Normally
analogue-to-digital con-verter (ADC) needs interfacing
through a microprocessor to convert analogue data into
digital format. This requires hardware and necessary
software, resulting in increased complexity and hence
the total cost. The circuit of A-to-D converter
shown here is configured around ADC 0808, avoiding the
use of a microprocessor. The ADC 0808 is an 8-bit A-to-D
converter, having data lines D0-D7. It works on the
principle of successive approximation. It has a total
of eight analogue input channels, out of which any one
can be selected using address lines A, B and C. Here,
in this case, input channel IN0 is selected by grounding
A, B and C address lines. Usually the control signals
EOC (end of conversion), SC (start conversion), ALE
(address latch enable) and OE (output enable) are interfaced
by means of a microprocessor. However, the circuit shown
here is built to operate in its continuous mode without
using any microprocessor. Therefore the input control
signals ALE and OE, being active-high, are tied to Vcc
(+5 volts). The input control signal SC, being active-low,
initiates start of conversion at falling edge of the
pulse, whereas the output signal EOC becomes high after
completion of digitisation. This EOC output is coupled
to SC input, where falling edge of EOC output acts as
SC input to direct the ADC to start the conversion.
As the conversion starts, EOC signal goes high. At next
clock pulse EOC output again goes low, and hence SC
is enabled to start the next conversion. Thus, it provides
continuous 8-bit digital output corresponding to instantaneous
value of analogue input. The maximum level of analogue
input voltage should be appropriately scaled down below
positive reference (+5V) level. The ADC 0808 IC
requires clock signal of typically 550 kHz, which can
be easily derived from an astable multivibrator constructed
using 7404 inverter gates. In order to visualise the
digital output, the row of eight LEDs (LED1 through
LED8) have been used, wherein each LED is connected
to respective data lines D0 through D7. Since ADC works
in the continuous mode, it displays digital output as
soon as analogue input is applied.
