Datasheets : Crocodile Technology and real-PCB

The circuits for the various units can be drawn in Crocodile Technology, combined to build up a complete circuit and converted to a PCB using real-PCB.

Suppose, for example, that the system planned is a night light – a lamp should light when it is dark. A suitable block diagram might be:

The signal from the Light sensor will go low when it is dark, so the signal from the Inverter will go high, which will switch on the Darlington driver and the Bulb will light.

Read the data sheets for the planned subsystems and make sure they will work as planned. In the case of the present system, looking at the data sheet for the Light sensor we find that we can simplify the circuit by using a Dark sensor, and missing out the Inverter:

There are three ways in which the circuit, based on the data sheets units, can be developed in Crocodile Technology.

• Crocodile Technology includes ‘Standard Blocks’ that are similar to many of the units in the data sheets and can be quickly combined to produce a system. This approach is convenient, but not all the units in the data sheets are available in Crocodile Technology.
• The blocks from the data sheets can be built up from separate components in Crocodile Technology. This is more time consuming.
• A system can be created with some ‘Standard Blocks’ and (for units not available in Crocodile Technology) some made up from components.
• It is possible to create and save ‘Custom Blocks’ to build new units.

In the present example, a Darlington driver and a bulb (Lamp) are available in Crocodile Technology, but no Dark Sensor, so the third or fourth approach is suitable.

Looking ‘inside’ the Lamp block (the nearest equivalent in Crocodile Technology Standard Blocks to the Bulb in the data sheets) we find that it includes a driver, so all we need to add to complete the circuit is the Dark sensor. There is no equivalent Standard Block for this in Crocodile Technology, so this unit has to be made up from separate components.

Next, use the simulation facility in Crocodile Technology to check that your circuit is working as planned (in this case, that the lamp is on when it is dark and off when it is light).

The circuit diagram can now be converted directly to a PCB (File > Export > real-PCB Netlist …). During the conversion process it is possible to select between alternative components. In the case of the present example, the current in a 6V bulb is typically around 60 mA. However, because the current surges at switch on (hotlink to Bulb data sheet), a safety margin of ´10 is recommended, so the driver needs to be able to provide up to 600mA. The ‘standard’ BC548B NPN transistor in Crocodile Technology can deliver a maximum current of 200mA and so is unsuitable. It is advisable to select the BCX38B (which can provide up to 800mA).

The PCB produced in this case will be something like:

It is prudent to check the PCB against the original circuit design before spending a lot of time making the PCB. Print out the PCB and the circuit diagram. Check that each track on the PCB corresponds to a connection on the circuit diagram (a good way to do this is to mark each track and connection with a highlighter).

The connection to the bulb is via the 2-pin terminal block.

The software used to produce the files for these notes was Crocodile Technology version 410.

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