how can we drive 100 relays with 40 digital I/O

 

One transistor per relay.
Transistor relay pairs in a 4x25 network.
25 digital pins to the 25 horizontal wires of the network.
4 digital pins to the vertical wires of the network.
One microcontroller
A lot of time writing the program.

 

What determines the relay sequence?

 

Each pin drives one relay. Each realy drives one relay. Now you have 80 relays. 20 2nd tier drive a 3rd relay.

 

use a matrix method, 40 i/o lines = 20 by 20 so you can control upto 400 relays!

make sure the relays have diodes in series with the coils.
In the diagram below, replace the switch with your relay coil or better still an opto coupler.


http://www.google.co.uk/imgres?imgu...onUcfcA6m80QXI1YGoAg&ved=0CD8Q9QEwBQ&dur=1310

 

^-Awesome!