Cybot Light I/O Board

This is the circuit diagram of the Light I/O Board. Basically it's just an interconnection board between the processor and the sensors, however it has a handful of components fitted, the major one of which is an LM311 comparator chip. This is used for the line-following mode, so it doesn't do anything yet, as well as the LM311 it uses a 47K 'pull-up' resistor, the output of an LM311 is designed so it can be OR'ed with other ones, so it requires this resistor to make it work.

The only functioning parts at stages 8 & 9 are the resistors R21, R22 and capacitors C10, C11, these measure the output from the LDR's by measuring the time it takes for the capacitors to charge up. The LDR's resistance varies with the amount of light falling on them (the more light, the lower the resistance), one end connects to 5V and the other through R21 or R22 to the input of the processor - the capacitors are charged through the LDR's. It's a standard technique to give a cheap crude A2D conversion, taking the LH side as an example - you first set pin 4 on the socket as an output, and set it low, this discharges C10 via the 300 ohm resistor R22, next you set pin 4 to be an input and you start incrementing a counter while you keep checking if the pin is high yet. As C10 charges via the LDR a point will be reached where the input pin reaches it's threshold and goes high, at this point you stop the counter and it includes a value representing the analogue value of the LDR. It's simple, but it works pretty well, in fact analogue joystick ports in PC's work in a similar fashion!.

The other resistor R20, is simply a current limiter from the 6V rail feeding the LED's which are part of Cybots antenna in later issues, they are both connected in series (which means they won't be able to operate independently), and are fed from the processor via pin 6 of the connector.

Cybot graphics used by kind permission of Eaglemoss
Last Updated 18/02/02 You can reach me by email at: nigelg@lpilsley.co.uk