was connected to the inhibit-input of the higher multiplexer and its complement went to the inhibit input of the lower multiplexer.
5) The A/D converter: An easily available and cheap A/D converter ADC0804 was employed to digitize the analog voltages. A reference voltage of 2/3VCC or 3.33V was chosen giving a least count of 13mV. A full 5V reference was deliberately not chosen keeping in mind the non-linear effects on the op-amp outputs at saturation voltages. The start of conversion signal was sent from the PC to initiate a conversion, and an end of conversion interrupt was received by the PC, which in turn asserted the read signal to enable the output latches and read in the digital data.
The data acquisition circuit showing the channel selction logic, the analog multiplexers, the ADC, and the parallel port interface.
6) Power supply: The circuit is capable of functioning from a single 5V supply, but several sensors produce outputs which need voltage shifting down to near zero point. So an additional -5V was also supplied to make this task easier. The supply is made using a 9-0-9 centre-tap transformer, with a bridge rectifier. These along with the filtering capacitors were placed in a separate adaptor casing and the +9V,-9V outputs are connected to the system using two plugs. On the circuit board, a 7805 and a 7905 voltage regulators were used to produce the +5V and -5V needed by the circuit. Two 100uF capacitors were connected as bypass capacitors to suppress digital switching noise.
The schematic of the +5/-5V supply used.