How to implement an Analog to Digital converter using basic electronics?(The cicuit diagram and a description)?

I need to implement a dual slope integrating type analog to digital converter using basic electronics. Micro controllers (PIC s) or complete a/d converter chips cannot be used. I need to implement this using op amps, BCD to seven segment decoders, counters, seven segment displays,transistors,timer ICs (NE 555), comparators,multiplexers and other basic electronic components such as resistors and capacitors. Here the integrating part has to be implemented using only the basic electronics(Op amps,capacitors and resistors etc).Can any one help me to find a complete circuit diagram(proteus or circuit maker) for this and a description about its operation.