This document describes a binary temperature monitor project using an 8-bit analog-to-digital converter. The project utilizes an ADC0808 8-bit A/D converter IC to convert the analog output of an AD594 thermocouple amplifier to an 8-bit binary number displayed from 0-255. A 555 timer configured as an astable multivibrator provides the clock for the A/D conversion. The circuit was designed, simulated, constructed, and tested, showing accurate conversion of temperature readings to the equivalent 8-bit binary number.

1. Digital Design Project
Binary Temperature Monitor
Utilizing
8-Bit A-D
Converter
Denis P. Cote

2. Binary Temperature Monitor
Overview
Temperature Monitor :
-100º to 90ºC
8 Bit Binary Output
Display (0-255)
Core: ADC0808 8-Bit A/D
IC – National
Semiconductor
A/D Utilizes Successive
Approximation
Single Supply (Vcc)
Referenced to Ground
Conditioned Analog Input:
AD594 Type J
Thermocouple Amp IC
A/D Clocked via 555
Timer Configured as
Astable Multivibrator
Simple Design, Low Cost
& Expandable

3. Circuit Block Diagram
Voltage
Regulator
(7805)
ADC
0808
LED
Output
Resistor
ICAD594
Thermocouple
&
Heat Source
Supply
Voltage
12V
Clock
LM555
Analog In
Digital Out
Vref (+) = 5 Vdc
Vref (-) = GND

4. Schematic

5. Model Simulation - MULTISIM
A1
ADC
Vin
Vref+
Vref-
D0
D1
D2
D3
D4
D5
D6
D7
SOC
EOCOE
11
U1
DCD_HEX_BLUE
U2
DCD_HEX_BLUE
LED1
A
R2
180 Ω
RPACK8
17
16
18
19
1
2
3
4
5
6
7
8
0
A2
555_VIRTUAL
GND
DIS
OUTRST
VCC
THR
CON
TRI
R3
100Ω
R4
100Ω
VCC
5V
C1
100nF
XFC1
123
23
22
0
VCC
14
15
20
21
U5
DC 10MW
3.000 V
+ -
0
0
VDD
3V
VDD
VCC
5V
VCC
24
555 Timer Model:
Freq Out - ~29.6KHz
Square Wave at Vcc
Fout =1.44/(R3+2R4)C1
LSB
Analog In Simulation:
Vdd = Output of T/C Amp

6. Circuit Prototype
Clock Pulse
Freq. ~30KHz
Binary Out:
10010111
Top = MSB

7. Parts List & Cost
Item Tag Qty Part No. Vendor Description Unit Price
1 U1 1 ADC0808CCN-
ND
Digi Key / Kit IC ADC 8BIT MPU 8CH MUX 28-DIP $ 5.40
2 U2 1 AD594AQ-ND Digi Key IC THERMOCOUPLE A W/COMP 14CDIP $ 11.48
3 U3 1 LM555CNFS-ND Digi Key IC TIMER SINGLE 0-70DEG C 8-DIP $ 0.44
4 U4 1 NJM78L05A-ND Digi Key 5 Volt Positive Regulator $ 0.12
5 LED 1 67-1009-ND Digi Key LED ARRAY 10X25MM 10SEG RED DIFF $ 2.13
6 TC 1 JMTSS-062G-6 Omega Type J Junction Thermocouple Probe $ 24.95
7 R9 1 Resistor KIT 100 Ohm 1/8 watt carbon film resistor $ -
8 R1-R7 1 Resistor IC KIT 300 Ohm Resistor IC $ -
9 R11 1 Resistor KIT 1K Ohm 1/4 watt resistor $ -
10 R10, R12 1 Trimmer KIT 2K Ohm trimmer potentiometer $ -
11 C1, C3, C4, C5 1 Capacitor Radio Shack 0.1 uF ceramic capacitor $ 1.18
12 C2 1 Capacitor KIT 10 uF 25VDC elecrolytic capacitor $ -
13 D1 1 Rectifier Diode KIT 1N4007 Rectifier Diode $ -
14 S1 1 Dip Switch KIT Dip Switch $ -
Misc Total $ 45.70
Hook Up Wire
breadboard

8. Measured Performance
MultiSIM Model
Vin Binary Out Dec Hex
0 0 0 00
0.500 11001 25 19
1.000 110011 51 33
1.500 1001100 76 4C
2.000 1100110 102 66
2.500 1111111 127 7F
3.000 10011001 153 99
3.500 10110011 179 B3
4.000 11001100 204 CC
4.500 11100110 230 E6
5.000 11111111 255 FF
Sensitivity:
5 volts / 255points =
0.019607 V/Point
~20mV/Point
Error: 1/2 LSB & 1 LSB
Circuit
Vin Vin Actual
Volts Lo Hi Binary Dec Hex
Calculated
Vout Comment
0 0 0.009 0 0 00 0.00
0.500 0.487 0.497 11010 26 1A 0.51
1.000 0.986 1.003 110011 51 33 1.00
1.500 1.479 1.492 1001101 77 4D 1.51
2.000 1.983 2.002 1100110 102 66 2.00
2.500 2.471 2.49 10000000 128 80 2.51
3.000 2.982 3 10011001 153 99 3.00
3.400 3.392 3.404 10101110 174 AE 3.41 (Limit of trimmer pot reached)
0.00
0.00
5.000 5.000 5.010 11111111 255 FF 5.00 Vcc
minmax DD
Dx
VzVfs
Vin
Comparison of
Model
Vs.
Circuit Output

9. Measured Performance
ADC0808 - Actual Vs Model
Linearity
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
0 1 2 3 4 5 6
Input Voltage
BinaryOutput(0-255)
MultiSIM Model
Circuit

10. Future Improvements
Increase number of Analog Inputs (0-7)
Robust Display: Decimal or Hex
Enable Multiplexing of Inputs
Parallel DIO output to PC – Data Logger
Disadvantages:
– Low Resolution (256 points)
– Limited to Max input equivalent to Vcc
– Higher Resolution = Chip Replacement

11. Questions/Discussion

12. Back Up
1 8 15 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28
Project Research (3/10 - 3/30/2008)
Project Selection (3/30/2008)
Project Approval
Design / Model
Part Procurement
Construction
Commission
Final Report Out
8-Bit ADC Digital Design Project (Digital Circuits 2) Project Timeline - 3 month outlook Denis Cote
Task
March April May June
ADC0808
Datasheet
Data Sheets:
Analog Devices
AD594
LM555
Datasheet
Schedule: