START-STOP circuit

Posted on Sep 9, 2012

We are going to build a START-STOP drive (as commonly found in professional tools), that is a circuit with one switch to run the output (an LED in this case) and one switch to stop it. Nutchip feature two kind of inputs: general purpose and fixed inputs. General-purpose programmable inputs (namely IN1 ... IN4) work according to the truth table we program into on-chip memory. A free - not connected to anything - input is always at logic level 1. Connecting the pin to ground (GND, the negative power supply rail) an input will assaume logic level 0. When we want to use a key (or a switch), whe connect it to an input on one side, and to GND on the other.

START-STOP circuit
Click here to download the full size of the above Circuit.

This technique ensures a logic level 0 when the key is pressed (or switch closed) , and a logic level 1 otherwise. Inputs are an important feature because the truth table can read them, steering the Nutchip to do the appropriate actions. Every state lists the input combinations and the state to jump to when the input conditions are meet. The schematic diagram is similar to the one shown in our previous article, with the addition of two key (SW1 and SW2) connected to inputs IN1 and IN2. This is all we need to test the circuit; later we can add a relay in addition to the LED, in order to drive more meaningful load - like an electric motor or a lamp. But now, here it is our schematic. If you have already prepared the board for the experiment from part 1, it's only a matter of adding a copule of keys (ask your dealer for normally open buttons) to it. The kind shown in the photo required soldering some short wire to button pins, in order to fit breadboard pits. If you don't have a couple of buttons at hand, for testing purpose you can "simulate" them touching briefly the GND pis with wire tips. To START LED light, touch GND with the wire connected to Nutchip pin 8 (IN1); to STOP LED light, touch GND with the wire connected to pin 8 (IN2).

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