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Electronic Circuit Designing: Modular Approach (Part 5)

Step: 10) Assemble the modules and test the assembled unit

We have built the reusable modules and we have a good idea from the circuit diagram about how their pins should be connected together. At this point we don’t have to worry about the internal circuits of the module and all we need to take care of is the pin outs and connections between modules and other components. The previous circuit diagram is re-drawn here based on this idea.

 Circuit Diagram Of BOARD_A and BOARD_B With Pin Configuration

Fig. 1: Circuit Diagram of BOARD_A and BOARD_B With Pin Configuration

Now we can connect these modules along with the rest of the components in a single board or multiple boards and then interconnect the boards together. In this project I used two separate boards for connecting the entire things. Let us call the boards as BOARD_A and BOARD_B.

My plan is to include the RELAY module, INDICATOR module and the external connectors for audio-video and power in the BOARD_A and the rest of the circuit in BOARD_B. I’ve made such differentiation for the advantage of testing the audio-video switching process using the relays separately.  The following figure shows how the circuit is split across two boards; BOARD_A and BOARD_B.

Circuit Split Across Two Boards; BOARD_A and BOARD_BFig. 2: Circuit Split Across Two Boards; BOARD_A and BOARD_B

The BOARD_B includes the things enclosed in the green outline and the rest of the circuit forms the BOARD_A. The BOARD_A and BOARD_B is connected together using four pins marked as A, B, C and D. The above figure is redrawn more simply as shown below.

 BOARD_A and BOARD_B with Pin Connections

Fig. 3: BOARD_A and BOARD_B with Pin Connections

The pin function details regarding the BOARD_B and BOARD_A is given in the following table.




This pin connects the ground line from the BOARD_A to BOARD_B


This pin provides the regulated voltage generated inside the BOARD_B to BOARD_A


This pin of the BOARD_B drives the relays inside the BOARD_A


This pin connects the VCC line from BOARD_A to the BOARD_B

 Fig. 4: BOARD_B Pin Details

Let us build the BOARD_A and plug in the modules and test it. The BOARD_A includes a RELAY module, INDICATOR module, power supply and audio-video connectors. We are not directly solder these components into the BOARD_A but solder their corresponding connectors only. The connector pins are then interconnected by using solder tracks.

Circuit building on PCB

The RELAY module and INDICATOR module has male berg stick connector pins, and we can use female berg stick connector for plug in the modules. 

 Male And Female Berg Stick Connector

Fig. 5: Male And Female Berg Stick Connector

We can use RMC male connector for plug in the external power supply. 9V DC adaptor connector terminated in a female 2 pin RMC is shown below.

DC Adapter Connector With 2 Pin RMC Female

Fig. 6: DC Adapter Connector With 2 Pin RMC Female

Commonly available female audio-video connectors can be used for connecting the audio-video lines from the set top box and DVD player. The audio-video connector is a little bit costly and hence I’ve decided to reuse them also. I’ve terminated their pins in male berg stick connector pins so that I can plug it into any board having the female berg stick connector. The following image gives you an idea how I made the female audio-video connector reusable rather than permanently soldering into a board.

Audio-Video Connector

     Fig. 7: Audio-Video Connector

Now it is time to build the BOARD_A with the connectors for the modules, power supply, audio-video connectors and the interconnecting solder tracks. The image of the BOARD_A which I’ve built is shown in the following figures.

 BOARD_A Top Plane View

Fig. 8: BOARD_A Top Plane View

 BOARD_A Bottom Plane View

Fig. 9: BOARD_A Bottom Plane View

You might have noticed the female connector marked as ‘capacitor X’, which is nothing but an option for connecting a high value capacitor (>=1000 mfd) in parallel with the power supply. It is intended to check the sudden drop in voltage when the three relays fire at a time.  This is not at all necessary if you are using power supply with high ampere ratings.  

 Plugging and Testing

Let us plug in the modules and the connectors into the BOARD_A and test it without connecting the BOARD_B with it. The image taken after the modules have been plugged into the BOARD_A is shown below.

Assembled BOARD_A With Modules And Connectors

Fig. 10: Assembled BOARD_A with Modules and Connectors

Now the question is how you can test the BOARD_A. The BOARD_A switches the audio-video connectors by activating the relays and the relays are activated whenever they are driven by the zero voltage appears at the PIN C of the BOARD_A. With this knowledge if we directly apply a zero voltage at the PIN C of the BOARD_A after power-up the relays should get activated and the switching should happen. We can use a simple toggle switch plugged into the BOARD_B’s connector port for this purpose. The arrangement is shown in the following figure.

Connector Port Arrangement For BOARD_B

Fig. 11: Connector Port Arrangement for BOARD_B

Testing BOARD_A With Audio-Video Connectors

Fig. 12: Testing BOARD_A with Audio-Video Connectors